Effect of Remelting Temperature and Soaking Time on ... - MDPI

Download PDF
3 downloads 0 Views 8MB Size Report
Comment
4 days ago - School of Materials Science and Engineering, Harbin Institute of ... High mechanical properties of the thixoformed parts .... Tokyo, Japan), Quanta 200 FEI scanning electron microscope (FEI, Hillsboro, OR, USA) equipped with.
metals Article

Effect of Remelting Temperature and Soaking Time on Microstructure and Mechanical Properties of the Thixoformed Part of Nano-Sized SiCp/7075 Composite Jufu Jiang 1, *, Guanfei Xiao 1 , Yingze Liu 1 , Ying Wang 2, * and Xi Nie 1 1 2

*

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (G.X.); [email protected] (Y.L.); [email protected] (X.N.) School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China Correspondence: [email protected] (J.J.); [email protected] (Y.W.); Tel.: +86-18746013176 (J.J.); +86-15945697615 (Y.W.)

Received: 5 August 2018; Accepted: 7 September 2018; Published: 11 September 2018

 

Abstract: Semisolid billet of the 7075 aluminum matrix composite reinforced with nano-sized SiC particles was first fabricated by an ultrasonic-assisted semisolid stirring method and rheoforming technology. Then it was thixoformed into a cylinder part under different remelting temperatures and soak times. The effects of the remelting temperature and soaking time on the mechanical properties and microstructure of the thixoformed composite part were investigated. The results show that parts of good quality were thixoformed successfully. The microstructure of the top side wall of the thixoformed part consisted of near spheroidal grains. A large quantity of elongated grains occurred in the medium and bottom side walls and the bottom itself. With increasing remelting temperatures, the size of the solid grains of the thixoformed parts showed a trend of first to increase and afterwards to decrease. High density dislocations were found in the microstructure when the remelting temperatures were 590 ◦ C and 600 ◦ C. When the soaking time was 15 min, the severest deformation occurred in the thixoformed part. High mechanical properties of the thixoformed parts were achieved under conditions such as a remelting temperature between 590 ◦ C and 600 ◦ C and a soaking time between 10 min and 15 min. The fracture mode of the thixoformed part changed from transgranular fracture to intergranular fracture when the remelting temperature was elevated from 580 ◦ C to 610 ◦ C. After the thixoformed parts were treated by T6, the ultimate tensile strength (UTS) and elongation of the side wall were improved to 552 MPa and 7.9%, respectively. Dispersed MgZn2 precipitates created by T6 heat treatment led to an improvement of the mechanical properties. Keywords: nano-sized SiC particles; 7075 aluminum alloy; microstructure; mechanical properties; thixoforming

1. Introduction Semisolid processing (SSP) developed by M.C. Flemings and D.B. Spencer from the MIT is a typical near-net-shape technology [1,2]. It has exhibited an obvious advantage versus conventional casting and forging [3]. Two typical forming technologies are involved in SSP, rheoforming and thixoforming. The rheforming is referred to as a forming technology in which a semisolid slurry with spheroidal solid grains and liquid phase is carried directed into the die cavity and formed into the final part. The thixoforming is defined as a forming technology in which a semisolid slurry with spheroidal solid grains and liquid is first fabricated, solidified into solid billet, quantitatively cut, reheated to semisolid temperature, and formed into the final parts. In the thixoforming, fabricated semisolid Metals 2018, 8, 709; doi:10.3390/met8090709

www.mdpi.com/journal/metals

Metals 2018, 8, 709

2 of 17

billet with spheroidal grains surrounded by liquid phase undergoes a remelting process and forming process. The fabricated methods of semisolid billet include mechanical stirring [4,5], electromagnetic stirring [6,7], strain induced melt activated (SIMA) [8–10], as well as recrystallization and partial remelting (RAP) [11,12]. Thixoforming exhibits low resistance to deformation and a good ability to fill die cavity because of the spheroidal grains surrounded by liquid phase. The materials used in the thixoforming process involve aluminum alloys, magnesium alloys, high melting point alloys, and composites. Especially for difficult-deform magnesium alloy, thixoforming was found to be a promising route to obtain parts with high performance [13,14]. Casting and wrought aluminum alloys are perfectly applicable for thixoforming of high-performance products. A large amount of research was focused on thixoforming of a variety of cast and wrought aluminum alloys. Research on 7075 aluminum alloy compound forming technology proposed via combining thixoforming and plastic deformation showed that the harness was almost uniform in spite of the different microstructures in the formed parts [15]. Examining the influence of the processing route on the microstructure of thixoformed A356 alloy, Kaio et al. [16] found that samples prepared by the equal channel angular pressing (ECAP) route were suitable for thixoforming. The effect of different process parameters on thixoextrusion of A357 aluminum alloy was investigated [17]. It was concluded that A357 alloy with high solid fraction more than 0.85 allowed more stable material flow at higher speeds. The mechanical properties of the thixoformed A380 aluminum alloy were improved significantly compared to cast alloy [18]. A390 aluminum alloy with 40–50% fraction liquid were near-net shaped into complex automotive components by using thixoforming [19]. The highest yield strength and elongation were obtained in the thixoformed 7075 wrought alloys in a simple graphite die [20]. In addition, 6061 [21], 2014 [22] and 6082 [23] wrought aluminum alloys were also used for thixoforming and showed good thixoformability. Magnesium alloys, as another typical light alloy, were used as materials for thixoforming. The mechanical properties of thixoformed AM60B alloys fabricated by the cyclic extrusion compression (CEC) method were higher than those of thixoformed AM60B alloys prepared by the compression and partial remelting method [24]. Higher mechanical properties were obtained in the thixoformed product of AZ80 magnesium alloy fabricated by the new SIMA method compared to conventional SIMA and semisolid isothermal treatment [25]. High melting point alloys are also used as thixoforming materials. Omar et al. [26] revealed that the HP9/4/30 steel product were thixoformed successfully in the range of 1470–1480 ◦ C. Forming load was also highly repeatable as long as the flow was kept laminar, as reported by Pierret et al. [27]. Composites are also suitable for thixoforming. Cheng et al. [28] reported the effect of solution treatment on the microstructure and properties of thixoformed Mg2 Sip /AM60B composite. They found that the tensile properties increased with increasing solution time from 0 h to 6 h. Semisolid processing shows an obvious advantage in preparing and forming composites reinforced with ceramic particles [29–31]. The reinforcements in these composites are mainly micro-sized ceramic particles. In recent years, nano-sized ceramic particles such as SiC and Al2 O3 have been used to reinforce a metal matrix, so-called nano-sized ceramic particle reinforced aluminum matrix composites (NCPRAMCs) [32–34]. An electromagnetic stirring method was employed to fabricate NCPRAMCs reinforced with a variety of nano-sized ceramic particles such as TiC, Al2 O3, and B4 C [35–40]. In addition, adaptive-network-based fuzzy inference system (ANFIS) and particle swarm optimization (PSO) were used to optimize the process parameters such as mold temperature, mix time, size of reinforcement, and volume fraction of reinforcement [35–40]. How to disperse uniformly the nano-sized ceramic particles is a challenging problem for the fabrication of NCPRAMCs. A new method called ultrasonic-assisted semisolid stirring (UASS) was proposed to solve the dispersion of nano-sized SiC particles during the fabrication of the composite semisolid slurry. In order to fabricate high-quality semisolid slurry of 7075 aluminum matrix composite reinforced with nano-sized SiC particles, the optimal parameters including stirring temperature of 620 ◦ C and stirring time of 20 min were obtained, as reported by Jiang et al. [41]. Furthermore, the effect of stirring time

Metals 2018, 8, 709

3 of 17

and pressure on mechanical properties and microstructure were reported in previous research of Jiang et al. [42]. Besides rheoforming, thixoforming is also a promising technology for forming NCPRAMCs [43]. Thixoforming has exhibited some obvious advantages such as lower resistance to deformation of materials and easier formation of structure components of a complex shape as compared to conventional forging technology. In addition, it also gives an advantage of achieving higher mechanical properties compared to conventional casting technology. However, research on thixoforming of aluminum matrix composite reinforced with nano-sized ceramic particles has been seldom reported. Therefore, the present research aimed to investigate thixoforming of 7075 aluminum matrix composite reinforced with nano-sized SiC particles in order to determine the effect of remelting temperature and soaking time on the microstructure and mechanical properties of the thixoformed parts produced from the nano-sized SiCp/7075 nanocomposite. 2. Materials and Methods Wrought 7075 aluminum alloys supplied by Northeast Light Alloy Co. Ltd. of China (Harbin, China) were used as matrix material of the composite. Its chemical composition was analyzed with an Axios pw4400 X-ray fluorescence spectrometer (PANalytical B.V., Almelo, The Netherlands) and contained 6.0 wt% Zn, 2.3 wt% Mg, 1.56 wt% Cu, 0.26 Si wt%, 0.27 wt% Mn, 0.17 wt% Cr, 0.03 wt% Ti and balance of Al. Nano-sized SiC particles with an average size of 80 nm were used as reinforcement of the composite. It was supplied by Xuzhou Jiechuang New Materials Co. Ltd. of China (Xuzhou, China). The solidus of 546 ◦ C and liquidus of 637 ◦ C temperatures were determined by using a Differential Thermal Analyzer (Mettler-Toledo, Zurich, Switerland) [41]. Solid fractions at various temperatures were obtained by integrating the data of the differential scanning calorimetry (DSC) converted from DTA data [36]. The 7075 aluminum alloy was first melted at 650 ◦ C via a molybdenum crucible placed in a resistance furnace. In order to compensate for the loss of Mg element due to evaporation or formation of Mg oxides, a small pure Mg ingot accounting for 0.2 wt% of the total weight of 7075 alloy was added into the melt. The melt was held isothermally for 10 min. Then nano-sized SiC particles were added into the melt of 7075 alloy. The melt with nano-sized SiC particles was treated for 20 min via a 2 kw ultrasonic device at a frequency of 20 kHz. After that, the 7075 alloy with nano-sized SiC particles was stirred continuously with a molybdenum stirrer when it was cooled from 650 ◦ C (above the liquidus temperature) to 620 ◦ C (a given semisolid temperature). When the 7075 alloy with nano-sized SiC particles was cooled to 620 ◦ C, it was stirred isothermally for 20 min to fabricate a semisolid slurry of composite. The semisolid billet of nano-sized SiCp/7075 composite was fabricated by rheoforming the semisolid slurry via a 2000 kN hydraulic press and a die with a preheated temperature of 300 ◦ C [41]. Figure 1 shows the macrograph of the fabricated semisolid billet of the nano-sized SiCp/7075 composite. Thixoforming experiments were carried out on a designed die with a preheating temperature of 400 ◦ C. The die schematic diagram and real product for the thixoforming cylinder part were given in the previous research of Jiang et al. [42]. The applied load and pressure-holding time during the thixoforming process were 2000 kN and 10 s, respectively. Three groups of thixoforming experiments were performed. In the first group, semisolid billets soaked for 5 min, 10 min, 15 min, 20 min, and 25 min respectively were thixoformed at 580 ◦ C. In the second group, semisolid billets soaked for 5 min, 10 min, 15 min, 20 min, and 25 min respectively were thixoformed at 600 ◦ C. In the third group, semisolid billets soaked for 10 min were thixoformed at 580 ◦ C, 590 ◦ C, 600 ◦ C, and 610 ◦ C, respectively. The microstructural specimens were cut from the locations as shown in Figure 2a, then ground with 200, 400, 600, 800, 1200, and 2000 grit papers, and polished with 0.1 µm diamond paste. The specimens were etched for about 10 s by Keller’s reagent (4 mL HF, 6 mL HCL, 8 mL HNO3 and 82 mL water) and observed by using an Olympus GX71 optical microscope (Olympus Coporation, Tokyo, Japan), Quanta 200 FEI scanning electron microscope (FEI, Hillsboro, OR, USA) equipped with an energy dispersive X-ray spectrometer (EDX) and Jeol2100 (Tokyo, Japan) transmission electron microscope (TEM) and a Talos F200x (FEI, Hillsboro, OR, USA) TEM with Energy Dispersive X-ray

Metals 2018, 8, 709

4 of 17

Metals 2018, 8, x FOR PEER REVIEW Metals 2018, 8, x FOR PEER REVIEW

4 of 18 4 of 18

spectrometers (EDS). Tensile specimens were cut from the locations as shown in Figure 2b and Figure 2b and then machined into standard tensile specimens according to ASTM Standard Test Figure 2b and then standard tensile specimens according to ASTM then machined into machined standard into tensile specimens according to ASTM Standard TestStandard MethodsTest for Methods for Tension Testing of Metallic Materials, E8M [44]. Some tensile specimens were directly Methods for Tension TestingMaterials, of MetallicE8M Materials, E8M [44]. Some tensile specimens weretested directly Tension Testing of Metallic [44]. Some tensile specimens were directly at tested at room temperature. The other specimens were first heat treated in solution for h at 465 °C ◦ C66with tested at room temperature. other specimens weretreated first heat treated for in solution h at 465 °C room temperature. The other The specimens were first heat in solution 6 h at 465for ageing with ageing for◦ 18 h at 125 °C and then used for the tensile test. with for C 18and h atthen 125 °C and fortest. the tensile test. for 18ageing h at 125 used forthen the used tensile

Figure 1. Macrograph of the fabricated semisolid billet of the nano-sized SiCp/7075 composite via Figure 1. Macrograph of the fabricated fabricated semisolid semisolid billet billet of of the the nano-sized nano-sized SiCp/7075 SiCp/7075 composite via rheoforming of the semisolid slurry prepared by ultrasonic-assisted semisolid stirring (UASS). rheoforming of the semisolid slurry prepared by ultrasonic-assisted ultrasonic-assisted semisolid semisolid stirring stirring (UASS). (UASS).

Figure 2. Sampling schematic diagram of the (a) microstructural specimens and (b) the tensile specimens. Figure 2. Sampling schematic diagram of the (a) microstructural specimens and (b) the tensile Figure 2. Sampling schematic diagram of the (a) microstructural specimens and (b) the tensile specimens. 3. Results and Discussion specimens.

Figureand 3 shows the macrograph of the thixoformed part of the nano-sized SiCp/7075 composite. 3. Results Discussion 3. andinDiscussion AsResults indicated Figure 3, a cylinder part was thixoformed successfully due to good filling ability of composite. Figure 3 shows the macrograph of the was thixoformed part of the nano-sized SiCp/7075 the semisolid billet. the Good surface quality observedpart in the thixoformed A half-sectional composite. Figure 3 shows macrograph of the thixoformed of the nano-sizedpart. SiCp/7075 As indicated in Figure 3, a cylinder part was thixoformed successfully due to good filling ability of macrograph 4. Aspart shown Figure 4, the macrostructure ofgood the thixoformed part As indicated is inshown Figurein 3, Figure a cylinder wasin thixoformed successfully due to filling ability of the semisolid billet. Good surface quality was observed in the thixoformed part. A half-sectional is very dense and noGood obvious porosity was was found in the macrostructure. In addition, is noticeable the semisolid billet. surface quality observed in the thixoformed part. Aithalf-sectional macrograph is shown in Figure 4. As shown in Figure 4, the macrostructure of the thixoformed part that slight clusters of in nano-sized SiCshown particles are found the macrostructure. It illustrates part that macrograph is shown Figure 4. As in Figure 4, theinmacrostructure of the thixoformed is very dense and no obvious porosity was found in the macrostructure. In addition, it is noticeable absolutely perfect the nano-sized SiC in particles in the 7075 alloy is very difficult to is very dense and dispersion no obviousofporosity was found the macrostructure. In matrix addition, it is noticeable that slight clusters of nano-sized SiC particles are found in the macrostructure. It illustrates that obtain although transient cavitation and acousticare streaming by an ultrasonic and high that slight clusters of nano-sized SiC particles found increated the macrostructure. It wave illustrates that absolutely perfect dispersion of the nano-sized SiC particles in the 7075 alloy matrix is very difficult and controllable of the semisolid slurrySiC canparticles disperseinthe particles effectively. absolutely perfectviscosity dispersion of the nano-sized thenano-sized 7075 alloySiC matrix is very difficult to obtain although transient cavitation and acoustic streaming created by ander ultrasonic wave and and Therefore, some slight clusters were found in the matrix due to large Van Waals forces to obtain although transient cavitation and acoustic streaming created by an ultrasonic wave and high and controllable viscosity of the semisolid slurry can disperse the nano-sized SiC particles electrostatic forces between the nano-sized SiC particles 4). high and controllable viscosity of the semisolid slurry(Figure can disperse the nano-sized SiC particles effectively. Therefore, some slight clusters were found in the matrix due to large Van der (SEM) Waals Figure Therefore, 5 gives the optical scanning electron microscope effectively. some slightmetalloscope clusters were (OM) found and in the matrix due to large Van der Waals forces and electrostatic forces between the nano-sized SiC particles (Figure 4). microstructure in different locations of the the nano-sized thixoformed part of the nano-sized forces and electrostatic forces between SiC particles (Figure 4). SiCp/7075 composite Figure 5 gives the optical metalloscope (OM) and scanning electron microscope (SEM) ◦ soaked for 10 590 optical C. As shown in Figure 5a,d,and the microstructure consists of solid (SEM) grains Figure 5 min givesat the metalloscope (OM) scanning electron microscope microstructure in different locations of the thixoformed part of the nano-sized SiCp/7075 composite microstructure in different locations of the thixoformed part of the nano-sized SiCp/7075 composite soaked for 10 min at 590 °C. As shown in Figure 5a,d, the microstructure consists of solid grains soaked for 10 min at 590 °C. As shown in Figure 5a,d, the microstructure consists of solid grains

Metals 2018, 8, 709 Metals 2018, 8, x FOR PEER REVIEW Metals 2018, 8, x FOR PEER REVIEW

5 of 17 5 of 18 5 of 18

and and liquid liquid phase. phase. Fine, Fine, near near spheroidal spheroidal solid solid grains grains and and elongated elongated solid solid grains grains along along the the flowing flowing and liquid phase. Fine, near spheroidal solid grains and elongated solid grains along the flowing direction of the semisolid slurry were found in the microstructure of the thixoformed thixoformed part. The fine direction of the semisolid slurry were found in the microstructure of the part. The fine direction of the semisolid slurry were found in the microstructure of the thixoformed part. The fine microstructure of the semisolid slurry fabricated by the ultrasonic-assisted semisolid stirring microstructure of slurry fabricated by the semisolid stirring method microstructure ofthe thesemisolid semisolid slurry fabricated byultrasonic-assisted the ultrasonic-assisted semisolid stirring method led to solid the fine solidofgrains of the thixoformed part [41]. led to the fine grains the thixoformed part [41]. method led to the fine solid grains of the thixoformed part [41].

Figure 3. Macrographs of the thixoformed part of the nano-sized SiCp/7075 composite: (a) front view Figure 3. Macrographs nano-sized SiCp/7075 SiCp/7075 composite: Macrographs of the thixoformed part of the nano-sized composite: (a) (a) front front view view and (b) top view. and (b) top view.

Figure 4. Half-sectional macrograph of the thixoformed part of the nano-sized nano-sized SiCp/7075 SiCp/7075 composite. composite. Figure 4. Half-sectional macrograph of the thixoformed part of the nano-sized SiCp/7075 composite.

The microstructure of fine grains is beneficial for improving the mechanical properties microstructureconsisting consisting of fine grains is beneficial for improving the mechanical The microstructure consisting of fine grains is beneficial for improving the mechanical of the thixoformed part. In addition, can be seen that thebe solid grains location A almost a near properties of the thixoformed part.itIn addition, it can seen that in the solid grains in keep location A properties of the thixoformed part. In addition, it can be seen that the solid grains in location A spheroidal (Figure 5a). However, the solid in thethe locations B, C, and D were obviously almost keepshape a near spheroidal shape (Figure 5a).grains However, solid grains in the locations B, C, almost keep a near spheroidal shape (Figure 5a). However, the solid grains in the locations B, C, elongated the flowing direction slurry (Figure 5b–d). The high resolution and D werealong obviously elongated alongof thesemisolid flowing direction of semisolid slurry (Figure 5b–d).SEM The and D were obviously elongated along the flowing direction of semisolid slurry (Figure 5b–d). The exhibited a homogeneous dispersion of nano-sizeddispersion SiC particles 5e). There are four deformation high resolution SEM exhibited a homogeneous of(Figure nano-sized SiC particles (Figure 5e). high resolution SEM exhibited a homogeneous dispersion of nano-sized SiC particles (Figure 5e). mechanisms in the semisolid compression liquid flow (LF), flow ofprocess, liquid incorporating solid There are four deformation mechanisms process, in the semisolid compression liquid flow (LF), There are four deformation mechanisms in the semisolid compression process, liquid flow (LF), grainsof(FLS), between solid grains plastic deformation solid(SSS), grainsand (PDS) [45]. flow liquidsliding incorporating grains (SSS), (FLS),and sliding between solid of grains plastic flow of liquid incorporating solid grains (FLS), sliding between solid grains (SSS), and plastic The top side wall (location of the cylinder part haswall a free surfaceA) without limits of part die during the deformation of solid grainsA) (PDS) [45]. The top side (location of the cylinder has a free deformation of solid grains (PDS) [45]. The top side wall (location A) of the cylinder part has a free medium stage of thixoforming [42]. Its deformation mode belongs to liquid flow (LF), flow of liquid surface without limits of die during the medium stage of thixoforming [42]. Its deformation mode surface without limits of die during the medium stage of thixoforming [42]. Its deformation mode incorporating solid grains (FLS), slidingincorporating between solidsolid grains (SSS).(FLS), Therefore, the solid grains belongs to liquid flow (LF), flowand of liquid grains and sliding between belongs to liquid flow (LF), flow of liquid incorporating solid grains (FLS), and sliding between almostgrains remain spheroidal shape deformation. However, deformation modes locations B, solid (SSS). Therefore, theafter solid grains almost remainthe spheroidal shape after in deformation. solid grains (SSS). Therefore, the solid grains almost remain spheroidal shape after deformation. C, and D belong to plastic deformation of solid B, grains (PDS). As a consequence, this resulted the However, the deformation modes in locations C, and D belong to plastic deformation of in solid However, the deformation modes in locations B, C, and D belong to plastic deformation of solid elongated grains deformation. grains (PDS). As aafter consequence, this resulted in the elongated grains after deformation. grains (PDS). As a consequence, this resulted in the elongated grains after deformation. In addition, it can be seen that the liquid fraction in the microstructure decreases with a In addition, it can be seen that the liquid fraction in the microstructure decreases with a location change from A to D. The liquid fraction in location A is obviously higher than those in location change from A to D. The liquid fraction in location A is obviously higher than those in locations B, C, and D. A similar phenomenon was found in the microstructure of the rheoformed locations B, C, and D. A similar phenomenon was found in the microstructure of the rheoformed

Metals 2018, 8, 709

6 of 17

In addition, it can be seen that the liquid fraction in the microstructure decreases with a location change from A to D. The liquid fraction in location A is obviously higher than those in locations B, C, and D. A similar phenomenon was found in the microstructure of the rheoformed cylinder parts of Metals 2018, 8, x FOR PEER REVIEW 6 of 18 nano-sized SiCp/7075 composite [42]. The difference between them is that the extent of change of the liquidcylinder fractionparts in the part is less than that[42]. in the part. This is attributed ofthixoformed nano-sized SiC p/7075 composite Therheoformed difference between them is that theto the lowerextent remelting temperature in the thixoforming process compared to the process shown of change of the liquid fraction in the thixoformed part is less thanrheoforming that in the rheoformed This isresearch attributed[42]. to the lower remelting temperature in the thixoforming process compared to in a in thepart. previous The amount of liquid phase influences the deformation mechanism the rheoforming process shown in the previous research [42]. The amount of liquid phase difference location. In location A, liquid phase is aggregated obviously due to lower flowing resistance influenceswith the solid deformation a difference location. A, liquid phase is as compared grains,mechanism resulting ininthe occurrence of FLS In or location SSS deformation mechanisms. aggregated obviously due to lower flowing resistance as compared with solid grains, resulting in Compared with location A, the amount of liquid phase in locations B, C, and D decreases obviously the occurrence of FLS or SSS deformation mechanisms. Compared with location A, the amount of due to its segregation in location A. Consequently, the deformation mechanism of PDS is dominant liquid phase in locations B, C, and D decreases obviously due to its segregation in location A. during the flow of semisolid slurry. Consequently, the deformation mechanism of PDS is dominant during the flow of semisolid slurry.

Figure 5. Microstructure in differentlocations locationsof of the the thixoformed thixoformed part the nano-sized SiCp/7075 Figure 5. Microstructure in different partofof the nano-sized SiCp/7075 composite soaked for 10 min at 590 °C: (a) Optical microscopy (OM) image in location A; (b) ◦ composite soaked for 10 min at 590 C: (a) Optical microscopy (OM) image in location A;OM (b) OM image in location B; (c) OM image in location C; (d) OM image in location D and (e) scanning electron image in location B; (c) OM image in location C; (d) OM image in location D and (e) scanning electron microscopy (SEM) image in location C. microscopy (SEM) image in location C.

Metals 2018, 8, 709

7 of 17

Metals 2018, 8, x FORthe PEER REVIEW 7 of 18 Figure 6 shows OM microstructure in location B of the thixoformed parts of the nano-sized SiCp/7075 composite soaked for 10 min at different remelting temperatures. As indicated in Figure 6, Figure 6 shows the OM microstructure in location B of the thixoformed parts of the nano-sized the solid grains are elongated obviously along the flowing direction. The grain size of the thixoformed SiCp/7075 composite soaked for 10 min at different remelting temperatures. As◦ indicated in Figure part slightly changes when the remelting temperature increases from 580 C to 590 ◦ C. However, 6, the solid grains are elongated obviously along the flowing direction. The grain size of the the grain size of the thixoformed partwhen increases obviously when theincreases remelting temperature is °C. elevated thixoformed part slightly changes the remelting temperature from 580 °C to 590 ◦ to 600However, C. This the is due to size the fact thatthixoformed elevated remelting temperature coarsening grain of the part increases obviouslyleads whento the remeltingof the microstructure ofisthe semisolid during remelting process. Therefore, the coarsened temperature elevated to 600 billet °C. This is due the to the fact that elevated remelting temperature leads to solid coarsening of the microstructure of the semisolid billet during the remelting process. Therefore, the grains remain in the microstructure of the thixoformed parts. Grain size decreases upon remelting ◦ coarsened solid grains remain in the microstructure of the thixoformed parts. Grain size decreases at a temperature of 610 C. The coarsening trend and melting trend together influence the grain remelting at a temperature of 610 °C. The coarsening trend and melting trend together size ofupon the semisolid billet during the remelting process [46]. When the remelting temperature is influence the grain size of the semisolid billet during the remelting process [46]. When the ◦ 610 C, the melting trend is dominant. Therefore, this leads to a decrease of grain size of the semisolid remelting temperature is 610 °C, the melting trend is dominant. Therefore, this leads to a decrease of billet during theofremelting process. thethe semisolid was After thixoformed, the grain grain size the semisolid billetAfter during remeltingbillet process. the semisolid billet size was of the thixoformed parts slightly decreased. On the whole, the size of the solid grains in the semisolid thixoformed, the grain size of the thixoformed parts slightly decreased. On the whole, the size of the slurry exhibits first an increase, followed byexhibits a decrease with increasing remelting temperatures. This was solid grains in the semisolid slurry first an increase, followed by a decrease with increasing remelting was attributed to and the double coarsening trend and the melting attributed to thetemperatures. double effectThis of coarsening trend meltingeffect trendofcaused by increasing remelting trend caused by increasing temperatures. addition,are it isslightly also noticeable thatinnano temperatures. In addition, it is the alsoremelting noticeable that nanoIn particles clustered Figures 5 particles are slightly clustered in Figures 5 and 6. This was attributed to large Van der Waals force and 6. This was attributed to large Van der Waals force and electrostatic force between nano-sized SiC and electrostatic force between nano-sized SiC particles. Therefore, absolutely homogeneous particles. Therefore, absolutely homogeneous dispersion of nano-sized is not possible to obtain due to dispersion of nano-sized is not possible to obtain due to the large Van der Waals and electrostatic the large Van der Waals and electrostatic forces between the nano-sized SiC particles. forces between the nano-sized SiC particles.

6. OM microstructure locationBBof of the the thixoformed thixoformed parts of of thethe nano-sized SiCp/7075 FigureFigure 6. OM microstructure in in location parts nano-sized SiCp/7075 composite soaked for 10 min at different remelting temperatures: (a) 580 ◦°C, (b) 590 °C, (c) ◦ C, and composite soaked for 10 min at different remelting temperatures: (a) 580 C, (b) 590 ◦ C, (c)600 600°C, and (d) 610 °C. ◦ (d) 610 C.

Figure 7 shows the mechanical properties of the thixoformed parts of the nano-sized SiCp/7075

Figure 7 shows the mechanical properties of the thixoformed parts of the nano-sized SiCp/7075 composite soaked for 10 min at different remelting temperatures. As indicated in Figure 7, the composite soaked for 10 min at different remelting temperatures. As indicated in Figure 7, the ultimate ultimate tensile strength (UTS) of the side wall of the thixoformed part only slightly increases when tensilethe strength (UTS) of the increases side wall of 580 the °C thixoformed part only increases when the remelting temperature from to 590 °C. However, the slightly UTS decreases gradually remelting temperature increases from 580 ◦ C to 590 ◦ C. However, the UTS decreases gradually upon further increase of the remelting temperature. The highest UTS of 313 MPa was obtained at a remelting temperature of 590 ◦ C. The highest elongation of 5.4% of the thixoformed part was achieved at a

Metals 2018, 8, x FOR PEER REVIEW

8 of 18

upon further increase of the remelting temperature. The highest UTS of 313 MPa was obtained at a Metals 2018, 8, 709 8 of 17 remelting temperature of 590 °C. The highest elongation of 5.4% of the thixoformed part was achieved at a remelting temperature of 600 °C. It can be noted that the UTS and elongation of the ◦ C. It side wall of the thixoformed arecan higher thanthat those atUTS the bottom of the thixoformed part. of Asthe to remelting temperature of 600part be noted the and elongation of the side wall the mechanical properties at than the bottom ofthe thebottom thixoformed the highest and elongation of thixoformed part are higher those at of theparts, thixoformed part.UTS As to the mechanical the thixoformed were achieved at a remelting temperature ofand 600 elongation °C. According to thixoformed the UTS and properties at the parts bottom of the thixoformed parts, the highest UTS of the ◦ C. According elongation of the thixoformed parts, the optimal remelting temperatures 590 °C and parts were achieved at a remelting temperature of 600 to the should UTS andbeelongation of 600 the °C. thixoformed parts, the optimal remelting temperatures should be 590 ◦ C and 600 ◦ C. The liquid fraction influences the deformation degree of solid grains during the thixoforming process. It can be noted that the liquid fraction increases increases with with an increase increase of of remelting remelting temperature. temperature. ◦ ◦ ◦ ◦ 590 °C, C, 600600 °C. and 610 610 °C, the fractions are 0.19, When the the remelting remeltingtemperatures temperaturesare are580 580°C,C, 590 C. and C, liquid the liquid fractions are 0.3, [41]. Therefore, theliquid fractions at remelting temperatures of 580 °C 0.19,0.38 0.3,and 0.380.49 andrespectively 0.49 respectively [41]. Therefore, theliquid fractions at remelting temperatures of ◦ C are ◦ C and ◦ C. Theextent and °C are thanless those atthose 600 °Catand °C. The between solid grains becomes 580 ◦590 C and 590less than 600610 610 contact contact extent between solid grains large duelarge to a small liquidof phase atphase a lowat remelting temperature. As a consequence, larger becomes due toamount a small of amount liquid a low remelting temperature. As a consequence, deformation occursoccurs in the solid leading to more strain strain strengthening. larger deformation in thegrains, solid grains, leading to obvious more obvious strengthening.

Figure 7. Mechanical properties of the thixoformed parts of the nano-sized SiCp/7075 composite soaked for 10 min at different remelting Figure 7. Mechanical properties of the temperatures. thixoformed parts of the nano-sized SiCp/7075 composite soaked for 10 min at different remelting temperatures.

Figure 8 gives evidence of the dislocations created in the thixoformed composite parts at different remelting temperatures. More dislocations were created due in to large deformation,composite which can improve Figure 8 gives evidence of the dislocations created the thixoformed parts at the UTS by hindering grain sliding.More As shown in Figure 8, a created large amount high deformation, density dislocations different remelting temperatures. dislocations were due tooflarge which ◦ C, 590 ◦ C, and 600 ◦ C. were found in the microstructure of the thixoformed composite parts at 580 can improve the UTS by hindering grain sliding. As shown in Figure 8, a large amount of high An obvious dislocation wall wasin created in the microstructure of the thixoformed composite part°C, at density dislocations were found the microstructure of the thixoformed composite parts at 580 ◦ 610 °C, C. and This600 was°C. attributed to dynamic recovery occurring deformation process, leading to a 590 An obvious dislocation wall was created in the microstructure of the thixoformed decreased dislocation density. It is also consistent with the decreased UTS of the thixoformed composite composite part at 610 °C. This was attributed to dynamic recovery occurring in the deformation ◦ at 610 Cleading (Figureto7).a decreased Especially,dislocation high density dislocations were noted in thethe thixoformed composite process, density. It is also consistent with decreased UTS of the ◦ C. Furthermore, at 590 ◦ C. This illustratesatthat highest 7). UTS of the sidehigh walldensity was obtained at 590were thixoformed composite 610 the °C (Figure Especially, dislocations noted in the ◦ C is larger than those as shown in Figure 6, the degree of the part of at the 590 side thixoformed composite atdeformation 590 °C. This illustrates thatthixoformed the highest UTS wall was obtained ◦ C, 600 ◦ C, and 610 ◦ C. In addition, fine grains also improve the UTS of the thixoformed part 580 °C. at 590 Furthermore, as shown in Figure 6, the deformation degree of the thixoformed part at 590 according the Hall–Petch [47]. °C is largertothan those at 580effect °C, 600 °C, and 610 °C. In addition, fine grains also improve the UTS of The microstructure of the thixoformed part soaked for different times at 600 ◦ C is presented in the thixoformed part according to the Hall–Petch effect [47]. Figure 9. As indicated in Figure 9, a large number of elongated grains were found in the microstructure. Furthermore, the elongated extent of the solid grains with a soaking time of 15 min is the largest, illustrating the largest deformation degree. This led to improvement of UTS and elongation of the thixoformed part (see Figure 10). Therefore, the highest mechanical properties of the thixoformed part are achieved with a soaking time of 15 min. When the soaking time is 20 min and 25 min, obvious liquid phase segregation was found in the microstructure, which led to a reduction of mechanical properties. Unlike the mechanical properties of the thixoformed part with elevated remelting temperature, no obvious regulation was found in the mechanical properties of the thixoformed part soaked for different times at 600 ◦ C.

Metals 2018, 8, 709 Metals 2018, 8, x FOR PEER REVIEW

9 of 17 9 of 18

FigureFigure 8. Transmission electron microscopy (TEM) images in location B of the composite 8. Transmission electron microscopy (TEM) images in location B ofthixoformed the thixoformed ◦ C, (b) 590 ◦ C, (c) 600 ◦ C, (d) 610 ◦ C. parts at different remelting temperatures (a) 580 composite parts at different remelting temperatures (a) 580 °C, (b) 590 °C, (c) 600 °C, (d) 610 °C. The the an thixoformed part soaked times at 600part °C issoaked presented Figure 11microstructure and Table 1 of give EDX analysis resultfor of different the thixoformed forin10 min ◦ Figure 9. As indicated in Figure 9, a large number of elongated grains were found in the at 590 C. This illustrates the existence of nano-sized SiC in the nano-sized SiCp/7075 composite. microstructure. Furthermore, the elongated extent of the solid grains with a soaking time of 15 min is than Furthermore, it can be noticed that nano-sized SiC particles in the grain boundaries are more the largest, illustrating the largest deformation degree. This led to improvement of UTS and those in the grains. Mg and Zn elements in the grain boundaries are less than those in the interior elongation of the thixoformed part (see Figure 10). Therefore, the highest mechanical properties of of the grains. This indicates the η phase (MgZn2 ) mainly exists in the grain boundaries, leading to the thixoformed part are achieved with a soaking time of 15 min. When the soaking time is 20 min some and consumption of Mgliquid and Zn elements in thewas grain boundaries. It is noticeable that 25 min, obvious phase segregation found in the microstructure, which ledCu toelement a aggregated in the grain boundaries. reduction of mechanical properties. Unlike the mechanical properties of the thixoformed part with The solidremelting phase consisted of the α-Al phase containing more element properties and less of other elevated temperature, no obvious regulation was found in of theAl mechanical the elements thixoformed part as soaked for different times at 600 °C. alloying such Zn, Mg, and Cu. The chemical composition of the solid phase is given in Table 1. The liquid phase consisted of the α-Al phase containing less Al element and more other alloying elements such as Zn, Mg, and Cu and the second phase such as η phase (MgZn2 ) and T phase (Al2 Mg3 Zn3 ). The chemical composition of the liquid phase is shown in Table 1. High-resolution TEM micrograph and EDX results are presented in Figure 12. Tables 2 and 3 exhibit the elements in locations A and B. As indicated in Table 1, the phase contains C and Si elements, implying the existence of nano-sized SiC particles. However, the matrix phase (α-Al) does not contain Si element. As shown in Figure 12, nano-sized SiC particles are distributed uniformly in the microstructure. It also illustrates that the ultrasonic-assisted semisolid stirring method is suitable for fabricating a semisolid slurry of the nano-sized SiCp/7075 composite.

Metals 2018, 8, 709 Metals 2018, 8, x FOR PEER REVIEW

10 of 17 10 of 18

OM microstructure thixoformed part forfor different timestimes at 600at°C: (a) ◦5C: min, Figure Figure 9. OM9.microstructure of of thethe thixoformed partsoaked soaked different 600 (a) 5 min, Metals 2018, 8,(b) x FOR PEER REVIEW 11 of 18 10 min, (c) 15 min, (d) 20 min, and (e) 25 min.

(b) 10 min, (c) 15 min, (d) 20 min, and (e) 25 min.

◦ C. Figure 10. Mechanical properties of the thixoformed part soaked for different different times times at at 600 600 °C.

Figure 11 and Table 1 give an EDX analysis result of the thixoformed part soaked for 10 min at 590 °C. This illustrates the existence of nano-sized SiC in the nano-sized SiCp/7075 composite. Furthermore, it can be noticed that nano-sized SiC particles in the grain boundaries are more than those in the grains. Mg and Zn elements in the grain boundaries are less than those in the interior of the grains. This indicates the η phase (MgZn2) mainly exists in the grain boundaries, leading to some consumption of Mg and Zn elements in the grain boundaries. It is noticeable that Cu element

590 °C. This illustrates the existence of nano-sized SiC in the nano-sized SiCp/7075 composite. Furthermore, it can be noticed that nano-sized SiC particles in the grain boundaries are more than those in the grains. Mg and Zn elements in the grain boundaries are less than those in the interior of the grains. This indicates the η phase (MgZn2) mainly exists in the grain boundaries, leading to some consumption Metals 2018, 8, 709of Mg and Zn elements in the grain boundaries. It is noticeable that Cu element 11 of 17 aggregated in the grain boundaries.

Figure 11. Positions Positions of energy dispersive X-ray spectrometry (EDX) analysis of the thixoformed thixoformed part ◦ soaked for 10 min at 590 °C. C. Table 1. Energy dispersive X-ray spectrometry (EDX) results in different locations of the thixoformed

The solid phase consisted ◦of the α-Al phase containing more of Al element and less of other part soaked for 10 min at 590 C. Metals 2018,elements 8, x FOR PEER 12 of in 18 alloying suchREVIEW as Zn, Mg, and Cu. The chemical composition of the solid phase is given Table 1. The liquid phase consisted of the phase Mg containing Al element and more other Elements Al α-AlZn Cu less Si C High-resolution TEM micrograph and EDX results are phase presented inasFigure 12. (MgZn Tables2)2 and and T3 alloying elements such as Zn, Mg, and Cu and the second such η phase wt% 88.91 5.51 3.08 0.88 0.58 1.04 exhibit the elements in locations A and B. As indicated in Table 1, the phase contains C and Si Position A phase (Al2Mg3Zn3). The chemicalat% composition the liquid 90.86 of2.32 3.49 phase 0.38is shown 0.57 in Table 2.38 1. elements, implying the existence of nano-sized SiC the matrix wt% 86.13 4.77particles. 2.56 However, 1.12 2.81 2.62 phase (α-Al) does Position B not contain Si element. As shown in Figure 12, nano-sized SiC particles are distributed uniformly in at% 86.13 1.97 2.84 0.47 2.70 Table 1. Energy dispersive X-ray spectrometry (EDX) results in different locations 5.88 of the thixoformed wt% 89.52 5.07 2.77 0.98 0.25 1.40 the microstructure. It min alsoat illustrates part soaked forPosition 10 590 °C. that the ultrasonic-assisted semisolid stirring method is suitable C 91.07 2.07 0.41 0.24 3.18 for fabricating a semisolid slurryat% of the nano-sized SiCp3.04 /7075 composite. Elements Al Zn Mg Cu Si C wt% 88.91 5.51 3.08 0.88 0.58 1.04 Position A at% 90.86 2.32 3.49 0.38 0.57 2.38 wt% 86.13 4.77 2.56 1.12 2.81 2.62 Position B at% 86.13 1.97 2.84 0.47 2.70 5.88 wt% 89.52 5.07 2.77 0.98 0.25 1.40 Position C at% 91.07 2.07 3.04 0.41 0.24 3.18

Figure of the the thixoformed thixoformed part part soaked soaked for for 10 10 min min at at 590 590 ◦°C. Figure 12. 12. High-resolution High-resolution TEM TEM micrograph micrograph of C. ◦ C. Table 2. Table 2. Element Element in in location location A A of of the the thixoformed thixoformed part part soaked soaked for for 10 10 min min at at 590 590 °C.

Element Element Carbon Carbon Aluminum Aluminum Copper Copper Chromium Chromium Manganese Iron Manganese Silicon Iron Silicon

Series Series K-series K-series K-series K-series K-series K-series K-series K-series K-series K-series K-series K-series K-series K-series

Net (wt%) (norm. (norm. wt%) wt%) Net (wt%) 3216 2.49 2.49 3216 2.49 2.49 269,060 87.22 87.22 269,060 87.22 87.22 5101 2.74 2.74 5101 2.74 2.74 1031 0.47 0.47 1031 0.47 0.47 4941 2.08 2.08 8962 3.90 3.90 4941 2.08 2.08 3469 1.10 1.10 8962 3.90 3.90 Sum 100 100 3469 1.10 1.10 Sum 100 100

(norm. at%) at%) (norm. 5.69 5.69 88.84 88.84 1.18 1.18 0.25 0.25 1.04 1.92 1.04 1.08 1.92 100 1.08 100

Errorin in wt% wt% Error 0.332 0.332 7.942 7.942 0.352 0.352 0.14 0.14 0.29 0.45 0.29 0.13 0.45 0.13

Table 3. Element in location B of the thixoformed part soaked for 10 min at 590 °C. Element

Series

Net

(wt%)

(norm. wt%)

(norm. at%)

Error in wt%

Metals 2018, 8, 709

12 of 17

Table 3. Element in location B of the thixoformed part soaked for 10 min at 590 ◦ C. Element

Series

Net

(wt%)

(norm. wt%)

(norm. at%)

Error in wt%

Carbon Aluminum Chromium Manganese Copper

K-series K-series K-series K-series K-series

69,567 1,174,789 596 749 10,023 Sum

12.21 86.43 0.06 0.08 1.22 100

12.21 86.43 0.06 0.08 1.22 100

23.96 75.52 0.03 0.04 0.45 100

1.18 7.86 0.08 0.09 0.19

Figure 13 gives the tensile fracture at the bottom of the thixoformed parts soaked for 10 min at different remelting temperatures. As indicated in Figure 13a, some obvious small-sized dimples and quasic-leavage steps are simultaneously found in the fracture of the thixoformed part. This illustrates a mixedMetals fracture of the ductile fracture and the brittle fracture occurring in the tensile specimens. 2018, 8, x FOR PEER REVIEW 13 of 18

Figure Figure 13. Tensile fracture at the bottom thixoformed parts soaked formin 10 atmin at different 13. Tensile fracture at the bottomof of the the thixoformed parts soaked for 10 different ◦ ◦ ◦ ◦ remelting temperatures: (a) 580 C, (b) 590 C, (c) 600 C, and (d) 610 C. remelting temperatures: (a) 580 °C, (b) 590 °C, 600 °C, and (d) 610 °C. 14 shows OM imagesof of the the fracture of of thethe bottom of thixoformed part soaked 10 FigureFigure 14 shows the the OM images fracture bottom of thixoformed partfor soaked for ◦ min at different remelting temperatures. The fracture type of the thixoformed part at 580 °C is 10 min at different remelting temperatures. The fracture type of the thixoformed part at 580 C is transgranular fracture (Figure 14a). Intergranular fracture and transgranular fracture occurred transgranular fracture (Figure 14a). Intergranular fracture and transgranular fracture occurred together together in the thixoformed◦ part at 590 °C (Figure 14b). When the remelting temperature is elevated in the thixoformed part at 590 C (Figure 14b). When the remelting temperature is elevated to 600 ◦ C to 600 °C and 610 °C, obvious intergranular fracture was found in the fracture macrograph (Figure ◦ and 610 C, obvious fracture was in the fracture (Figure 14c,d). This is dueintergranular to the effect of increasing liquidfound film thickness and liquidmacrograph segregation. When the 14c,d). This is due to thetemperature effect of increasing liquid film thickness and liquid thefilm remelting remelting is elevated, increased liquid fraction leads tosegregation. an increase When of liquid thickness and liquid segregation. The strength in the areas of the liquid film and liquid segregation is temperature is elevated, increased liquid fraction leads to an increase of liquid film thickness and lower than the solid grain’s strength, resulting in easier occurrence of fracture compared with solid grains.

Metals 2018, 8, 709

13 of 17

liquid segregation. The strength in the areas of the liquid film and liquid segregation is lower than the solidMetals grain’s strength, resulting in easier occurrence of fracture compared with solid grains. 2018, 8, x FOR PEER REVIEW 14 of 18

Figure 14. Macrograph of the fracture at the bottom of the thixoformed part soaked for 10 min at Figure 14. Macrograph of the fracture at the bottom of the thixoformed part soaked for 10 min at different remelting temperatures: (a) 580 °C, (b) 590 °C, (c) 600 °C, and (d) 610 °C. different remelting temperatures: (a) 580 ◦ C, (b) 590 ◦ C, (c) 600 ◦ C, and (d) 610 ◦ C.

Figure 15 gives the effect of T6 treatment (i.e., solid solution and artificial aging) on the Figure 15 gives the effect of T6 treatment (i.e., solid solution and artificial aging) on the mechanical mechanical properties of the rheoformed composite parts, the thixoformed composite parts, and the properties of the rheoformed composite parts, the thixoformed composite parts, and the thixoformed thixoformed 7075 aluminum alloy parts. As shown in Figure 15, the mechanical properties were 7075improved aluminum alloy parts. in Figure 15, the mechanical properties were significantly afterAs theshown thixoformed composite parts were treated by T6. The UTSimproved and significantly the thixoformed compositepart parts wereT6 treated byMPa T6. and The5.4%, UTSrespectively. and elongation elongationafter of the side wall of the thixoformed without were 313 of the sidethe wall of the thixoformed without were MPa and 5.4%, respectively. After the After thixoformed parts werepart treated by T6,T6the UTS313 and elongation of the side wall were improvedparts to 552were MPa treated and 7.9%, The UTS of the thixoformed composite part improved with T6 thixoformed byrespectively. T6, the UTS and elongation of the side wall were to was 76.4% higher than that ofThe theUTS thixoformed composite part withoutpart T6. The of thehigher 552 MPa and 7.9%, respectively. of the thixoformed composite withelongation T6 was 76.4% composite part with T6 was higher of the thixoformed compositecomposite part than thixoformed that of the thixoformed composite part46.3% without T6.than Thethat elongation of the thixoformed without T6. The UTS of the bottom of the thixoformed part was increased from 310 MPa to 549 MPa. part with T6 was 46.3% higher than that of the thixoformed composite part without T6. The UTS of the The elongation increased from 5.3% to 7.8%. The extents of increase were 77% and 47.2%, bottom of the thixoformed part was increased from 310 MPa to 549 MPa. The elongation increased from respectively. The UTS of the thixoformed parts with and without T6 was improved compared to the 5.3%thixoformed to 7.8%. The extents of increase were 77% and 47.2%, respectively. The UTS of the thixoformed parts of 7075 aluminum alloy with and without T6 due to the addition of nano-sized partsSiC with and without T6 was improved to the by thixoformed parts 7075 aluminum particles. The increasing dislocationcompared density created the coefficient ofof thermal expansion alloy with (CTE) and without T6 due to the addition of nano-sized SiC particles. The increasing dislocation density mismatch between the matrix and nano-sized particles enhances the strength of the composite created the coefficient of thermal expansion (CTE) mismatch between the matrixalso and nano-sized [48].by The obstacle of uniformly distributed SiC particles to the movement of dislocation plays an important role inthe the strength improvement of strength according the obstacle Orowan mechanism [49]. distributed In addition, SiC particles enhances of the composite [48]. toThe of uniformly the improvement of strength of the thixoformed parts could be related to the the improvement fracture toughness of particles to the movement of dislocation also plays an important role in of strength the interface between the nano-sized SiC and the Al matrix. The pillar splitting technique as according to the Orowan mechanism [49]. In addition, the improvement of strength of the thixoformed Matteo Ghidelli et al. could provide a feasible method to measure the fracture partsreported could beby related to the fracture toughness of the interface between the nano-sized SiC and the Al toughness [50]. The elongation of the thixoformed parts with and without T6 was reduced compared matrix. The pillar splitting technique as reported by Matteo Ghidelli et al. could provide a feasible with the thixoformed parts of 7075 aluminum alloy with and without T6. The UTS of the method to measure the fracture toughness [50]. The elongation of the thixoformed parts with and thixoformed parts without T6 is less than that of the rheoformed parts without T6 [42]. However, the without was reduced compared withwith the T6 thixoformed of 7075 with and UTS T6 of the thixoformed composite parts is even moreparts than that of thealuminum rheoformedalloy composite without of the thixoformed parts without T6 isparts less than that without of the rheoformed part T6. withThe T6. UTS The elongations of the thixoformed composite with and T6 were less parts without [42].ofHowever, the UTS of the thixoformed thanT6 those the rheoformed composite part with and composite without T6. parts with T6 is even more than that

Metals 2018, 8, 709

14 of 17

Metals 2018, 8, x FOR PEER REVIEW

15 of 18

of the rheoformed composite part with T6. The elongations of the thixoformed composite parts with Metals 2018, 8, x T6 FORwere PEERless REVIEW and without than those of the rheoformed composite part with and without T6. 15 of 18

Figure 15. Effect of T6 on the mechanical properties of the rheoformed composite parts, the thixoformed composite and thixoformed 7075 aluminum alloycomposite parts. 1 means wall of the Figure 15. T6 T6 onparts, the properties of the rheoformed parts, side the thixoformed Figure 15. Effect Effectof of on mechanical the mechanical properties of the rheoformed composite parts, the thixoformed composite part, 2 means bottom of the alloy thixoformed composite part, 3 of means side wall of composite parts, and thixoformed 7075 aluminum parts. 1 means side wall the thixoformed thixoformed composite parts, and thixoformed 7075 aluminum alloy parts. 1 means side wall of the the thixoformed 7075 aluminum alloy, 4 means bottom composite of the thixoformed 7075 aluminum 5 composite part, 2 means ofbottom the thixoformed part, 3part, means side side wallalloy, of the thixoformed composite part,bottom 2 means of the thixoformed composite 3 means wall of means side wall of the rheoformed composite part and 6 means the bottom of the rheoformed thixoformed 70757075 aluminum alloy,alloy, 4 means bottom of theofthixoformed 7075 aluminum alloy, 5alloy, means the thixoformed aluminum 4 means bottom the thixoformed 7075 aluminum 5 composite part. side wall of the rheoformed composite part and 6 means the bottom of the rheoformed composite part. means side wall of the rheoformed composite part and 6 means the bottom of the rheoformed composite part.

make phase (MgZn uniformly in thein matrix, as shown In addition, addition, T6 T6treatment treatmentcan can make phase (MgZn 2) precipitate uniformly the matrix, as 2 ) precipitate in Figure 16. Some16. long striped ηstriped phases η(MgZn also foundalso in the TEMinmicrograph (Figure 16). shown in Figure Some long phases (MgZn 2) were found the TEM micrograph 2 ) were In addition, T6 treatment can make phase (MgZn2) precipitate uniformly in the matrix, as These η 16). phases (MgZn dissolved in the 7075 alloy after thematrix thixoformed composite parts (Figure These η Some phases 2) were dissolved in matrix the 7075 alloy after the thixoformed 2 ) were(MgZn shown in Figure 16. long striped η phases (MgZn 2) were also found in the TEM micrograph ◦ were solution treated for 6 h at treated 465 C. for They a supersaturated solution of α Al when the composite were solution 6 dissolved hformed at 465 °C. formed asolid supersaturated solution (Figure 16).parts These η phases (MgZn2) were in They the 7075 alloy matrix after thesolid thixoformed solution treated thixoformed parts were quenched. parts These were η phases (MgZn2 )These precipitated uniformly in of α Al when the solution treated thixoformed quenched. η phases (MgZn 2) composite parts were solution treated for 6 h at 465 °C. They formed a supersaturated solid solution the 7075 alloy matrix. The uniformly dispersed η phases strengthen the alloy, leading to improvement precipitated uniformly in thetreated 7075 alloy matrix. The uniformly dispersedThese η phases strengthen the of α Al when the solution thixoformed parts were quenched. η phases (MgZn 2) of the leading mechanical properties. of the mechanical properties. alloy, to improvement precipitated uniformly in the 7075 alloy matrix. The uniformly dispersed η phases strengthen the

alloy, leading to improvement of the mechanical properties.

◦ C with T6. Figure Figure 16. TEM TEM micrograph micrograph of the thixoformed part soaked for 10 min at 590 °C

Figure 16. TEM micrograph of the thixoformed part soaked for 10 min at 590 °C with T6. 4. Conclusions Conclusions 4.

Nano-sized SiCp/7075 SiCp/7075 composite composite cylinder cylinder parts parts with with good good surface surface quality quality and and dense dense 4. • Conclusions Nano-sized microstructure were obtained via thixoforming technology. The deformation mechanism of microstructure were obtained via thixoforming technology. The deformation mechanism of the  Nano-sized SiCp/7075 composite cylinder parts with good surface quality and dense the top side wall depends mainly on the liquid flow (LF), the flow of liquid incorporating solid top side wall depends mainlyvia onthixoforming the liquid flow (LF), theThe flowdeformation of liquid incorporating microstructure were obtained technology. mechanism ofsolid the grains (FLS), and sliding between solid grains (SSS). The plastic deformation (PDS) mechanism mechanism grains (FLS), and sliding between solid grains (SSS). The plastic deformation (PDS) top side wall depends mainly on the liquid flow (LF), the flow of liquid incorporating solid dominates the the deformation deformation of of the the middle side the bottom side wall, the itself. dominates middle side wall, wall, wall, and and(PDS) the bottom bottom itself. grains (FLS), and sliding between solid grains (SSS).the Thebottom plasticside deformation mechanism • The optimal process parameters obtained according to the UTS and elongation results are soaking  The optimal parameters obtained according the UTS results are dominates theprocess deformation of the middle side wall, the to bottom side and wall,elongation and the◦ bottom itself. ◦ C. time between 10 min 10 and 15and min15and remelting temperatures between 590 590 C and 600 600 soaking time between min min and remelting temperatures between °C and  The optimal process parameters obtained according to the UTS and elongation results are The The highest UTS UTS and elongation of the of thixoformed composite part without T6 were T6 313 MPa and °C. highest elongation the thixoformed composite part without 313 soaking time betweenand 10 min and 15 min and remelting temperatures between 590 °Cwere and 600 5.4% respectively. MPaThe andhighest 5.4% respectively. °C. UTS and elongation of the thixoformed composite part without T6 were 313  A mixed fracture morphology was found in the SEM image of tensile fracture due to the MPa and 5.4% respectively. simultaneous existence of some was small-sized dimples andimage quasi-cleavage The due fracture of  A mixed fracture morphology found in the SEM of tensilesteps. fracture to the the compositeexistence part thixoformed at 580 °C is dimples characterized by transgranular fracture. A mixed simultaneous of some small-sized and quasi-cleavage steps. The fracture of

the composite part thixoformed at 580 °C is characterized by transgranular fracture. A mixed

Metals 2018, 8, 709

•

•

15 of 17

A mixed fracture morphology was found in the SEM image of tensile fracture due to the simultaneous existence of some small-sized dimples and quasi-cleavage steps. The fracture of the composite part thixoformed at 580 ◦ C is characterized by transgranular fracture. A mixed fracture of transgranular fracture and intergranular fracture was found in the composite part thixoformed at 590 ◦ C. The intergranular fracture was dominant in the composite parts thixoformed at 600 ◦ C and 610 ◦ C due to increasing liquid film thickness and liquid segregation. The mechanical properties were improved significantly after the thixoformed composite parts were treated by T6. The highest UTS of 552 MPa and the highest elongation of 7.9% were obtained in the thixoformed composite parts treated by T6. Uniform precipitation of η phase (MgZn2) after T6 led to improvement of the mechanical properties of the thixoformed composite part. The UTS of the thixoformed parts with and without T6 was improved compared with the thixoformed 7075 alloy parts with and without T6 due to the addition of nano-sized SiC particles. The UTS of the thixoformed parts with T6 was higher than that of the rheoformed composite part.

Author Contributions: J.J. designed most experiments, analyzed the results, and wrote this manuscript. G.X., Y.L., and X.N. performed most experiments. Y.W. helped analyze the experimental data and gave some constructive suggestions of how to write the manuscript. Acknowledgments: This research was funded by the Natural Science Foundation of China (NSFC) by Grant No.51375112 and the fund of the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology under Grant No.SKLAB02015003. Conflicts of Interest: The authors declare no conflicts of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13.

14.

Spencer, D.B. Rheology of liquid-solid mixtures of lead-tin. Ph.D. Thesis, Massachusetts Institute of Technology, Massachusetts, MA, USA, 1971. Spencer, D.B.; Mehrabian, R.; Flemings, M.C. Rheological behavior of Sn-15 pct Pb in the crystallization range. Metall. Trans. B 1972, 3, 1925–1932. [CrossRef] Flemings, M.C. Behavior of metal alloys in the semisolid state. Metall. Trans. A 1991, 22, 957–981. [CrossRef] Haghayeghi, R.; Zoqui, E.J.; Halvaee, A.; Emamy, M. An investigation on semi-solid Al-7Si-0.3Mg alloy produced by mechanical stirring. J. Mater. Process. Technol. 2005, 169, 382–387. [CrossRef] Li, D.N.; Luo, J.R.; Wu, S.S.; Xiao, Z.H.; Mao, Y.W.; Song, X.J.; Wu, G.Z. Study on the semi-solid rheocasting of magnesium alloy by mechanical stirring. J. Mater. Process. Technol. 2002, 129, 431–434. [CrossRef] Wang, C.L.; Chen, A.T.; Zhang, L.; Liu, W.C.; Wu, G.H.; Ding, W.J. Preparation of an Mg-Gd-Zn alloy semisolid slurry by low frequency electro-magnetic stirring. Mater. Des. 2015, 84, 53–63. [CrossRef] Bai, Y.L.; Xu, J.; Zhang, Z.F.; Shi, L.K. Annulus electromagnetic stirring for preparing semisolid A357 aluminum alloy slurry. Trans. Nonferrous Met. Soc. China 2009, 19, 1104–1109. [CrossRef] Young, K.P.; Kyonka, C.P.; Courtois, J.A. Fine Grained metal composition. U.S. Patent 4415374, 30 March 1982. Saklakoglu, N.; Saklakoglu, I.E.; Tanoglu, M.; Oztas, O.; Cubukcuoglu, O. Mechanical properties and microstructural evaluation of AA5013 aluminum alloy treated in the semi-solid state by SIMA process. J. Mater. Process. Technol. 2004, 148, 103–107. [CrossRef] Bolouri, A.; Shahmiri, M.; Kang, C.G. Coarsening of equiaxed microstructure in the semisolid state of aluminum 7075 alloy through SIMA processing. J. Mater. Sci. 2012, 47, 3544–3553. [CrossRef] Kirkwood, D.H.; Sellars, C.M.; Elias Boyed, L.G. Thixotropic Materials. European Patent 0305375 B1, 28 October 1992. Bolouri, A.; Kang, C.G. Correlation between solid fraction and tensile properties of semisolid RAP processed aluminum alloys. J. Alloys Compd. 2012, 516, 192–200. [CrossRef] Chen, Q.; Yuan, B.G.; Lin, J.; Xia, XS.; Zhao, Z.D.; Shu, D.Y. Comparisons of microstructure, thixoformability and mechanical properties of high performance wrought magnesium alloys reheated from the as-cast and extruded states. J. Alloys Compd. 2014, 584, 63–75. [CrossRef] Chen, Q.; Zhao, Z.D.; Chen, G.; Wang, B. Effect of accumulative plastic deformation on generation of spheroidal structure, thixoformability and mechanical properties of large-size AM60 magnesium alloy. J. Alloys Compd. 2015, 632, 190–200. [CrossRef]

Metals 2018, 8, 709

15.

16. 17. 18. 19.

20. 21.

22. 23. 24.

25. 26. 27. 28.

29.

30. 31.

32. 33. 34.

35. 36.

16 of 17

Chen, Q.; Chen, G.; Ji, X.H.; Han, F.; Zhao, Z.D.; Wan, J.; Xiao, X.Q. Compound forming of 7075 aluminum alloy based on functional integration of plastic deformation and thixoformation. J. Mater. Process. Technol. 2017, 246, 167–175. [CrossRef] Kaio, N.C.; Cecília, T.W.P.; Eugênio, J.Z. Influence of the processing route on the microstructure of aluminum alloy A356 for thixoforming. Mater. Charact. 2013, 85, 26–37. Forn, A.; Vaneetveld, G.; Pierret, J.C.; Menarguess, S.; Baile, M.T.; Camillom, M.; Rassili, A. Thixoextrusion of A357 aluminium alloy. Trans. Nonferrous Met. Soc. China 2010, 20, s1005–s1009. [CrossRef] Parshizfard, E.; Shabestari, S.G. An investigation on the microstructural evolution and mechanical properties of A380 aluminum alloy during SIMA process. J. Alloys Compd. 2011, 509, 9654–9658. [CrossRef] Kapranos, P.; Kirkwood, D.H.; Atkinson, H.V.; Rheinlander, J.T.; Bentzen, J.J.; Toft, P.T.; Debel, C.P.; Laslaz, G.; Maenner, L.; Blais, S.; et al. Thixoforming of an automotive part in A390 hypereutectic Al–Si alloy. J. Mater. Process. Technol. 2003, 135, 271–277. [CrossRef] Chayong, S.; Atkinson, H.V.; Kapranos, P. Thixoforming 7075 aluminium alloys. Mater. Sci. Eng. A 2005, 390, 3–12. [CrossRef] Abolhasani, D.; Ezatpour, H.R.; Sajjadi, S.A.; Abolhasani, Q. Microstructure and mechanical properties evolution of 6061 aluminum alloy formed by forward thixoextrusion process. Mater. Des. 2013, 49, 784–790. [CrossRef] Birol, Y. Thixoforming of EN AW-2014 alloy at high solid fraction. J. Mater. Process. Technol. 2011, 211, 749–756. [CrossRef] Birol, Y. Comparison of thixoformability of AA6082 reheated from the as-cast and extruded states. J. Alloys Compd. 2008, 461, 132–138. [CrossRef] Wang, L.P.; Jiang, W.Y.; Chen, T.; Feng, Y.C.; Zhou, H.Y.; Zhao, S.C.; Liang, Z.Q.; Zhu, Y. Spheroidal microstructure formation and thixoforming of AM60B magnesium alloy prepared by SIMA process. Trans. Nonferrous Met. Soc. China 2012, 22, s435–s444. [CrossRef] Jiang, J.F.; Wang, Y.; Qu, J.J.; Du, Z.M.; Luo, S.J. Preparation and thixoforging of semisolid billet of AZ80 magnesium alloy. Trans. Nonferrous Met. Soc. China 2010, 20, 1731–1736. [CrossRef] Omar, M.Z.; Palmiere, E.J.; Howe, A.A.; Atkinson, H.V.; Kapranos, P. Thixoforming of a high performance HP9/4/30 steel. Mater. Sci. Eng. A. 2005, 395, 53–61. [CrossRef] Pierret, J.C.; Rassili, A.; Vaneetveld, G.; Lecote-Beckers, J. Stability of steel thixoforming process. Trans. Nonferrous Met. Soc. China 2010, 20, s937–s942. [CrossRef] Cheng, F.L.; Chen, T.J.; Qi, Y.S.; Zhang, S.Q.; Yao, P. Effects of solution treatment on microstructure and mechanical properties of thixoformed Mg2 Sip/AM60B composite. J. Alloys Compd. 2015, 636, 48–60. [CrossRef] Guan, L.N.; Geng, L.; Zhang, H.W.; Huang, L.J. Effects of stirring parameters on microstructure and tensile properties of (ABOw + SiCp)/6061Al composites fabricated by semi-solid stirring technique. Trans. Nonferrous Met. Soc. China 2011, 21, s274–s279. [CrossRef] Alhawari, K.S.; Omar, M.Z.; Ghazali, M.J.; Salleh, M.S.; Mohammed, M.N. Wear properties of A356/Al2 O3 metal matrix composites produced by semisolid processing. Procedia Eng. 2013, 68, 186–192. [CrossRef] Zhang, H.W.; Geng, L.; Guan, L.N.; Huang, L.J. Effects of SiC particle pretreatment and stirring parameters on the microstructure and mechanical properties of SiCp/Al-6.8Mg composites fabricated by semi-solid stirring technique. Mater. Sci. Eng. A 2010, 528, 513–518. [CrossRef] Koli, D.K.; Agnihotri, G.; Purohit, R. A review on properties, behaviour and processing methods for Al-nano Al2 O3 composites. Procedia Mater. Sci. 2014, 6, 567–589. [CrossRef] Mazahery, A.; Shabani, M.O. Characterization of cast A356 alloy reinforced with nano SiC composites. Trans. Nonferrous Met. Soc. China 2012, 22, 275–280. [CrossRef] Kandemir, S. Microstructure and mechonicol properties of A357/SiC nonocomposites fobricated by ultrosonic covitotion-based dispersion of ball-milled nanoparticles. J. Compon. Mater. 2017, 51, 395–404. [CrossRef] Shamsipour, M.; Pahlevani, Z.; Shabani, M.O.; Mazahery, A. Optimization of the EMS process parameters in compocasting of high-wear-resistant Al-nano-TiC composites. Appl. Phys. A 2016, 122, 457–471. [CrossRef] Tofigh, A.A.; Rahimipour, M.R.; Shabani, M.O.; Davami, P. Application of the combined neuro-computing, fuzzy logic and swarm intelligence for optimization of compocast nanocomposites. J. Compon. Mater. 2015, 49, 1653–1663. [CrossRef]

Metals 2018, 8, 709

37.

38. 39.

40.

41.

42. 43.

44. 45. 46. 47. 48. 49.

50.

17 of 17

Shamsipour, M.; Pahlevani, Z.; Shabani, M.O.; Mazahery, A. Squeeze casting of electromagnetically stirred aluminum matrix nanocomposites in semi-solid condition using hybrid algorithm optimized parameters. Kovove Mater. 2017, 55, 33–43. [CrossRef] Tofigh, A.A.; Shabani, M.O. Efficient optimum solution for high strength Al alloys matrix composites. Ceram. Int. 2013, 39, 7483–7490. [CrossRef] Shabani, M.O.; Rahimipour, M.R.; Tofigh, A.A.; Davami, P. Refined microstructure of compo cast nanocomposites: the performance of combined neuro-computing, fuzzy logic and particle swarm techniques, Neural Computing and Applications. Neural. Comput. Appl. 2015, 26, 899–909. [CrossRef] Shabani, M.O.; Heydari, F.; Tofigh, A.A.; Rahimipour, M.R.; Razavi, M.; Mazahery, A.; Davami, P. Wear properties of rheo-squeeze cast aluminum matrix reinforced with nano particulates. Prot. Met. Phys. Chem. Surfaces 2016, 52, 486–491. [CrossRef] Jiang, J.F.; Wang, Y. Microstructure and mechanical properties of the semisolid slurries and rheoformed component of nano-sized SiC/7075 aluminum matrix composite prepared by ultrasonic-assisted semisolid stirring. Mater. Sci. Eng. A 2015, 639, 350–358. [CrossRef] Jiang, J.F.; Wang, Y. Microstructure and mechanical properties of the rheoformed cylindrical part of 7075 aluminum matrix composite reinforced with nano-sized SiC particles. Mater. Des. 2015, 79, 32–41. [CrossRef] Kandemir, S.; Atkinson, H.V.; Weston, D.P.; Hainsworth, S.V. Thixoforming of A356/SiC and A356/TiB2 nanocomposites fabricated by a combination of green compact nanoparticle incorporation and ultrasonic treatment of the melted compact. Metall. Mater. Trans. A 2014, 45A, 5782–5798. [CrossRef] ASTM International. ASTM Standard E8M, Standard Test Methods for Tension Testing of Metallic Materials [Metric]; ASTM International: West Conshohocken, PA, USA, 2008. Chen, C.P.; Tsao, C.Y.A. Semi-solid deformation of non-dendrtic structure-I. Phemonological behavior. Acta Mater. 1997, 45, 1955–1968. [CrossRef] Jiang, J.F.; Luo, S.J. Reheating microstructure of refined AZ91D magnesium alloy in semi-solid state. Trans. Nonferrous Met. Soc. China 2004, 14, 1074–1081. Kubotak, K.; Mabuchi, M.; Higashi, K. Review processing and mechanical properties of fine-grained Mg alloys. J. Mater. Sci. 1999, 34, 2255–2262. [CrossRef] Zhong, X.L.; Wong, W.L.E.; Gupta, M. Enhancing strength and ductility of magnesium by integrating it with aluminum nanoparticles. Acta Mater. 2007, 55, 6338–6344. [CrossRef] Karbalaei, M.; Mirzaee, O.; Baharvandi, H.R. Fabrication and study on mechanical properties and fracture behavior of nanometric Al2 O3 particle-reinforced A356 composites focusing on the parameters of vortex method. Mater. Des. 2013, 46, 199–205. [CrossRef] Ghidelli, M.; Sebastiani, M.; Johanns, K.E.; Pharr, G.M. Effects of indenter angle on micro-scale fracture toughness measurement by pillar splitting. J. Am. Ceram. Soc. 2017, 100, 5731–5738. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).