materials Article
Improving Mechanical Properties of PVPPA Welded Joints of 7075 Aluminum Alloy by PWHT Guowei Li, Furong Chen *, Yongquan Han and Yahong Liang School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China;
[email protected] (G.L.);
[email protected] (Y.H.);
[email protected] (Y.L.) * Correspondence:
[email protected]; Tel.: +86-471-6578-861; Fax: +86-471-6575-752 Received: 3 February 2018; Accepted: 28 February 2018; Published: 5 March 2018
Abstract: In this study, 7075 aluminum alloy with a thickness of 10 mm was successfully welded with no obvious defects by pulsed variable polarity plasma arc (PVPPA) welding. The mechanical properties of PVPPA welded joints have been researched by post weld heat treatment (PWHT). The results indicate that the heat treatment strongly affects the mechanical properties of the welded joints. The tensile strength and the microhardness of the welded joints gradually improved with the increase of the solution temperature. With the increase of the solution time, the tensile strength, and microhardness first dramatically increased and then decreased slightly. The best tensile strength of 537.5 MPa and the microhardness of 143.7 HV were obtained after 490 ◦ C × 80 min + 120 ◦ C × 24 h, and the strength was nearly 91.2% of that of the parent metal, and increased about 35% compared with as-welded. The improvement of strength and microhardness was mainly due to the precipitation of η0 phase. Keywords: PVPPA welding; heat treatment; 7075 aluminum alloy; microstructure; mechanical properties
1. Introduction Al-Zn-Mg-Cu aluminum alloys have been widely used in the spacecraft and industrial fields due to their usefulness in providing lightweight structure, high strength-to-weight ratio, high strength, high toughness, corrosion resistance, etc. [1,2]. Most of the structural components in machines, pressure vessels, transport vehicles, earthmoving equipment, etc., are made by welding method. The welding of 7-series aluminum alloys is difficult for researchers because of the porosity, slag inclusion, and cracks that are easily exhibited in the weld metal zone (WMZ). Also, the loss in mechanical properties as compared to the base material is very significant. These factors make the joining of these alloys by conventional welding processes unattractive [3,4]. Compared with other welding processes, high weld quality and high productivity can be obtained by variable polarity plasma arc (VPPA) welding. The advantages of VPPA welding are attributed mainly to the fully penetrated keyhole-mode welding, where the hydrogen cannot be trapped in the pool, tenacious oxide film can be removed from the workpiece surface, and better fluidity of the metal can be guaranteed in the weld pool [5]. VPPA welding has been used successfully in fabricating a space shuttle’s external tank which is made from aluminum alloy [6,7]. Based on VPPA welding, a new method of PVPPA welding has been developed in our previous work, where pulsed current was added in VPPA welding and showed an obvious improvement of strength [8]. The cycling of welding current from a high level to a low level was conducted at a regular PVPPA welding frequency. The other advantage of PVPPA welding is that the current can be controlled by a square wave with a long positive time and short reversed time, and the welding heat can be adjusted by changing the positive current and reversed current. Thus, PVPPA welding was adopted in this study.
Materials 2018, 11, 379; doi:10.3390/ma11030379
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For welding of metals, as is well known, the tensile strength and elongation of as-welded joints are lower than those of the base metal. Fortunately, many researchers have found that the mechanical properties of welded joints can be improved by PWHT in some metals [9,10]. Wang et al. (2017) [11] researched the effects of aging treatments on the mechanical properties of Al-Cu-Li alloy joints and found that the strength coefficient of the joints increased from 0.64 to 0.90 after post weld double aging treatment. Fadaeifard et al. (2016) [12] found that the hardness in all zones increased after PWHT and the elastic modulus improved from 69.93 to 81 GPa in gas tungsten arc welded AA6061-T6 alloy. Lin et al. (2017) [13] investigated the microstructure evolution after PWHT and the cryogenic fracture toughness of AA2219 variable polarity tungsten inert gas (VPTIG) welding joints, and the results showed that the strength improved while the plasticity and fracture toughness decreased slightly after PWHT. Therefore, PWHT should be an efficient way to improve the mechanical properties of welded aluminum alloy. However, to the knowledge of the authors, there has been no study on the effect of PWHT on the PVPPA welded joints of 7075 aluminum alloy. Therefore, the objective of this study is to investigate the effect of PWHT on a PVPPA welded joint of 10 mm thick 7075 aluminum alloy plate. 2. Experimental The experimental material was 7075 aluminum alloy (7075-T651). The welding wire was ϕ1.2 mm ER5183 aluminum magnesium alloy. The chemical compositions of the 7075 aluminum alloy (supplied by the Southwest Aluminum (Group) Co., Ltd., Chongqing, China) and 5183 welding wire (supplied by Zheng Zhou ChuanWang Welding Consumables, Zhengzhou, China) are listed in Table 1. Table 2 shows the mechanical properties of the 7075 aluminum alloy. Table 1. Chemical compositions of 7075 aluminum alloy and ER5183 welding wire (wt-%). Elements Materials 7075 5183
Zn
Mg
5.1∼6.1 2.1∼2.9 0.25 4.3∼5.2
Cu
Cr
Mn
Ti
Fe
Si
Impurity
Al
1.2∼2.0 0.1
0.18∼0.28 0.05∼0.25
0.3 0.5∼1.0
0.2 0.15
0.5 0.4
0.4 0.4
0.15 0.15
Bal. Bal.
Table 2. Mechanical properties of 7075 aluminum alloy. Alloy
Tensile Strength (MPa)
Yield Strength (MPa)
Elongation (%)
Hardness (HV)
7075
589.2
535.3
12
150
The welding samples were cut to dimensions of 100 × 80 × 10 mm, and the edge of the specimen was welded L-S (longitudinal-short). Before welding, the surfaces of the samples were sanded using 1000 grit SiC paper, followed by washing away oil pollution in acetone liquid. The VPPA-300 welding power supply was used in the experiment. Pure (99.9%) argon gas was used as the shielding gas and plasma gas. The welding parameters are listed in Table 3. The PWHT parameters are listed in Table 4. The schematic of the tensile specimen is shown in Figure 1. The Chinese GB/T 2651-2008 was adopted in the experiment. Detailed weld microstructural examination was carried out for the cross-sections of the generated welds using a SN-3400 scanning electron microscope (SEM, Hitachi Limited, Tokyo, Japan), phase analysis of the welded joint was completed on a D/Max 2500 X-ray diffractometer (XRD, Rigaku Corporation, Tokyo, Japan), and the composition was analyzed by a 51-XMX1023 energy spectrometer (XES, Hitachi Limited, Tokyo, Japan). The distribution of the second phases and the micro-area composition in the welded joint were observed on a Tecnai G2 F20 transmission electron microscope (TEM, FEI Company, Hillsboro, OR, USA). The tensile tests were carried on a microcomputer-controlled electro-hydraulic servo universal testing machine (ShangHai SANS Measuring instrument manufacturing Co., Ltd., Shanghai, China). The Vickers microhardness
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was measured by a HXD-1000™ microhardness tester (Shanghai Taiming Optical Instrument Co. Ltd., Shanghai, China) with load 200 g and time 15 s. Table 3. Welding parameters of PVPPA. Positive Polarity Current (A)
Welding Parameters
240∼260
value
Reversed Polarity Current (A)
Welding Speed (mm·min−1 )
Wire Feeding Speed (mm·min−1 )
280∼300
150
220
Plasma Gas Flow Rate (L·min−1 ) 2.0
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Welding Parameters
Protection Gas Flow Rate (L·min−1 )
Tungsten Time Ratio High Frequency Electrode Table 3. Welding parameters of PVPPA (ms) (Hz) Neck-In (mm)
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Low Frequency (Hz)
Positive 15
Reversed Welding Wire Feeding Plasma Gas 3 21:4 50 1 Polarity Polarity Speed Speed Flow Rate −1 −1 −1 Current (A) Current (A) (mm·min ) (mm·min ) (L·min ) value 240∼260 280∼300 220 2.0 Table 4. Parameters150 of PWHT. Protection Gas Tungsten Welding Time Ratio High Frequency Low Frequency Flow Rate Electrode ◦ C) Parameters Number (ms) (Hz) Time (min) (Hz) Solution Temperature ( Solution −1 (L·min ) Neck-in (mm) value 15 3 460 21:4 50 1 1
value Welding
Parameters
2 Number 1 2 3 4 5 6 7 8 9 10
3 4 5 6 7
Table 470 4. Parameters of PWHT
480 Solution Temperature (°C) 460 490 470 500 480 490 500
8 9
60 Solution Time (min)
490 490
10
60
40 60
40
80 60 100 80 120
100 120
Figure 1. Schematic of tensile specimen.
Figure 1. Schematic of tensile specimen. 3. Results
3. Results 3.1. Microstructure
3.1. Microstructure
3.1.1. As-Welded
The schematic illustration and metallographic microstructures of the corresponding zone on the 3.1.1. As-Welded welded plate are shown in Figure 2. Figure 2a is the schematic illustration of the welded joint. Figure 2b
The isschematic illustration and metallographic of microstructure the corresponding zone on the microstructure of the parent metal, where it can microstructures be seen that the rolling of the parent metal unchanged after welding. Figure2a 2cis shows the equiaxedillustration grain shape of the welded plate areisshown in Figure 2. Figure the schematic ofthe theheat welded joint. zone (HAZ), which indicates that the full recrystallization is affected by the welding heat. Figure 2baffected is the microstructure of the parent metal, where it can be seen that the rolling microstructure The microstructure of the WMZ mainly consists of columnar dendrites, as shown in Figure 2d. The of the parent is unchanged after Figure 2cand shows the is equiaxed grain3a. shape of the phase metal composition of the welded joint welding. was identified by XRD, the result shown in Figure heat affected which indicatesis that the full recrystallization affected by of the welding It can zone be seen(HAZ), that the phase composition mainly composed of α-Al and T (Mgis 32(AlZn) 49) phase as-welded condition. TheWMZ morphology of the T phase,of as columnar analyzed by BSEM (Backscatter Scanning heat. Thethe microstructure of the mainly consists dendrites, as shown in Figure 2d. Microscope), was distributed in the interdendritic microstructure and exhibits a mainly The phaseElectron composition of the welded joint was identified by XRD, and the result is shown in Figure 3a. dotted and strip-shaped appearance, as the white positions shown in Figure 3b. The white positions
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It can be seen that the phase composition is mainly composed of α-Al and T (Mg32 (AlZn)49 ) phase of the as-welded condition. The morphology of the T phase, as analyzed by BSEM (Backscatter Scanning Electron Microscope), was distributed in the interdendritic microstructure and exhibits a Materials 2018, 11, 11, x FOR PEER REVIEW of 12 Materials 2018, x FOR PEER REVIEW of412 mainly dotted and strip-shaped appearance, as the white positions shown in Figure 43b. The white positions (Section A) was analyzed by EDS (Energy Disperse Spectroscopy), and the results were (Section A) A) was analyzed andthe theresults resultswere were shown (Section was analyzedbybyEDS EDS(Energy (Energy Disperse Disperse Spectroscopy), Spectroscopy), and shown in in Figure 3b.3b. The results ofof the XRD analysis composition Al,Zn, Zn,Mg, Mg, and Cu elements, shown in Figure 3b. The results of the XRD and analysis andcomposition the phaseofof composition ofCu Al, Zn, Mg, and Cu Figure The results the XRD analysis andthe the phase phase Al, and elements, confirmed that the bright phase (AlZn) 49 confirmed that the bright phaseisphase isTT(Mg (Mg32is 32 (AlZn) 49)) phase. elements, confirmed that the bright T (Mg (AlZn) ) phase. 32 49 heat heat heat heat weld metal metal zone affected affected weld affected affected parent metal zone parent zone zone parent metal parentmetal metal zone
(a)(a)
Figure 2. Optical microstructures of welded joint: (a) Schematic illustration of the welded joint;
Figure 2. (b) Optical metal; microstructures welded (a) Schematic illustration of the welded joint; (c) heat affectedofzone; and (d)joint: weld metal zone. Figureparent 2. Optical microstructures of welded joint: (a) Schematic illustration of the welded joint; (b) parent metal; (c) heat affected zone; and (d) weld metal zone. (b) parent metal; (c) heat affected zone; and (d) weld metal zone.
Figure 3. XRD pattern (a) and BSE micrograph (b) of the welded joint.
3.1.2. Effect of Solution Temperature Figure 3. XRD pattern (a) and BSE micrograph (b) of the welded joint. Figure 3. XRD pattern (a) and BSE micrograph (b) of the welded joint. The effect of the solution temperature on the microstructure was researched. Figure 4 shows the 3.1.2. Effect of Solution Temperature X-ray diffraction patterns of the welded joints after PWHT, with the solution at different temperatures 3.1.2. Effect Temperature forof 60Solution min followed by aging at 120 °C for 24 h. The peak value of the T phase decreased with the The effect of the solution temperature on the microstructure was researched. Figure 4 shows the increase of the solution temperature. In other words, the content of the T phase reduced with the X-ray diffraction patterns oftemperature the welded joints after PWHT, with the solution at different temperatures The effect ofofthe the evidence microstructure Figure 4 shows the increase thesolution solution temperature. Theon direct is seen in was Figureresearched. 5, which shows the BSE for 60 min followed by aging at 120 °C for 24 h. The peak value of the T phase decreased with the X-ray diffraction patterns the welded joints after PWHT, the solution at different temperatures micrographs of the of welded joints after PWHT. It can be seenwith that the volume fraction and the size of increase of the solution temperature. In other words, the content of the T phase reduced with the ◦ phase decreased with the increase Figures 4 and 5 indicate that the with the for 60 minTfollowed by aging at 120 C of forthe24solution h. Thetemperature. peak value of the T phase decreased increase the solution temperature. The direct evidenceofissolution seen intemperature. Figure 5, which the BSE more of T phase dissolved in the matrix with the increase Withshows the solution increasemicrographs of the solution temperature. InPWHT. other words, the that content of thefraction T phase with the of over the welded after canthe besize seen the volume andreduced the size temperature 490 °C, joints the volume fraction Itand of T phase have hardly changed. The η′ of increaseT of the solution temperature. The direct evidence is seen in Figure 5, which shows the BSE phase decreased with the increase of the solution temperature. Figures 4 and 5 indicate that the (MgZn2) phase precipitated in matrix during the PWHT, as can be seen in Figure 4.
more Tofphase dissolvedjoints in theafter matrix with the increase of solution temperature. With theand solution micrographs the welded PWHT. It can be seen that the volume fraction the size of T temperature over 490 °C, the volume fraction and the size of T phase have hardly changed. The η′ more T phase decreased with the increase of the solution temperature. Figures 4 and 5 indicate that the (MgZn2) phase precipitated in matrix during the PWHT, as can be seen in Figure 4. phase dissolved in the matrix with the increase of solution temperature. With the solution temperature over 490 ◦ C, the volume fraction and the size of T phase have hardly changed. The η0 (MgZn2 ) phase precipitated in matrix during the PWHT, as can be seen in Figure 4.
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Figure of the the welded weldedjoints jointsafter afterPWHT PWHTwith withsolution solutionatat different temperatures Figure 4. 4. XRD XRD patterns patterns of different temperatures forfor 60 Figure 4. XRD patterns of120 the welded joints after PWHT with solution at different temperatures for 60 min followed by aging at °C for 24 h. ◦ min followed by aging at 120 C for 24 h. 60 min followed by aging at 120 °C for 24 h.
Figure 5. BSE micrographs of welded joints after PWHT with solution at different temperatures for
Figure BSE micrographs welded after with temperatures 60 5. min followed by aging of at 120 °C forjoints 24 h: (a) 460 PWHT °C; (b) 470 °C; solution (c) 480 °C;at (d)different 490 °C; and (e) 500 °C. for ◦ ◦ ◦ ◦ ◦ 60 min followed by aging at 120 C for 24 h: (a) 460 C; (b) 470 C; (c) 480 C; (d) 490 C; and (e) 500 ◦ C.
Figure 5. BSE micrographs of welded joints after PWHT with solution at different temperatures for 60 min followed by aging at 120 °C for 24 h: (a) 460 °C; (b) 470 °C; (c) 480 °C; (d) 490 °C; and (e) 500 °C.
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Effect of of Solution Time 3.1.3. Effect 3.1.3. Effect of Solution Time Figure 66 shows shows the the X-ray X-ray The effect of the solution time on the microstructure was also studied. Figure The effect of the timejoints on the microstructure also studied. Figure 6 shows thetimes X-ray ◦ C for diffraction patterns of solution the welded welded joints after PWHT with withwas the solution solution at 490 diffraction patterns of the after PWHT the at °C for different diffraction the welded after PWHT with the solutionofatTof 490 for different times ◦C followed bypatterns agingatatof 120 for2424 h. The phases mainly composed T °C phase and η0 phase. followed by aging 120 °C for h.joints The phases areare mainly composed phase and η′ phase. The followed by aging at 120 °C for 24 h. The phases are mainly composed of T phase and η′ phase. The corresponding micrographs of the welded joints after PWHT areshown shownininFigure Figure7.7.ItIt can canThe corresponding BSEBSE micrographs of the welded joints after PWHT are be corresponding BSE micrographs of the welded are shown in Figure 7. It can be seen from Figures 6 and 7 that the content and thejoints scale after of thePWHT T phase phase decreased with the increase decreased the increase of seen from Figures 6 and 7 that content and the scale of the T phase decreased with the increase the solution time. However, However, thethe volume fraction and scale the solution time. the volume fraction and scale of the T phase hardly changed when theof the solution time. However, the volume fraction and scale of the T phase hardly changed when the solution time was over 80 min, as can be seen from Figure 7. solution time was over 80 min, as can be seen from Figure 7.
Figure ◦ C for Figure 6. 6. XRD XRD patterns patterns of of the the welded welded joints joints after after PWHT PWHT with with solution solution at at 490 490 °C for different different times times Figure 6.byXRD patterns of the welded joints after PWHT with solution at 490 °C for different times followed aging at 120 °C for 24 h. ◦ followed by aging at 120 C for 24 h. followed by aging at 120 °C for 24 h.
Figure 7. Cont.
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Figure 7. BSE micrographs of welded joints after PWHT with solution at 490 for different times ◦ C °C Figure for Figure 7. 7. BSE BSE micrographs micrographs of of welded welded joints joints after after PWHT PWHT with with solution solution at at 490 490 °C for different different times times followed by aging at 120 °C for 24 h: (a) 40 min; (b) 60 min; (c) 80 min; (d) 100 min; and (e) 120 min. ◦ followed by aging at 120 C for 24 h: (a) 40 min; (b) 60 min; (c) 80 min; (d) 100 min; and followed by aging at 120 °C for 24 h: (a) 40 min; (b) 60 min; (c) 80 min; (d) 100 min; and (e) (e) 120 120 min. min.
3.2. Mechanical Properties 3.2. Mechanical Properties 3.2.1. Effect of Solution Temperature 3.2.1. Effect of 3.2.1. Effect of Solution Solution Temperature Temperature Figure 8 shows the effect solution temperature tensile strength welded joints. Figure effect of of thethe solution temperature onon thethe tensile strength of of thethe welded joints. Figure 88 shows shows the the effect of the solution temperature on the tensile strength of the welded joints. The tensile strength of the as-welded joint is 397.9 MPa, which is about 67.5% of the parent metal. The is 397.9 397.9 MPa, MPa, which which is is about about 67.5% 67.5% of of the the parent parent metal. metal. The tensile tensile strength strength of of the the as-welded as-welded joint joint is During PWHT, the tensile strength increased to the peak value and then decreased with the increase During PWHT, the tensile strength increased to the peak value and then decreased with the increase During PWHT, the tensile strength increased to the peak value and then decreased with the increase of the solution temperature. The tensile strength the welded joint 406.2 is 406.2 MPa with the solution of The tensile strength of of the of the the solution solution temperature. temperature. The tensile strength of the welded welded joint joint is is 406.2 MPa MPa with with the the solution solution temperature of 460 °C, which is slightly higher than as-welded. With the continued increase of the ◦ temperature C, which With the temperature of of 460 460 °C, which is is slightly slightly higher higher than than as-welded. as-welded. With the continued continued increase increase of of the the solution temperature, the tensile strength of the welded joint gradually improved. The highest tensile solution solution temperature, temperature, the the tensile tensile strength strength of of the the welded welded joint joint gradually gradually improved. improved. The The highest highest tensile tensile strength of 523.5 MPa was obtained at 490 which nearly is nearly 88.8% of the parent metal, and was it was ◦ C,°C, strength which strength of of 523.5 523.5 MPa MPa was was obtained obtained at at 490 490 °C, which is is nearly 88.8% 88.8% of of the the parent parent metal, metal, and and it it was increased by 31.6% compared to the untreated welded joint. However, the tensile strength of the increased However, the the tensile increased by by 31.6% 31.6% compared compared to to the the untreated untreated welded welded joint. joint. However, tensile strength strength of of the the welded joint with a solution temperature of 500 °C decreased slightly compared with °C. Figure ◦ decreased ◦490 Figure welded 8 welded joint joint with withaasolution solutiontemperature temperatureofof500 500 C °C decreasedslightly slightlycompared comparedwith with490 490C. °C. Figure 8 indicates thattensile the tensile strength of the welded joint is evidently improved by PWHT. The same indicates that the strength of the welded joint is evidently improved by PWHT. The same trend 8 indicates that the tensile strength of the welded joint is evidently improved by PWHT. The same trend was also observed in thefor results for the microhardness of thejoints. welded joints. was in the results thefor microhardness of the welded trendalso wasobserved also observed in the results the microhardness of the welded joints.
Figure 8. Tensile strengths of welded joints between as-welded and PWHT with solution at different Figure 8. Tensile strengths of welded joints between as-welded and PWHT with solution at different Figure 8. Tensile for strengths weldedby joints between as-welded temperatures 60 minof followed aging at 120 °C for 24 h.and PWHT with solution at different temperatures for 60 min followed by aging at 120 °C for 24 h. temperatures for 60 min followed by aging at 120 ◦ C for 24 h.
Figure 9 shows the microhardness distribution of the welded joints. The microhardness distribution Figure 9 shows the microhardness distribution of the welded joints. The microhardness distribution of Figure the as-welded joint in Figure 9 presents a typical W shape. TheThe center of the joint exhibits the 9 shows the microhardness distribution of theWwelded microhardness distribution of the as-welded joint in Figure 9 presents a typical shape.joints. The center of the joint exhibits the minimum value of microhardness, and the HAZ of the welded joint has an obvious trough of of the as-welded joint in Figure 9 presents a typical W shape. The center of the joint exhibits the minimum value of microhardness, and the HAZ of the welded joint has an obvious trough of microhardness. The PWHT has a significant effect on the joint. First, the microhardness changes from minimum valueThe of PWHT microhardness, and theeffect HAZ welded jointmicrohardness has an obvious trough of microhardness. has a significant onof thethe joint. First, the changes from a W shape to aThe V shape, where the microhardness gradually decreased from the parent metal to the microhardness. PWHT has a significant effect on the joint. First, the microhardness changes a W shape to a V shape, where the microhardness gradually decreased from the parent metal to the center of the welded joints after PWHT. After PWHT, the microhardness of the welded joint was center of the welded joints after PWHT. After PWHT, the microhardness of the welded joint was enhanced obviously. As a matter of fact, 7075 aluminum alloy is a precipitation-strengthened alloy. enhanced obviously. As a matter of fact, 7075 aluminum alloy is a precipitation-strengthened alloy. The microhardness could be enhanced by the MgZn2 phase which precipitated in matrix during the The microhardness could be enhanced by the MgZn2 phase which precipitated in matrix during the
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from a W shape to a V shape, where the microhardness gradually decreased from the parent metal to the center of the welded joints after PWHT. After PWHT, the microhardness of the welded joint was enhanced obviously. As a matter of fact, 7075 aluminum alloy is a precipitation-strengthened alloy. The microhardness could beREVIEW enhanced by the MgZn2 phase which precipitated in matrix during Materials 2018, 11, x FOR PEER 8 of 12 the aging process, and led to the increased microhardness of the HAZ. Secondly, the microhardness aging process, led to the increased microhardness of the HAZ. the Secondly, the microhardness Materials 2018, 11, xand FOR PEER REVIEW 8 of of 12 of the welded joint was improved greatly by PWHT. Taking microhardness of the center, the welded joint was improved greatly by PWHT. Taking the microhardness of the center, for for example, the microhardness of as-welded is about 126.7 HV, while the highest value of 141.8 HV example, the microhardness as-welded is about 126.7ofHV, highestthe value of 141.8 HV of is aging process, and led to theofincreased microhardness the while HAZ. the Secondly, microhardness is achieved by the solution treated at 490 ◦ C for 60 min followed by aging at 120 ◦ C for 24 h. Finally, achieved by joint the solution treated atgreatly 490 °Cby forPWHT. 60 min Taking followedthe bymicrohardness aging at 120 °Cofforthe 24 center, h. Finally, the welded was improved for the microhardness of the welded joints first increased and andthen thendecreased decreased during the PWHT, and the the microhardness of the welded joints firstisincreased duringvalue the PWHT, and example, the microhardness of as-welded about 126.7 HV, while the highest of 141.8 HVthe is ◦ C. highest microhardness was obtained when the solution temperature was 490 highest microhardness was obtained when the 60 solution temperature was 490 °C.°C for 24 h. Finally, achieved by the solution treated at 490 °C for min followed by aging at 120 the microhardness of the welded joints first increased and then decreased during the PWHT, and the highest microhardness was obtained when the solution temperature was 490 °C.
Figure 9. Microhardness distribution of welded joints between as-welded and PWHT with solution
Figure 9. Microhardness distribution of welded joints between as-welded and PWHT with solution at at different temperatures for 60 min followed by aging at 120 °C for 24 h. different temperatures for 60 min followed by aging at 120 ◦ C for 24 h. Figure 9. Microhardness distribution of welded joints between as-welded and PWHT with solution
3.2.2.atEffect of Solution Timefor 60 min followed by aging at 120 °C for 24 h. different temperatures
3.2.2. Effect of Solution Time
Figure 10 shows the effect of the solution time on the tensile strength of the welded joints. 3.2.2. Effect of Solution Time The tensile of the welded was improved evidently by PWHT. With the increase of thejoints. Figure 10 strength shows the effect of joints the solution time on the tensile strength of the welded solution time, tensile strength first dramatically increased then decreased slightly. The tensile Figure 10the shows effect joints of the solution time onevidently theand tensile the the welded joints.of the The tensile strength of thethe welded was improved bystrength PWHT.of With increase strength ofthe thetensile welded joint is 443.4 MPawas with the solution time 40 min, which isslightly. slightly The time, tensile strength ofstrength the welded joints improved evidently bythen PWHT. With the increasehigher of the solution first dramatically increased andof decreased The tensile than as-welded. With the increase of the solution time, the tensile strength of the welded joint solution time, the tensile strength first dramatically increased and then decreased slightly. The tensile strength of the welded joint is 443.4 MPa with the solution time of 40 min, which is slightly higher than gradually The highest tensile abouttime 537.5 at 80 min,iswhich is nearly strength ofimproved. the welded joint 443.4 MPa strength with thewas solution of MPa 40 min, which slightly as-welded. With the increase ofisthe solution time, the tensile strength of the welded jointhigher gradually 91.2% of the parent metal, aboutof 139.7 higher thanthe as-welded. When the time joint was than as-welded. With theand increase the MPa solution time, tensile strength of solution the welded improved. The highest tensile strength was about 537.5 MPa at 80 min, which is nearly 91.2% of the over 80 min, the tensile strength decreased slightly,was but was still significantly higher as-welded. gradually improved. The highest tensile strength about 537.5 MPa at 80 min, than which is nearly parent metal, and about 139.7 MPa higher than as-welded. When the solution time was over 80 min, 91.2% of the parent metal, and about 139.7 MPa higher than as-welded. When the solution time was the tensile slightly, but was still significantly higher thanhigher as-welded. over 80strength min, thedecreased tensile strength decreased slightly, but was still significantly than as-welded.
Figure 10. Tensile strengths of welded joints between as-welded and PWHT with solution at 490 °C for different time followed by aging at 120 °C for 24 h. Figure 10. Tensile strengths of welded joints between as-welded and PWHT with solution at 490 °C
◦ for Figure 10. Tensile strengths of welded joints between as-welded PWHT withafter solution at 490 Figure 11 shows the results of the microhardness of the and welded joints PWHT withCthe for different time followed by aging at◦120 °C for 24 h. different time by aging at 120 C forby 24aging h. at 120 °C for 24 h. As can be seen from Figure 11, solution at 490followed °C for different times followed
the microhardness of the improved evidently by PWHT, exhibiting a V shape on Figure 11 shows thewelded results joint of thewas microhardness of the welded joints after PWHT with the solution at 490 °C for different times followed by aging at 120 °C for 24 h. As can be seen from Figure 11, the microhardness of the welded joint was improved evidently by PWHT, exhibiting a V shape on
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Figure 11 shows the results of the microhardness of the welded joints after PWHT with the solution ◦ C for different times followed by aging at 120 ◦ C for 24 h. As can be seen from Figure 11, at 490 Materials 2018, 11, x FOR PEER REVIEW 9 of 12 the microhardness of the welded joint was improved evidently by PWHT, exhibiting a V shape on the figure. The microhardness first increased and then decreased with the increase of the solution solution time. The The maximum maximum value value of of 143.7 143.7 HV HV at at the the center center of the welded joints was obtained obtained at 80 min, which which is about 17 HV higher than as-welded. With the solution time over 80 min, the microhardness decreased slightly. slightly.
Figure 11. Microhardness distribution of welded joints between as-welded and PWHT with solution Figure 11. Microhardness distribution of welded joints between as-welded and PWHT with solution at at 490 °C for different times followed by aging at 120 °C for 24 h. 490 ◦ C for different times followed by aging at 120 ◦ C for 24 h.
4. Discussion 4. Discussion The weldability of high strength aluminum alloy is very poor, and defects such as hot cracking, The weldability of highdefects strength aluminum alloy is very poor, and defects such as hot cracking, porosity, and other welding often appear in welded joints, limiting their extensive application. porosity, and other welding defects in welded joints, limiting extensivearc application. The preferred welding processes ofoften 7075 appear aluminum alloy frequently use atheir gas tungsten welding The preferred welding processes of 7075 aluminum alloy frequently use a gas tungsten arc or laser welding process due to their comparatively easy application [14], but it needs to bewelding welded or laser welding process due to their comparatively easy application [14], but it needs to be welded two or more times when the aluminum is thicker, and the cost of the laser welding equipment is high. two morefinds times when aluminum is thicker, and the cost of mm the laser welding equipment is high. Thisor study that 7075the aluminum alloy with a thickness of 10 can be successfully welded with This study finds that 7075 aluminum alloy with a thickness of 10 mm can be successfully welded with no obvious defects using PVPPA welding. Usually, the tensile strength of the welded joint reaches no obvious defectsmetal usingwhen PVPPA welding. Usually, the tensilemethod. strengthInofthis thework, welded reaches 60% 60% of the parent using the traditional welding thejoint tensile strength of the parent metal when using the traditional welding method. In this work, the tensile strength of of the welded joint by PVPPA welding reached 67.5% of the parent metal. Therefore, high weld the welded joint by PVPPA welding reached 67.5% of the parent metal. Therefore, high weld quality quality and high productivity can be obtained by PVPPA welding in one stroke. and high productivity be methods, obtained by However, as withcan other thePVPPA tensile welding strength in ofone the stroke. welded joint by PVPPA welding is However, as with other methods, the tensile strength of the welded joint by PVPPA welding is poor. In order to improve the strength of the welded joint even more, the researchers looked for other poor. In order to improve the strength of the welded joint even more, the researchers looked for other methods, and PWHT was found to be an effective method for improving the mechanical properties methods, PWHT wasand found be an effective method the mechanical of weldedand joints [15–17], nowtoconfirmed by our results.for Asimproving shown in Figures 8 and 9, properties the tensile of welded joints [15–17], and now confirmed by our results. As shown in Figures 8 and 9, the tensile strength and the microhardness were enhanced by PWHT. The reason for the improvement of the strength and the microhardness were enhanced by PWHT. The reason for the improvement of the mechanical properties of the welded joint can be concluded as the result of precipitation strengthening. mechanical properties of the welded joint can be concluded as the result of precipitation strengthening. Figure 12a shows the TEM image of the as-welded joint. It can be seen that a small amount of unclear Figure 12a shows TEM image as-welded joint. possibly It can bebecause seen that small amount of precipitated phasethe precipitated in of thethe as-welded matrix, thea welded joint has unclear precipitated phase precipitated the as-welded possiblyFigure because welded joint insufficient time to precipitate due to thein faster cooling ratematrix, after welding. 12bthe shows the TEM has insufficient time to precipitate due to the faster cooling rate after welding. Figure 12b shows the image of the welded joints of PWHT at 490 °C for 80 min followed by aging at 120 °C for 24 h. It can ◦ C for 80 min followed by aging at 120 ◦ C for 24 h. TEM image of the welded joints of PWHT at 490 be seen that the nanoscale precipitated phase was distributed uniformly in the matrix, and the It can be seen that the nanoscale precipitated was distributed uniformly in the matrix, and the quantity increased obviously compared to thephase as-welded. quantity increased obviously compared to the as-welded.
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Figure 12. 12. Comparison Figure Comparison of of the the TEM TEM micrographs micrographs of of welded welded joints joints between between as-welded as-welded and and PWHT PWHT with with Figure 12. Comparison of thefollowed TEM micrographs of welded joints between as-welded and PWHT with solution at 490 °C for 80 min by aging at 120 °C for 24 h: (a) as-welded and (b) 490 ◦ ◦ ◦ C, 80 solution at 490 C for 80 min followed by aging at 120 C for 24 h: (a) as-welded and (b) 490 °C, 80 min. min. solution at 490 °C for 80 min followed by aging at 120 °C for 24 h: (a) as-welded and (b) 490 °C, 80 min.
Figure 13 shows the TEM images corresponding to Figure 12. The presence of η′0 is confirmed by Figure shows the images corresponding to Figure 12. The presence of is confirmed confirmed by by Figure 13 13 the TEM TEM images corresponding toin Figure 12.13a). The The presence of η η′ is examination ofshows the SAED patterns on [011] Al (as shown Figure high resolution images of examination of the patterns on [011] (as shown shown in in Figure Figure 13a). The The high high resolution resolution images examination the SAED SAED patterns [011]Al Al images of of the weld afterofFourier transform areon shown in(as Figure 13b, and the 13a). interplanar distance d is calculated the weld after Fourier transform are shown in Figure 13b, and the interplanar distance d is calculated thecorresponding weld after Fourier transform Figure 13b, and the interplanar d is12b calculated as to the d valueare of shown MgZn2.inTherefore, the precipitated phasedistance in Figure can be as to the MgZn Therefore, the the precipitated precipitated phase phase in in Figure Figure 12b as corresponding corresponding the d d2 value value of of MgZn22.. Therefore, 12b can can be be determined as thetoMgZn phase. Dispersion precipitation is the main reason for the enhancement determined as the MgZn phase. Dispersion precipitation is the main reason for the enhancement 2 determined as treatment the MgZn[18–20]. 2 phase. Dispersion precipitation is the main reason for the enhancement following heat following following heat heat treatment treatment [18–20]. [18–20]. —
—
1—11— 111 —
022— 022 — η’ 111— η’ 111
Figure 13. TEM images of welded joints after PWHT with solution at 490 °C for 80 min followed by Figureat13. TEM images joints and after(b) PWHT withimage. solution at 490 °C for 80 min followed by aging 120 °C for 24 h: of (a)welded SAED image HRTEM Figure 13. TEM images of welded joints after PWHT with solution at 490 ◦ C for 80 min followed by aging at 120 °C for 24 h: (a) SAED image and (b) HRTEM image. aging at 120 ◦ C for 24 h: (a) SAED image and (b) HRTEM image.
The solution temperature and time exhibited an important effect on the mechanical properties The solution temperature and exhibited an important effect the mechanical properties of the welded joints. As shown in time Figure 8, the tensile strength andon microhardness dramatically The solution temperature and time exhibited an important effect on the mechanical properties of of the welded joints. As shown in Figure 8, the tensile strength and microhardness dramatically increased as the solution temperature reached 490 °C, and the maximum values of the tensile strength the welded joints. As shown in Figure 8, the tensile strength and microhardness dramatically increased increased as the solution reached °C, and valuesin ofthe the matrix, tensile strength and microhardness weretemperature obtained. During the490 PWHT, thethe T maximum phase dissolved and the as solution temperature reached 490 ◦with C, maximum of temperature. theintensile strength and andthe microhardness During theand PWHT, the Tofphase dissolved the matrix, the content and scale of were the T obtained. phase decreased thethe increase thevalues solution Thisand means microhardness were obtained. During the PWHT, the T phase dissolved in the matrix, and the content content scale of the phase with the increase thethe solution This that theand content MgT and Zndecreased solute elements increasedofin matrixtemperature. and resulted inmeans more and of the phase decreased with the increase of the solution temperature. This means that the that scale the content of Mg and Zn elements increased in the resulted in more precipitation of T MgZn 2 during the solute aging heat treatment. Therefore, thematrix greaterand content of MgZn 2 led content of Mg and Zn solute elements increased in the matrix and resulted in more precipitation of precipitation of MgZn2 during the aging Therefore, over the greater of MgZn 2 led to the higher mechanical properties. Withheat the treatment. solution temperature 490 °C,content the T phase showed MgZn during the aging heat treatment. Therefore, the greater of effect MgZn tomechanical theshowed higher 2higher 2 led to the With the solution temperature 490 °C, the Tthe phase no change, asmechanical shown in properties. Figure 5. The solution time also had content theover same on ◦ C, the T phase showed no change, mechanical properties. With the solution temperature over 490 no change,ofasthe shown in joints. FigureThe 5. tensile The solution time had the same effect on increased the mechanical properties welded strength andalso microhardness dramatically as the as shown in Figure 5. The solution time also had the same effect on the mechanical properties of the properties of the welded joints. The tensile strength and microhardness dramatically increased aswas the solution time reached 60 min, and the maximum value of the tensile strength and microhardness welded joints. The tensile strength and microhardness dramatically increased as the solution time solution time reached 60 min, the maximum value thesolution tensile strength andtensile microhardness obtained at 80 min. With theand continued increase of ofthe time, the strength was and reached and maximum value of the tensile and microhardness was obtained atand 80 obtained60atmin, 80 changed min.the With the continued increase ofstrength theduring solution time, tensile strength microhardness little. The changing of the T phase PWHT canthe explain why the volume min. With thethe continued increase the solution time, the tensile and explain microhardness microhardness changed little. Theofchanging ofwith the Tthe phase during PWHT can whythe thechanged volume fraction and scale of the T phase decreased increase ofstrength the solution time. With solution little. The changing of the T phase during PWHT can explain why the volume fraction and the scale of fraction and scale T phase decreased with the increase of the With the solution time over 80 the min, theof T the phase was unchanged, as shown in Figure 7. solution With thetime. continued increase of the T phase decreased with the increase of the solution time. With the solution time over 80 min, the T time over 80 min, the T phase was unchanged, as shown in Figure 7. With the continued increase of the solution temperature and time, the tensile strength and microhardness showed little change. phase was the unchanged, as shown in Figure 7.strength Withstrength the continued increasemay of the solution temperature the solution temperature andoftime, the tensile and microhardness little change. However, slight decrease the tensile and microhardness beshowed due to grain growth However, the slight decrease of the tensile strength and microhardness may be due to grain growth [21]. [21].
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and time, the tensile strength and microhardness showed little change. However, the slight decrease of the tensile strength and microhardness may be due to grain growth [21]. 5. Conclusions The 7075 aluminum alloy was successfully welded by the PVPPA welding with no obvious defects. The microstructure and mechanical properties of PVPPA welded joints have been investigated by PWHT. The major conclusions can be summarized as follows: (1)
(2)
(3)
7075 aluminum alloy with a thickness of 10 mm can be successfully welded with no obvious defects by PVPPA welding. The tensile strength of the welded joint using PVPPA welding reached 67.5% of the parent metal. The solution temperature and time have an important effect on the mechanical properties of the welded joint. During PWHT, the tensile strength and microhardness first increased and then decreased slightly with the increase of the solution temperature and solution time. The change of the mechanical properties was due to the dissolution of the T phase and the precipitation of η0 phase during PWHT. The mechanical properties of the welded joint using PVPPA welding can be improved by the PWHT. The highest tensile strength of 537.5 MPa was obtained when the solution was treated at 490 ◦ C for 80 min followed by aging at 120 ◦ C for 24 h, which is about 91.2% of the parent metal. The maximum microhardness (143.7 HV) at the center of welded joints was also obtained under this heat treatment.
Acknowledgments: This work was financially supported by National Natural Science Foundation of China (grant No. 51665044) and the Grassland Talent Foundation of Inner Mongolia, China (No. CYYC5033), which is gratefully acknowledged. Author Contributions: Guowei Li and Furong Chen conceived and designed the experiments; Guowei Li and Yahong Liang performed the experiments; Furong Chen and Yongquan Han analyzed the data; Yongquan Han and Yahong Liang contributed materials and analysis tools; Guowei Li wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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