A Review on Dissimilar Friction Stir Welding of ...

Materials and Manufacturing Processes

ISSN: 1042-6914 (Print) 1532-2475 (Online) Journal homepage: http://www.tandfonline.com/loi/lmmp20

A Review on Dissimilar Friction Stir Welding of Copper to Aluminum: Process, Properties, and Variants Kush P. Mehta & Vishvesh J. Badheka To cite this article: Kush P. Mehta & Vishvesh J. Badheka (2016) A Review on Dissimilar Friction Stir Welding of Copper to Aluminum: Process, Properties, and Variants, Materials and Manufacturing Processes, 31:3, 233-254, DOI: 10.1080/10426914.2015.1025971 To link to this article: http://dx.doi.org/10.1080/10426914.2015.1025971

Accepted author version posted online: 23 Mar 2015. Published online: 23 Mar 2015. Submit your article to this journal

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Date: 08 October 2016, At: 06:13

Materials and Manufacturing Processes, 31: 233–254, 2016 Copyright # Taylor & Francis Group, LLC ISSN: 1042-6914 print=1532-2475 online DOI: 10.1080/10426914.2015.1025971

A Review on Dissimilar Friction Stir Welding of Copper to Aluminum: Process, Properties, and Variants Kush P. Mehta and Vishvesh J. Badheka Department of Mechanical Engineering, School of Technology, Pandit Deendayal Petroleum University, Gandhinagar, Gujarat, India Copper and aluminum materials are extensively used in different industries because of its great conductivities and corrosion resistant nature. It is important to join dissimilar materials such as copper and aluminum to permit maximum use of the special properties of both the materials. The joining of dissimilar materials is one of the most advanced topics, which researchers have found from last few years. Friction stir welding (FSW) technology is feasible to join dissimilar materials because of its solid state nature. Present article provides a comprehensive insight on dissimilar copper to aluminum materials joined by FSW technology. FSW parameters such as tool design, tool pin offset, rotational speed, welding speed, tool tilt angle, and position of workpiece material in fixture for dissimilar Cu–Al system are summarized in the present review article. Additionally, welding defects, microstructure, and intermetallic compound generation for Cu–Al FSW system have been also discussed in this article. Furthermore, the new developments and future scope of dissimilar Cu–Al FSW system have been addressed. Keywords Aluminum; Copper; Dissimilar; Friction; Materials; Welding.

together. FSW is a solid state welding process, invented and patented by The Welding Institute (TWI), London, UK, in 1991 [4–10]. Friction and stirring of material produced by the nonconsumable rotating tool consists of specially designed pin and shoulder as shown in Fig. 1. Tool pin is totally inserted between abutting surfaces of workpiece with suitable rotational speed until the shoulder makes a contact with the workpiece surfaces. Rubbing action between tool and workpiece surfaces generate a large amount of frictional heat which softens the workpiece materials. This softened material travelled from front to back and top to bottom of the pin when the tool is travelled along with the certain rotational speed [4–10]. The basic process principle of dissimilar FSW technology is shown in Fig. 1. This article comprehensively reviews recent work along several significant aspects of dissimilar Cu–Al FSW system such as process parameters and its effect, material flow and microstructure changes, defects, mechanical properties, and variant of FSW.

INTRODUCTION Joining of dissimilar materials by any welding process is always difficult because of the enormous differences in mechanical and metallurgical properties. The joints of dissimilar materials are increasingly employed in different sectors of industries due to its technical and economic advantages. Copper (Cu) and aluminum (Al) materials have good electrical and thermal conductivities which make it useful for electrical and thermal applications. Electrical connectors, bus-bars, foil conductor in transformers, capacitor and condenser foil windings, refrigeration tubes, heat-exchangers tubes, and tube-sheets, etc., are some common applications of Cu and Al joints. Fusion welding processes are not recommended to join Cu and Al together because of a tendency to form large intermetallic compounds (IMCs). These IMCs are very hard and brittle, which causes many defects. The solidification and liquefaction cracking are some common problems associated with fusion welding [1]. Solid state welding processes such as friction welding, ultrasonic welding, cold rolling, explosive welding, diffusion welding, and friction stir welding (FSW) are feasible methods by which Cu–Al dissimilar materials can be joined together [2, 3]. Since, the last few years, researchers have focused on FSW technology to join Cu–Al materials

FSW TOOL FSW tool is a heart of the process. The functions of the FSW tool are heating and softening of base materials, extruding the base materials from front to back and from top to bottom of the tool, and finally make the bonding of the softened material to form a solid state joint [11]. The tool material and tool geometry are the important components of FSW tool.

Received November 14, 2014; Accepted February 22, 2015 Address correspondence to Kush P. Mehta, Department of Mechanical Engineering, School of Technology, Pandit Deendayal Petroleum University, Raisan, Gandhinagar 382007, Gujarat, India. E-mail: [email protected]; or Vishvesh J. Badheka, E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lmmp.

Tool Material Tool material should be such that, the geometry and features remain unchanged during the process. 233

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K. P. MEHTA AND V. J. BADHEKA

diameter, shoulder surface angle, pin geometry, including its shape and size, and the nature of tool surfaces [11]. Tool design and geometry affects the heat input, force, and torque variations and plasticized material flow in FSW technology [7]. Different tool designs and geometries for dissimilar Cu–Al FSW system are discussed as below.

FIGURE 1.—Process principle of dissimilar friction stir welding.

Requirement of tool material is critical for higher melting workpiece materials. Important characteristics of tool material such as ambient and elevated temperature strength, elevated temperature stability, wear resistance, tool reactivity, fracture toughness, machinability, uniformity in microstructure, and density, availability of materials are required for getting success in the FSW process [6]. Simplest tool design and cost-effective tool material are the greatest advantages of dissimilar Cu–Al FSW system. Available literatures indicate that the tool steel (with tempered and quenched conditions and usually hardened from 45 HRC to 62 HRC) is commonly used as a tool material for dissimilar Cu–Al FSW system. The required hardness of the tool material depends on alloys and the thickness of the workpieces. Literature summary of different recommended tool materials for dissimilar Cu–Al FSW systems are shown in Table 1. Esmaeilia et al. [15, 16] described that the tool steel of H13 grade eroded off at a higher rotational speed for dissimilar brass-AA1050H16 FSW system. The consistent results are addressed by Agrawal et al. [37] for the Cu-AA6063 FSW system. The reason for this may be attributed to lower wear resistance and elevated temperature stability at higher rotational speed. The rubbing action with high strength alloys such as brass and AA6063 materials at a higher rotational speed may result in wear of the tool. Additionally, the sticking of Cu–Al mixed material on the surface of the tool after every welding run is a big issue which causes defects. However, this problem can be avoided by inserting the FSW tool into the fresh Al material after every experiment. Insertion of tool in fresh material helps to react Cu–Al mixed material with fresh Al material which in turn clean the tool pin and prevent the defects [15, 62]. Tool Design and Geometry The FSW tool has two basic parts: (I) pin and (II) shoulder. Important elements of these parts are shoulder

Tool shoulder. In FSW, the shoulder diameter is maximum responsible for heat generation. It has been found that the shoulder generates around 87% heat by rubbing action between the shoulder surface and the workpiece [9]. Tool shoulder diameter and geometry= surface features affect the quality of weld in FSW as it contributes to maximum heat generation. For achieving good quality FSW joint, the optimum shoulder diameter is one of the important parameters needed into consideration before the welding [5]. Tool shoulder diameter affects the peak temperature variation, material deformation, plunge load variation, mechanical properties, microstructural variation, and formation of IMCs in dissimilar Cu–Al FSW system. Akinlambi et al. [29] claimed that uniform mixing between Cu and Al with a proper material flow pattern can be obtained with 15 mm and 18 mm diameters while improper material mixing was observed with 25 mm diameter for 3.175 mm thick dissimilar (AA5754-C11000) FSW system. Maximum tensile strength (of 208 MPa) and a minimum tensile strength (of 171 MPa) are reported at 18 mm and 25 mm shoulder diameters, respectively. Moreover, the layer of IMCs is found thicker when the larger shoulder diameter (i.e., 25 mm) was applied. Higher hear input is responsible for the thick IMCs layer, material deformation, and mechanical properties which consequently deteriorate the material flow of Cu fragments and Al matrix. So, the microhardness varies as the shoulder diameter changes. Three shoulder geometries such as concave, convex, flat with special profile features like scrolled, ridges, grooves, concentrating circles (shown in Fig. 2) can be provided to increase material deformation and uniform mixing in FSW [5]. Selection of proper shoulder geometry=feature depends on the workpiece and tool materials as well as workpiece thickness. For dissimilar Cu–Al FSW system, the tool shoulder geometries and profile affects the material flow, formation of IMCs, and mechanical properties of the joint. Galvao et al. [13] reported that, scrolled shoulder is used to force Cu–Al mixed material downward, which, in turn give good surface morphology. But, large amount of IMCs are formed when scrolled profile is used. These IMCs are responsible to increase hardness and brittleness in the stir zone and that also causes defects. So, scroll surface profile should not be recommended to achieve sound defect free dissimilar Cu–Al FSW joint. Conical and flat surface shoulder features are favorable profiles for dissimilar Cu–Al FSW system. Conical shape around 2–10 cavity helps to push the material downward through centrifugal force which gives the proper material flow for joint formation [12–14, 22, 29, 69, 70]. Conical angle depends

235

A REVIEW ON DISSIMILAR FRICTION STIR WELDING TABLE 1.—Summary of recommended tool design, process parameters and remarkable properties of different dissimilar Cu–Al FSW systems. Workpiece materials

Thickness (mm)

Tool design

Process parameters

Remarkable properties of joint

References

TM: H13 tool steel SSp: Scrolled, Conical 3 cavitry SD: 14 mm PSp: Cylindrical PD: 3 mm SPR: 4.66:1 TM: Hot worked alloy steel (1.2344), 45 HRC SD: 15 mm SSp: Concave 5 PSp: Tapered (18 ) slotted PD: RD: 5 mm, TD: 4 mm SPR: 3:1 PL: 2.85 TM: Tool Steel SSp: Concave SD: 12 mm PSp: Cylindrical PD: 3 mm SPR: 4:1

RS: 750 and 1000 rpm WS: 160 and 250 mm=min TTA: 2 DSP: 0.05 (scrolled) APL: 700 kgf

IMCs: CuAl2, Cu9Al4 Cu(Al) solid solution Hardness: 700 HV

[12–14]

RS: 450 rpm WS: 20 mm=min TTA: 1.5 TPO: 1.6 mm DSP: 0.25 mm

IMCs: CuAl2, Cu9Al4, CuZn UTS: 100.1 MPa JE: 80% of Al YS: 79.3 MPa Hardness: 165 HV (top), 150 HV (bottom)

[15–19]

RS: 1100 rpm WS: 20 mm=min TPO: 0.2 mm

[20]

3

SSp: Concave SD: 16 mm PSp: Threaded cylindrical PD: 5.2 mm SPR: 3:1

RS: 1000 rpm WS: 80 mm=min TTA: 2.5 TPO: 2 mm

Oxygen free Cu (Cu-DHP, R240) and AA6082-T6

3

RS: 1000 rpm WS: 200 mm=min TTA: 3 TPO: 1.9 and 2.5 mm APL: 7 KN

Commercially pure Cu and AA6101

3

RS: 710 rpm WS: 355 mm=min TTA: 1.5

UTS: 135.5 MPa YS: 91.92 MPa EL: 3.1% Hardness: 115 HV

[23]

Commercially pure and Cu AA6061

3

[24]

3

RS: 1118 rpm WS: 60 mm=min TTA: 1.5 TPO: 2 mm RS: 1000 rpm WS: 50 mm=min TPO: 1 mm

UTS: 170 MPa Hardness: 170 HV (top), 200 HV (bottom)

Pure Cu and AA5052

IMCs: CuAl2, Cu9Al4, Cu(Al) laminae

[25]

Pure Cu and AA5052

3

RS: 1000 rpm WS: 100 mm=min TTA: 2 TPO: 0 mm

IMCs: CuAl2, Cu9Al4, Cu(Al) laminae, Cu3Al UTS: 127 MPa Hardness: 125 HV

[26]

Pure Cu and AA1060

3

TM: H13 Tool steel SSp: Conical 7 SD: 16 mm PSp: Cylindrical PD: 5 mm SPR: 3.2:1 PL: 2.9 mm TM: Tungsten Carbide SD: 20 mm PD: 7 mm SPR: 2.85 PL: 2.7 mm SD: 18 mm PSp: Square profile- 4 mm SPR: 4.5:1 PL: 2.75 mm SD: 12 mm PD: 3 mm PSp: Cylindrical non threaded SPR: 4:1 PL: 4.5 mm (1.5 mm barrier þ pin length) SD: 12 mm SSp: Concave PD: 3 mm PSp: Conical SPR: 4:1 TM: heat treated tool steel SD: 18 mm PD: 6 mm PSp: cylindrical PL: 2.7 mm

IMCs: CuAl2, Cu9Al4, Cu3Al2 UTS: 130 MPa JE: 75.6% of Al BF: 700N Hardness: 172.4 HV (top), 195.3 HV (bottom) UTS: 152 MPa JE: 74.14% of Al EL: 6.4% Hardness: 90 HV (top), 100 HV (middle), 120 HV (bottom) No IMCs formation

RS: 600 rpm WS: 100 mm=min TPO: 2 mm

IMCs: CuAl2, Cu9Al4 UTS: 130 MPa YS: 110 MPa Hardness: 200 HV

[27]

Oxygen free Cu (Cu-DHP, R240) and AA5083-H111

1

Brass and AA1050H16

3

Pure Cu (T2) and Al (5A02)

3

Pure Cu and AA1350

[21]

[22]

(Continued )

236

K. P. MEHTA AND V. J. BADHEKA TABLE 1.—Continued

Workpiece materials

Thickness (mm)

Pure Cu and 5A06

3

C11000 and AA5754

3.175

Soft annealed Cu-DHP and AA6083-T6

3.20

Tool design

Process parameters

Not presented by authors

RS: 950 rpm WS: 150 mm=min

TM: H13 tool steel, 53 HRC SSp: Concave SD: 18 mm PSp: Cylindrical threaded PD: 5 mm SPR: 3.6:1 PL: 2.6 mm SD: 18 mm PD: 6 mm PSp: Triflute type SPR: 3:1

RS: 950 rpm WS: 50 mm=min APL: 11.6 KN

Pure Cu (99.9%) and AA6063

4

TM: High speed steel, H13 tool steel SD: 15, 18 mm PSp: Cylindrical threaded PD: 7 mm SPR: 2.1:1, 2.5:1 PL: 3.7 mm TM: Tool steel PPs: Threaded cylindrical taper pin (2.8 pin angle) SD: 18 mm, 20 mm, 22 mm SSp: conical surface TM: H13 tool steel (quenched and tempered) SD: 20 mm PD: 4 mm SPR: 5:1 PL: 3.8 mm SD: 20 mm PSp: Unthreaded cylindrical profile PD: 6.5 mm PL: 4 mm SPR: 3.07:1 Not presented by authors

Pure Cu (T2) and AA5A06

4


4

C12200-H01 and AA1050-H16

4

Cu-b1 (C12200 H01) and AA1050 H16

4


5

TM: Heat treated H13 tool steel SD: 20 mm PSp: Cylindrical threaded PD: 6 mm SPR: 3.33:1 PL: 4.8 mm

Pure Cu and AA6082-T6

5

Pure Cu and AA5083

5

TM: Z38CD5heat treated steel (50 HRC hardness) SD: 15 mm PD: RD: 9.7 mm, TD: 3 mm SPR: 1.54:1 PL: 4.8 mm TM: Hear treated tool steel H13 SD: 20 mm SSp: Concave 6 PSp: Cylindrical non threaded

RS: 1000 rpm WS: 0.1 mm=revolution TTA: 2.5 Cold rolling 0.28 mm, width 10 mm

RS: 1000 rpm WS: 56 mm=min TPO: 0.5 mm


No IMCs formation UTS: 296 MPa JE: 100% of Cu and 96% of Al IMCs: CuAl2, Cu9Al4 UTS: 208 MPa JE: 86% of Al Hardness: 240 HV ER: 0.101 mX

IMCs: CuAl2, Cu9Al4, Cu3Al UTS: 335 MPa JE: 142.55% of Cu EL: 10% Hardness: 120 HV ER: 25 nXm IMCs: CuAl2, Cu9Al4 Hardness: 295 HV

References

[28]

[29–35]

[36]

[37]

RS: 1050 rpm WS: 45 mm=min TPO: 0.2 mm TTA: 2.8

UTS: 236 MPa

[38]

RS: 1420 rpm WS: 100 mm=min TTA: 2 TPO: 1.5-1.75 mm

UTS: 90 MPa JE: 75% of Al

[39]

RS: 900 rpm WS: 10 mm=min TPO: pin inserted totally in Al

IMCs: CuAl2, Cu9Al4 UTS: 82 MPa JE: 60% of Al

[40]

RS: 900 rpm IMCs: Al2Cu, Al4Cu9 WS: 100 mm=min Hardness: 125 HV TPO: pin inserted totally in Al RS: 600 rpm IMCs: CuAl2, Cu9Al4 UTS: 110 MPa WS: 100 mm=min JE: 91% TPO: 2 mm YS: 90 MPa EL: 13% Hardness: 100 HV (top), 110 HV (middle),120 HV (bottom) IMCs: CuAl2, Cu9Al4 RS: 800 rpm JE: 25% of Cu WS: 750 mm=min EL: 3% TTA: 2.5 TPO: 1 mm Hardness: 130 HV APL: 1500 kg RS: 800 rpm WS: 60 mm=min TTA: 3

IMCs: CuAl2, Cu9Al4 UTS: 225.6 MPa JE: 97.40% of Cu EL: 3%

[41]

[42, 43]

[44, 45]

[46–48]

(Continued )

237

A REVIEW ON DISSIMILAR FRICTION STIR WELDING TABLE 1.—Continued Workpiece materials

Pure Cu (99.99%) and AA6061-T6

Pure Cu and AA1100

Pure C and AA1100-H14

Pure Cu (99.972% purity) and AA5086-H116)

Electrolytic tough pitch Cu and AA6061-T6

Electrolytic tough pitch Cu and AA6061-T6

AA6060- T6 and Oxygen free Cu

Pure Cu (99.99%) and AA6061-T6 Pure Cu and Pure Al

Pure Cu and AA6061

Pure Cu and AA7070

Thickness (mm)

Tool design

PD: 5 mm SPR: 4:1 PL: 4.7 mm 6 TM: Carbon steel screw PD: 6.5 SD: 19 SPR: 2.92:1 PL: 5.8 6 SD: 20 mm PD: 6 mm PSp: cylindrical SPR: 3.33:1 PL: 5.7 mm 6 TM: hardened super high speed steel SD: 18 mm SSp: concave PD: 7.2 mm (RD), 5.5 (TD) PSp: tapered plain PL: 5.8 mm 6.3 TM: Chromium alloy steel PD: 6 mm PSp: Cylindrical threaded pin SD: 18 mm SSp: Concave 10 SPR: 3:1 PL: 5.9 mm 6.3 TM: tool steel M2grade (hardened to 62HRC) SD: 26.64 mm PD: 8 mm PSp: cylindrical threaded (pitch 1 mm left hand) 6.5 TM: tool steel SD: 26 mm PD: 8 mm PSp: Cylindrical threaded pin (1 mm pitch) SPR: 3.25:1 PL: 6.3 mm 10 TM: IN738LC SD: SSp: Concave 10 PD: 8 mm (RD), 6 mm (TD) PSp: tapered unthreaded pin 12.7 TM: Tool steel PSp: Cylindrical threaded PD: 12 mm 1.9 þ 0.9 SD: 12 mm (Lap joint) PD: 2.8 mm PL: 2.6 mm SPR: 4.28:1 1.6 þ 1.6 TM: H13 tool steel (Lap joint) SD: 10 mm PSp: Cylindrical threaded PD: 4 mm SPR: 2.5:1 PL: 1.6 mm 2þ2 TM: 2436 steel (Lap joint) SD: 17.5 mm

Process parameters


References


Intercalated vortex type microstructures

[49, 50]

RS: 815 rpm WS: 98 mm=min TPO: 1 mm Preheating Current: 45 A

UTS: 107.2 MPa JE: 75% of Al EL: 5% Hardness: 125 HV (top and bottom),162 HV (midle) IMCs: Al2Cu, AlCu and Al4Cu9 UTS: 113 MPa JE: 70.62%

[51, 52]

RS: 1075 rpm WS: 80 mm=min TPO: 2 mm

[53]

[54]

RS: 710 rpm WS: 69 mm=min TPO: 0.2 mm TTA: 2 DSP: 0.25 mm DT: 20 sec

IMCs: CuAl2 UTS: 206.7 MPa Hardness: 130 HV

RS: 1300 rpm WS: 40 mm=min TPO: 2 mm TTA: 2 to 4 APL: 788–850 kgf

IMCs: CuAl2, Cu9Al4, Cu4Al3, Cu3Al UTS: 117 MPa Hardness: 181 HV

RS: 2000 rpm WS: 40 mm=min TTA: 2 TPA: 1 mm

UTS: 135.6 MPa JE: 59% of Cu EL: 3.43% Hardness: 210 HV

RS: 750 WS: 150 mm=min TPO: 1.5 mm (Cu side) TTA: 2.5

IMCs: CuAl, CuAl2 UTS: 158 MPa Hardness: 700 HV ER: 0.39 mm


IMCs: CuAl2, Cu9Al4, Cu3Al Hardness: 760 HV Hardness: 150 HV

[60]

RS: 1400 rpm WS: 127 mm=min TTA: 3

IMCs: CuAl2, Cu9Al4 Tensile fracture load: 4 KN EL: 23%

[62]


Tensile shear fracture load: 4 KN

[63]

RS: 800 rpm WS: 50 mm=min DT: 25 sec

[55, 56]

[57]

[58, 59]

[61]

(Continued )

238


Workpiece materials

Thickness (mm)

Pure Cu and AA1100-H24

2þ2 (Lap joint)

Pure Cu and AA1100-H24

1þ2 (Lap joint)

Pure Cu and AA6061-T6

2þ2 (Lap joint)

Brass and AA5083

2.5 þ 2.5 (Lap joint)

Pure Cu (99.98%) and pure Al (99.64%)

3 þ 3 (Lap joint)

Cu-DHP (R240) and heat treatable AA6082-T6, Cu-DHP (R240) and non-heat treatable AA5083-H111

1 þ 6 (Lap joint)


3 þ 3 (Lap joint)

Pure Cu and AA1060

4þ3 (Lap joint)

Pure Cu and AA6060

3þ4 (Lap joint)

Pure Cu and AA5083

3 þ 2 (Lap joint)

Process parameters

Tool design

PSp: Non threaded cylindrical PD: 5 mm SPR: 3.5:1 PL: 3.5 mm TM: Tool steel SKD 61 SD: 10 mm PD: 3 mm SPR: 3.33:1 PL: 2.1= 2.2 mm TM: Tool steel SKD 61 SD: 10 mm PD: 3 mm SPR: 3.33:1 PL: 1.7 mm SD: 8 mm SSp: Concave PD: RD: 2 mm and TD: 1.5 mm PSp: Conical threaded pin SPR: 2:1 PL: 4 mm TM: 2436 steel alloy SD: 20 mm PSp: Cylindrical threaded PD: 6 mm SPR: 3.33:1 PL: 3.5 mm TM: High speed steel-M2 (oil quenched and tempered), RC62 SD: 15 mm PSp: tapered PD: RD: 5 mm, TD: 3.5 mm SPR: 3:1 PL: 5.7 mm SSp: Conical 8 SD: 9.5 mm PSp: cylindrical PD: 3 mm SPR: 3.1:1 PL: 1 mm TM: Heat treated tool steel SD: 20 mm PSp: Cylindrical PD: 8 mm SPR: 2.5:1 PL: 4 mm TM: Quenched and tempered tool steel SD: 15 mm PSp: Threaded PD: 5 mm SPR: 3:1 PL: 6.5 mm TM: tool steel (SPK quenched and tempered) SD: 25 mm PD: 5 mm PSp: cylindrical PL: 3.5 mm TM: Quenched and tempered tool steel SD: 19.1 mm SSp: concave 6


References

TTA: 2

Hardness: 110 HV


IMCs: CuAl, Cu9Al4 Tensile fracture load: 238 N Hardness: 133 HV

[64]

RS: 1002 rpm WS: 198 mm=min TTA: 3 Intermediate layer: Zn

IMCs: CuAl, Cu9Al4 Tensile fracture load: 526 N Hardness: 133 HV

[65]

RS: 1000 rpm WS: 30 mm=min DPS: 0.2 mm

IMCs: CuAl2, Cu9Al4

[66]

RS: 1120 rpm WS: 6.5 mm=min TTA: 1.5

Tensile shear fracture load: 3400 N Hardness: 120 HV

[67]


Hardness: 70 HV Maximum thermal stress: 10 MPa

[68]

RS: 600 rpm WS: 50 mm=min TTA: 0 APL: 4 KN

IMCs: CuAl2, Cu9Al4 UTS: 240 MPa Hardness: 180 HV


IMCs: CuAl2, Cu9Al4 Tensile shear fracture load: 2680 N Hardness: 130 HV


IMCs: CuAl2, Cu9Al4, CuAl Tensile shear fracture load: 2709 N Hardness: 90 HV

RS: 1120 rpm WS: 25 mm=min TTA: 2 Intermediate layer of MIL-A8625

IMCs formation prevented Tensile shear fracture load: 4673 N Hardness: 110 HV

[74]

RS: 825 rpm WS: 32 mm=min TTA: 3.5 DSP: 0.4 mm

IMCs: CuAl2, Cu9Al4 UTS: 204.51 MPa JE: 78% of Cu and 74% of Al

[75]

[69, 70]

[71]

[72, 73]

(Continued )

239

A REVIEW ON DISSIMILAR FRICTION STIR WELDING TABLE 1.—Continued Workpiece materials

Brass (CuZn34) and AA5083

Pure Cu and Al

Oxygen free Cu (Cu-DHP, R240) and AA5083-H111

Thickness (mm)

Tool design

PD: 5 mm PL: 3.8 mm 2.5 þ 2.5 TM: 2436 tool steel (Lap joint) SD: 20 mm SSp: concave PD: 6 mm PSp: non threaded cylindrical PL: 3.5 mm 3 þ 1.5 TM: H13 tool steel (Lap joint) PD: 2.9 mm PSp: cylindrical SD: 12 mm PL: 2.6 mm 1þ6 TM: H13 tool steel (Lap joint) SSp: Conical 8 cavitry SD: 10 mm PSp: Cylindrical PD: 3 mm SPR: 3.33:1

Process parameters


References

Hardness: 95 HV RS: 1120 rpm WS: 6.5 mm=min TTA: 1.5

IMCs: Al2Cu, Al4Cu9, and CuZn Tensile shear fracture load: 5400 N Hardness: 123 HV

[76]


IMCs: CuAl2 UTS: 230 MPa Hardness: 190 HV

[77]


IMCs: CuAl2, Cu9Al4

[14]

TM: tool material, SSp: shoulder surface profile, SD: shoulder diameter, PSp: pin surface profile, PD: pin diameter, SPR: shoulder to pin diameter ratio, PL: pin length, RD: root diameter, TD: tip diameter, RS: rotational speed, WS: welding speed, TTA: tool tilt angle, TPO: tool pin offset, APL: axial plunge load, DSP: depth of sinking pin, DT: dwell time, IMCs: intermetallic compounds, UTS: ultimate tensile strength, YS: yield strength, JE: joint efficiency, BF: bending force, EL: fracture to elongation, ER: electrical resistivity. Note: Hardness shows maximum hardness in joint area.

on the thickness of workpieces and the diameter of the shoulder. Optimum shoulder design for dissimilar Cu– Al FSW system may still consider for strenuous research interest because of limited research articles. Tool pin. Tool pin is responsible for plasticized material flow by stirring action in the joint area. Pin diameter, surface profile, and pin length are important parts of the tool pin. Pin length affects the penetration level of plasticized material in nugget=stir zone. Tool pin length is generally kept 0.2 to 0.3 mm less than the workpiece thickness so that the shoulder can get proper contact with the workpiece by giving appropriate axial plunge load [5]. Pin diameter and surface profile features affect the size of stir zone, microstructure, and material flow. Zhao et al. [38] studied experiments with three different tool pin profiles, threaded cylindrical, taper cylindrical, and straight cylindrical and concluded that, the taper pin is most effective to attain high strength dissimilar Cu–Al FSW joint. Selection of the optimum tool pin profile and its dimension is one of the active research areas for dissimilar Cu–Al FSW joint.

FIGURE 2.—Shoulder surface profile features [5].

The relation between pin and shoulder dimension is defined as shoulder to pin diameter ratio (SPR). SPR of dissimilar Cu–Al FSW system depends on the type and thickness of alloys being joined. However, general range of 2:1 to 5:1 is noted by researchers (refer Table 1). This mentioned range of SPR is relatively higher than the similar material FSW system. In a dissimilar system of Cu and Al materials (which is having different thermal conductivities and specific heats), the SPR should be such that the thermal input and the distribution of it can be maintained. So, by keeping relatively higher SPR, the thermal input can be raised in an appropriate way, and at the same time, the distribution of it can be managed by the other process parameters such as tool pin offset, position of the workpiece material, rotational speed, and welding speed. In addition to this, higher thickness of workpiece materials requires larger SPR because pin is more responsible for thermal input in case of higher thickness system. PROCESS PARAMETERS In FSW technology, the important process parameters are rotational speed, welding speed, axial plunge load, and tool tilt angle for similar material system. Other two parameters such as tool pin offset and position of different base materials in fixture are the additional parameters which affect the dissimilar FSW system along with mentioned similar material system’s parameters. Importance of these parameters is explained next in detail.

240


Tool Pin Offset Tool pin offset is the key parameter as far as the dissimilar Cu–Al FSW system is concerned. Tool pin offset is nothing but the displacement of FSW tool toward particular base material. Figure 3 (a) shows no pin offset that means it is zero when FSW tool is exactly at the center of joint line, whereas Fig. 3 (b) shows 1 mm displacement toward Al side that means pin offset is 1 mm. It is impossible to produce plastic deformation in Cu and Al together by keeping tool pin offset 0 mm (i.e., without tool pin offset) because of differences in melting points and thermal conductivities. Another logic behind the tool pin offset is to generate and distribute the heat in both the materials optimally which is difficult to produce by keeping ordinary tool pin offset (i.e., 0 mm pin offset). Shifting of the tool toward Al material produces more heat at Al side and less at Cu. Therefore, the higher coefficient of thermal expansion of Cu can take away a lesser amount of heat than Al which in turn helps to distribute thermal stresses optimally. The smaller pin offset generates large amounts of IMCs in the weld area which decreases the weld strength of the joint [22]. Ordinary tool pin insertion (i.e., 0 mm) gives poor joints and produces many defects [22, 34, 43, 60, 62]. Additionally, the surface morphology becomes poor when pin offset is too small. For Cu–Al FSW system, tool pin should be displaced toward Al side in such a way that only a few particles could be detached from Cu base material which can easily flow and mix in Al matrix. So, the sizes of Cu particles are being maintained through tool pin offset parameter. In addition to this, the larger tool pin offset toward an Al material gives more stirring at Al side which leads to form Al matrix and that provides a better path to the smaller Cu particles. These smaller Cu particles help to make good metallurgical bonding with an Al matrix by reducing the amount of IMCs. Firouzdor et al. [62] reported that by placing an extra sheet of Al adjacent to Cu in such a way that the tool pin offset can be adjusted in lap joint configuration. They also showed that, the joint strength can be increased, if pin offset is introduced in the dissimilar lap joint configuration. Moreover, they claimed that, uniform mixing between Al and Cu can be achieved by taking the pin offset toward softer (means Al) material. So, it is also

reason out that butt FSW is better than a lap welding for dissimilar system. The literature indicated that 1.5 to 2 mm pin offset is preferred to achieve good quality dissimilar Cu–Al FSW joint (refer Table 1). However, the optimum tool pin offset is depends on the tool design and thickness of the workpiece to be welded. Position of Cu and Al Base Materials Position of base material does not weigh in the similar material FSW system, but it is an important affecting parameter for dissimilar Cu–Al FSW system. The butt joint and lap joint configurations are affected by this parameter. Material flow in the joint area is strongly affected by advancing and retreating sides [6]. It is recommended that, Cu and Al are required to be positioned on advancing and retreating side, respectively, for butt joint configuration [13, 14, 42, 43] while Al and Cu are needed to be positioned on top and bottom, respectively, for lap joint configuration [63]. According to the Nune’s kinematic model, the way of material flows of retreating side is straight through current flow while it follows the whirlpool pattern at advancing side (explained very well by Mishra et al. [6] in Ch. 3). So, by keeping Cu on advancing side, the flow path of Cu particles can be improved through a whirlpool pattern that subsequently provides uniform distribution of Cu particles in Al matrix which leads to make sound bonding. Also, the harder material (i.e., Cu) is difficult move from retreating to advancing side which results in nonuniform material flow, if the Cu is placed at retreating side. This nonuniform dissimilar material flow causes obvious volume defects like tunnels and voids because of improper mixing [42, 43]. The surface tunnel is generated in most of the cases when Cu is kept on retreating side while defect free surface morphologies are reported when Cu is kept on advancing side [13, 14, 42, 43]. On the other hand, position of base materials for dissimilar Cu–Al lap joint is also considered as important parameter which affects the joint quality. Akbari et al. [63] studied the effect of base material (i.e., Cu and Al) position in lap FSW. They concluded that defect free joint can be achieved when Al is required to be kept on top and Cu at the bottom. Due to low thermal

FIGURE 3.—Concept of tool pin offset (a) 0 mm and (b) 1 mm.

A REVIEW ON DISSIMILAR FRICTION STIR WELDING

conductivity of Al relative to Cu, large amount of heat is generated in nugget when Al kept on top which results in formation of defect free fined grained stir zone. Also, the greater stirring can be supplied in Al by keeping it on top which can help to produce sound metallurgical bonding between Al matrix and Cu particles likely to be detached from the bottom Cu material. Rotational Speed The speed at which the FSW tool rotates is called rotational speed. Shojaeefard et al. [67] concluded that rotational speed contributes overall 40% in dissimilar Cu–Al FSW system. Rotational speed is an important process parameter of FSW, because it influences the large amount of frictional heat generation, plastic deformation of material, and forces on the tool which consequently influences the formation of IMCs, material flow, defect generation, the size of the stirred zone, and tool wear in the dissimilar FSW system. A mutation in a frictional heat generation also affects the formation of IMCs in dissimilar Cu–Al FSW. Higher rotational speed forms the large amount of IMCs because of higher heat input. On the other hand, stronger stirring action at high rotational speed is responsible to detach large Cu particles from Cu base material. These particles are not capable to make proper bonding with Al matrix and results in defects like cracks and voids [12, 16, 42, 43, 47, 75]. Moreover, increase in rotational speed accompanied by thickening of interfacial IMCs layer due to higher heat input at the joint interface [16]. The high rotational speed also increases the tool wear because of strong rubbing action which reduces tool life (for tool steel alloys) in case of high strength alloys like brass and AA7XXX. On the other hand, extremely low rotational speed results in imperfect joints because of low heat input [43]. Due to lack of heat production, slow rotation will not let the joint area reach suitable temperature for sufficient plastic deformation. Consequently, the stir zone cannot be plasticized appropriately, and an unsuitable flow results in defects, especially macrocracks and channel defects in dissimilar Cu–Al FSW [16, 43, 72, 75]. Extremely low rotational speed or higher rotational speed results in lower tensile strength and higher hardness values [16, 43, 75]. Welding Speed or Travel Speed Welding speed or travel speed is the speed at which tool travels through the joint line of workpiece. Welding speed is equally important for achieving good quality dissimilar Cu–Al FSW [30, 46, 53]. It affects the metallurgical bonding and material mixing of Cu and Al materials. Extremely high welding speed produces joint with incompletely welded interfaces because of lack of heat input [30]. Lower welding speed gives higher heat input and generates more amounts of IMCs [30, 46, 53]. High heat input softens the material largely which results in turbulent movement of plasticized material. Due to this reason, the stir zone is filled up with the turbulent flow of the Al matrix along with the different distribution of Cu particles which forms large amount of IMCs. These IMCs

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forms more cracks that consequently reduce the joint strength [46]. Higher welding speed gives lower heat input results in improper mixing of Cu–Al material and causes defects like voids [30, 46]. Another reason for these defects is differences in flow stress of Al and Cu materials. So, the optimum welding speed is required through which the flow stress of dissimilar materials can be managed. Decreasing the welding speed at constant rotational speeds led to similar trend to increasing the rotational speeds at constant welding speeds [72]. It is well documented that, the optimum combination between rotational speed and welding speed controls the formation of IMCs in dissimilar Cu–Al FSW system because it influences the heat input [12, 13, 75]. Galvao et al. [12] reported that the ratio of rotational speed to welding speed is directly influenced to the heat input which, in turn, affects the formation of IMCs and mechanical properties of dissimilar Cu–Al FSW system. Tool Tilt Angle Tool tilt angle is defined as ‘‘the angle at which the FSW tool is positioned relative to the workpiece surface, i.e., no or 0 tilted tool is positioned perpendicular to the workpiece surface’’ [55]. A suitable tilt angle ensures that the shoulders hold the stirred material by tool pin and move it properly from the top to bottom and front to the back under the FSW tool [5]. Tool tilt angle also helps to compress the plastically deformed material beneath the FSW tool. Therefore, the horizontal as well as vertical material flow inside the stir zone is influenced by tilt angle. For dissimilar Cu-AA6061-T651 FSW system, range of tilt angle 2 to 4 is recommended by Mehta et al. [55]. Larger tilt angle helps to fill up the defects through forging material downward instead of spreading it above the top surface (i.e., flash effect). In addition to this, higher tilt angle allows Cu particles to flow freely in Al matrix, at a higher axial plunge force, that improves the metallurgical bonding between Cu and Al. The increase in tilt angle causes temperature rise in stir zone which increases the amount of IMCs as well as types of IMCs. So, hardness of joint area increases as the tilt angle increases [55]. Downward Force or Plunge Force or Axial Force Force acts parallel to the spindle axis direction are called downward force or plunge force or axial force. Plunge force helps to maintain the contact of the tool at or beneath the material. Sufficient plunge force is required to achieve full penetration in the stir zone. Insufficient plunge force gives an inappropriate vertical flow of deformed material, whereas higher plunge force causes thinning of the deformed material and results in a flash-out effect. Large amount of axial plunge load (more than 600 kgf) is required for dissimilar Cu–Al FSW system to obtain sound joint (refer Table 1). The higher hardness of Cu relative to Al requires increased plunge force to forge material downward beneath the shoulder of the tool. Process parameters such as welding speed and tilt angle influences plunge force studied by Akinlabi et al. [30] and Mehta et al. [55], respectively.

242 MICROSTRUCTURES AND INTERMETALLIC COMPOUNDS Microstructures of similar materials FSW system are divided into four parts; area under the shoulder consists of two microstructures [stir zone and thermomechanically affected zone (TMAZ)] while outside of shoulder heat affected zone and parent metal microstructure. Same way, the dissimilar materials FSW system consists of these zones. These different microstructures subsequently affect the mechanical properties. Microstructures of dissimilar Cu–Al FSW systems reported by different authors are presented in Fig. 4. Stir zone of similar material FSW generally consists of an onion ring structure [5]. But, no such particular pattern is observed for dissimilar Cu–Al FSW system. Intercalated vortex type microstructure in stir zone is formed because of complex mesh and dissimilar materials. Stir zone of the dissimilar FSW system consists of


different sized irregular Cu particles distributed and mixed with Al matrix seems like Cu islands in Al matrix [see Fig. 4 (a), (b), and (c)]. The mixing of these irregular Cu particles in Al material, usually forms IMCs such as CuAl2, Cu9Al4, Cu3Al, and CuAl (see Table 1 for summary). These IMCs are very hard and brittle in nature [12, 16, 41, 42, 45, 72]. The reason for such complex microstructures is incompatibilities of materials such as melting points, chemical compositions, and holding time. The chemical reaction or phase transformation cannot occur, because the FSW process operates below the melting points of material which drives to form IMCs due to inter-diffusion through extreme deformation and intense stirring. The IMCs continuously presented in a layer form at the interface of Cu and Al materials [example shown in Fig. 4 (d) and (e)]. Continuous thin layer (between 0.5 to 4 mm) of IMCs

FIGURE 4.—Microstructures for different dissimilar Cu-Al FSW systems (a) TMAZ and stir zone at interface of Cu-AA6061 [55], (b) TMAZ and stir zone at Al [55], (c) SEM image of stir zone [55], ((d) and (e)) SEM image of Cu-Al interface layer and IMCs [20, 45].


is required to achieve sound, defect free dissimilar joint [42, 53, 58, 72]. There are two distinct TMAZs for dissimilar Cu–Al butt FSW joint, one at Cu–Al interface while another at Al side. TMAZ zones generally contain a high density grains with sub-boundaries [63]. TMAZ of Al side is usually wider in size because the FSW tool is displace more toward Al side which allow more stirring in Al material which deforms material and results in enlargement of zone. On the other hand, TMAZ of Cu–Al interface is narrow [16]. TMAZ of Cu–Al interface is the weakest zone considered for dissimilar Cu–Al butt FSW system because of the presence of hard and brittle IMCs in a layered form [30, 55]. The HAZ region is formed because of intense heat in which the grain size becomes coarse while no grain deformation occurred in it and that results in a minor decrement of hardness [16]. The HAZ generally found at Al side in most of the cases because of the displacement of tool toward Al while small or no HAZ is formed toward HAZ. The peak temperature is relatively low which does not affect the Cu grains. MECHANICAL PROPERTIES OF DISSIMILAR CU–AL FSW SYSTEM

Mechanical properties of dissimilar Cu–Al FSW joint are quite different than the similar materials FSW system due to imbalances in properties of workpiece materials. Tensile properties and hardness results for different, dissimilar Cu–Al FSW systems are summarized in Table 1. It is reported that the ultimate tensile strength (UTS) of dissimilar Cu–Al FSW system is mostly less than the base materials that means the joint efficiency is even lower than the low strength material. UTS depend on the microstructures and IMCs formation in the joint area which is affected by process parameters of dissimilar FSW. Most of the defects are generated because of improper material flow between Cu particles and Al matrix that is because of nonoptimized process parameters which subsequently results in reduction of UTS. However, a small Cu particles with proper material flow in Al matrix provides a strengthening effect that increases the UTS of joint. Therefore, the size of Cu particles plays an important role to obtain higher UTS. In addition to this, the strong stirring action provides grain refinement strengthening which also drives to increase the UTS. These strengthening effects are still not helpful to obtain higher UTS than the base materials because of unavoidable IMCs and inhomogeneous microstructure. The reason for generation IMCs has been already addressed in the ‘‘Process parameters’’ section. The IMCs formation in the stir zone reduces the ductility of the joint because of the inherent nature of IMCs and that consequently reduces the yield strength and percentage of fractures to elongation. The low fracture to elongation is attributed to brittle fracture or fine recrystallized grains, which is analyzed through fractrography in most of the studies. However, the mixed fracture mechanism of ductile and brittle pattern is present in the fractured tensile specimens such as dimples and flat surfaces, respectively. TMAZ zone is generally found the weakest zone for

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dissimilar Cu–Al FSW joint, because the fracture of a tensile specimen generally occurred in it. TMAZ of Cu side is attributed for brittle fracture because of the presence of IMCs in these areas [26, 33, 38, 55]. On the other hand, the TMAZ of Al side attributed for ductile fracture because of the very fine grain size caused by severing plastic deformation [20, 21, 46, 47, 75]. Improvement in tensile properties for dissimilar Cu–Al FSW can be another area of research for the future. Hardness across the weld section typically observed ‘‘W’’ shaped profile for similar FSW systems [6]. But, different kind of trend like ‘‘\’’ shaped hardness profile is observed for dissimilar Cu–Al FSW lap joint system in most of the literatures [16, 20, 21, 29, 41, 42, 66, 76]. Inhomogeneous hardness distribution is observed for dissimilar FS welded Cu–Al butt joint [16, 20, 27 42, 55, 73]. Harness profiles for different, dissimilar FSW systems are presented in Table 2. The hardness at the stir zone always found higher than both the base materials. The reason for this is the formation of hard IMCs, as well as the intense plastic deformation in the stir zone [20, 63, 76] which is result of Hall–Petch effect [76]. Moreover, solid solution strengthening and grain refinement strengthening are other proposed reasons for higher hardness in stir zone [20, 21]. The hardness in HAZ region is found lower than the other regions due to the grain coarsening occurred during welding that can be seen from Table 2 (Sr. No. 1, 2, 5, and 9) [16, 54]. For butt joint, the thin layer of Cu=Al interface that means at the TMAZ (Cu side) consist the maximum hardness which is presented in Table 2 (Sr. No. 6, 8, and 9). This is because of the presence of large IMCs at the Cu=Al interface in the form of layers [16, 41]. Akinlambi et al. [29] described that the dynamic recrystallization occurred during the welding may also responsible for higher hardness at Cu=Al this interfacial region. The grain refinement and mechanical twinning are the other responsible mechanism which increases the hardness at the interface [29]. On the other hand, for butt joint, the hardness also varies from top to bottom in stir zone and that can be seen from Table 2 (Sr. No. 5, 6, and 7). According to Xue et al. [42] and Xia-wei et al. [21], bottom part of stir zone possess higher hardness because of the presence of higher fraction of IMCs at bottom (see Table 2 Sr. No. 6 and 7, respectively) which is contradictory in the case of Esmaeili et al. [12] [refer Table 2 (Sr. No. 9)]. It depends on the distribution of Cu particles in Al matrix. Higher fragmentation of Cu particles at any zone increases the hardness of that zone, which can be controlled by the parameters of dissimilar FSW. Also, higher hardness at any region is may be attributed, due to two reasons: (I) the fraction of Cu is higher at that region and (II) the lamella structure is more homogenous and finer in that particular area relative to other areas. The parameters of FSW significantly affect the hardness of the joint because of the formation of IMCs and microstructures. Higher heat input conditions such as larger shoulder diameter [27], higher tilt angle [55], higher rotational speed [16], and lower welding speed [28] results in higher amounts of IMCs in stir zone that subsequently

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K. P. MEHTA AND V. J. BADHEKA TABLE 2.—Hardness distribution for different dissimilar Cu–Al FSW systems.

Sr. No.

Hardness profile across the weld

Dissimilar FSW system and description

Reference

1

AA5083-Brass lap joint

[76]

2

Pure Al-Cu lap joint [Line 1 indicate profile at Cu side, Line 2 indicate profile at joint interface, and Line 3 indicate profile below the interface]

[61]

3

AA7070-Cu lap joint [Al is placed on the Cu]

[63]

(Continued )

245

A REVIEW ON DISSIMILAR FRICTION STIR WELDING TABLE 1.—Continued Sr. No.



Reference

4

AA7070-Cu lap joint [Cu is placed on the Al]

[63]

5

5A02-Cu(T2) butt joint

[20]

6

AA1060-Cu butt joint

[42]

(Continued )

246


Sr. No.



Reference

7

AA1350-Cu butt joint

[21]

8

AA1050H16-C12200 butt joint

[73]

9

A1050H16-Brass butt joint

[16]

(Continued )

247

A REVIEW ON DISSIMILAR FRICTION STIR WELDING TABLE 1.—Continued Sr. No.



Reference

10

AA5754-C11000 butt joint [hardness distribution for three different shoulder diameters]

[27]

11

AA6061-T651-electrolytic tough pitch copper [hardness distribution for different tilt angles]

[55]

increases the hardness [example of hardness distribution for different shoulder diameters and tilt angles are shown in (Table 2 Sr. No. 10 and 11, respectively)]. Reducing the hardness of stir zone can be considered as further area of research for the dissimilar FSW system. WELDING DEFECTS OF DISSIMILAR CU–AL FSW SYSTEM The dissimilar Cu–Al FSW defects can be caused by several incorrect process parameters, such as tool design, rotational speed, welding speed, plunge depth, tilt angle, tool pin offset, and fixed position of the base metals. Additionally, a too wide welding gap and mismatch of workpiece plate thickness can also lead to the formation of welding defects [56, 58]. Most common welding defects occurred in dissimilar Cu–Al FSW systems are presented in Fig. 5. Voids and Tunnel Voids are volumetric, contain no material, and are aligned with the welding direction usually occurs at advancing side. Voids form when the amount of heat and pressure has not been adequate to fill the space behind and below the tool. This means that, the too high traverse speed, too low rotational speed, or too low plunge load are the main responsible parameters for generating voids [58]. Low pin offset and position of Cu at retreating side (in butt joint configuration) are governing parameters to generate big size voids because

of improper mixing of large Cu particles in Al matrix [22, 42, 43]. Additionally, the taper tool pin profile also causes voids in the root area in the stir zone because of nonuniform mixing of Cu particles at the bottom area of stir zone [56]. Figure 5 ((b), (c), and (d)) represent the voids in stir zone of Al–Cu FSW joint. If the presence of void is continuous throughout the entire weld beneath the top surface and above the root, it is called ‘‘tunnel defect.’’ Inappropriate design of tools and axial pressure are the main parameters of FSW of a similar system for the formation of this defect, but, the tool pin offset and position of material are leading parameters for dissimilar FSW system [17]. Additionally, it is reported that due to lack of heat production the surface tunnel forms when Cu is fixed at retreating side [43]. Furthermore, Mehta et al. [56] reported tunnel defect at larger tool pin offset with small shoulder diameter.

Macrocrack and Microcrack Macrocracks and Microcracks are another defects reported for dissimilar Cu–Al FSW system presented in Fig. 5 ((e), (f), and (g)), generally occurs because of the formation of brittle IMCs in stir zone. The brittle nature of IMCs increases the hardness in the stir zone, which commences different size cracks in this area [17]. Poor

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FIGURE 5.—Common welding defects; (a) FSW defects [58], ((b), (c), and (d)) voids [43, 62], (e) surface cracks [43], ((f) and (g)) microcracks [43, 73], ((h) and (i)) fragmental defect [15, 17].

metallurgical bonding between large Cu particles in Al matrix causes these defects (see Fig. 5 (f) and (g)) [43, 56]. Improper tool design, tool pin offset, and material positioning are driving parameters which cause these defects [17, 43, 56, 58].

Fragmental Defects Fragmental defects are not found in similar materials FSW system. The fragments of coarse Cu particles dispersed within the Al material is categorized as fragmental defect. The material flow affects the flow of


distribution of these Cu particles. Improper dispersion and flow distribution does not allow filling sharp edges and contacting surface of fragments which lead it to voids and microcracks. In addition to this fragmental defect lead to high hardness and low tensile strength in stir zone. Proper tool pin offset and low rotational speed can reduce these defects [17]. Lack of Penetration Lack of penetration (LOP) leaves the plates at the root of the weld un-joined, though they result in weak bonding. This type of defect is effectively a crack, which causes the structure to fracture easily due to the high stress concentration factor. It causes a reduction in tensile strength and loss of fatigue strength. The primary reason for LOP is a too short tool probe. It can be caused also by a too low plunge depth, plate thickness variation, improper tool design, or tool misalignment in relation to the butting surfaces. It is possible to detect LOP with radiographic, ultrasonic, eddy current, or dye penetrates testing (in through-thickness welds), but no reliable non-destructive test (NDT) method is available at the moment. The only definitive method is a bend test with the root in tension [56]. Pores Pores are generally found in stir zone either single or in line, form, which are 0.1–0.5 mm in diameter, and pore lines may be up to 9 mm in length. Incorrect welding parameters such as too small tool plunge depth cause this defect [56]. Additionally, the small tilt angle also cause pores in stir zone [55, 68]. VARIANTS OF FSW FOR DISSIMILAR CU–AL MATERIALS Recent novel developments of FSW technology for dissimilar Cu–Al system are discussed here as variants of the technology. Some variants earlier addressed for similar materials FSW system such as friction stir spot welding (FSSW) and under water friction stir welding (UFSW) are applied for dissimilar Cu–Al FSW system. Additionally, ultra-modern variants such as FS lap welding using intermediate layer, friction stir butt barrier welding (FSBBW), hybrid FSW (HFSW), friction stir diffusion bonding (FSDB), friction stir brazing (FSB), and microfriction stir spot welding (mFSW) are also discussed here for Cu–Al system. FSSW is an alternative of resistance spot welding (RSW), self-piercing rivets (SPR), and clinching. The disadvantages of these processes (RSW, SPR, and clinching) such as dressing of weld electrode, consumption of high energy, the cost of the electrodes=rivets can be avoided by FSSW technology [6]. Heideman et al. [78] studied the influence of FSSW parameters like tool pin length, shoulder plunge depth, weld time, and tool rotation speed for dissimilar AA6061-T6 and Cu system (1.5 mm thickness of both the materials). They claimed that rotational speed of 2000 rpm, pin length of 2.60 mm, plunge depth of 0.13 mm, and weld time 3 sec give weld strength of 2080 N when aluminum is kept

249

on the copper. In addition to this, they observed that Cu ring is extruded upwards into the Al sheet at the top which lead it to the interlocking and bonding between dissimilar materials and that are free from the IMCs. Shen et al. [79] obtained symmetrical joint of dissimilar AA5083 to deoxidized phosphorous Cu with no hook by refill FSSW technology. Furthermore, the composite band of 10 mm fine particle layer is observed at the interface of Cu=Al joint. UFSW applied for dissimilar AA6061-T6 to pure Cu system by Zhang et al. [66] and compared with normal FSW by keeping process parameters constant. They found that UFSW reduces the formation of IMCs and its layer by decreasing the peak temperature. Moreover, the oxidization of the base materials can be prevented by UFSW. Also, they concluded that the material flow significantly affected in the vertical direction in case of UFSW while it is affected horizontally for normal FSW. This is because of changes in the friction coefficient. It decreases when preheating of tool is present in normal FSW and results in weak friction toward the vertical plane consequently causing horizontal flow only. Recently, Akbari et al. [74] introduced the new material (anodized Al-MIL-A-8625 F containing coating of anodic sulfur with a layer of Cu thickness of 23 mm) as an intermediate layer between AA6060 and Cu base materials. This intermediate layer has prevented the formation of brittle IMCs and 25% increase in shear strength was reported than the joint without the use of the intermediate layer. The same technique is employed by Elrefaey et al. [64] for dissimilar pure Cu and AA1100-H24 FSW system by using 50 mm thick Zn intermediate layer. They have reported improved joint performance by limiting the formation of hard and brittle IMCs. Another technique suggested by Liu et al. [25] is FSBBW by which they improved the surface appearance of dissimilar AA5052 to Cu joint. They claimed that, the barrier sheet of AA5052 can be used to improve surface appearance without cracks, pits, grooves, and flashes whereas the Cu sheet is not recommended as barrier layer. In addition to this, they also reported that as the barrier sheet increases up to certain thickness the weld situations get better. Firouzdor et al. [62] modified a dissimilar lap joint by adding extra Al sheet adjacent to the Cu. Here, the barrier layer is added adjacent to Cu. This helps to introduce appropriate tool pin offset in lap joint configuration. Extra Al sheet and tool pin offset have significantly improved the weld quality of the joint. Furthermore, the productivity can also be increased because one can go for double welding speed after modifying the joint configuration. Girard et al. [44] proposed FSDB technique by which dissimilar materials of Cu and Al can be joined. FSDB is another advanced version of FSW in which the tool pin is totally inserted into the Al material and the joining is being done by interfacial chemical reaction with no mechanical mixing. The inter-diffusion reaction is caused by the frictional heating and stirring which led

250 it to the bonding. However, the formation of IMCs cannot be avoided by this method which results in low tensile strength. Zhao et al. [38] applied HFSW approach by introducing another friction stir tool as a preheating source at Cu base materials. They reported that, through preheating tool, 150–200 C temperature can be raised at Cu side which have improved weld efficiency and obtained a high quality joint by this method. They have obtained maximum tensile strength of 230 MPa with the help of this new technology for dissimilar AA5A06Cu-T2 FSW system. Same way, Yaduwanshi et al. [51] developed HFSW by incorporating plasma torch as preheating source to 200 C toward Cu side and that have improved the weld efficiency of dissimilar Cu-AA1000 FSW joint. Khal et al. [36] did HFSW approach for dissimilar AA6063-T6 and soft-annealed DHP Cu system. They have achieved more than 50% joint efficiency of the base materials by FSW followed by cold rolling technology. These huge rises in tensile strength are because of strain hardening effect into the composite materials. The cold rolling did strain hardening process for dissimilar materials. It is reported that cold rolling after the FSW can remove the defects like voids, cracks, etc. Additionally, they have concluded that the layer of IMCs can be partitioned by performing cold rolling after FSW. Zhang et al. [80] modified a FSW process and introduced the FSB technology. The Zn braze foil of 0.1 mm thin sheet was used between Al and Cu base materials. Then, the pin less tool was moved on the workpices which produce heat and that can be utilized to achieve dissimilar joint. The quality of joints made by FSB was excellent compared to joint made by furnace brazing. In addition to this, they also changed the lap configurations called stepped FS lap joint and exhibited the higher failure load compared to conventional lap joint configurations. The same technology is implemented by Kuang et al. [81] for dissimilar 1A99-pure Cu system, in which good mechanical properties achieved by avoiding intense stirring. The Zn intermediate layer helps to reduce IMCs through diffusional deformation. Teh et al. [82] applied mFSSW for joining Cu cable to C connector of Al material and improved the electrical resistivity and the strength of the joint by reducing IMCs through this solid state process. The mFSSW is the variant of FSW for thicknesses in order of 1000 m or less than that. Montazerolghaem et al. [83] achieved 70 mm thickness of dissimilar AA1100C11000 joint from 0.5 mm initial thickness through FSW followed by cold rolling. Cold rolling helped to remove flaws from the weld generated after FSW. In addition to this, Yusof et al. [84] obtained 0.8 mm ultra-thin FSW joint of AA5052-pure Cu materials without IMCs formation. Muthukumaran et al. [85] has mounted Cu tube on the FSW tool in such a way the tube act as a tool. Then after it is being touched on the Al sheet with a higher rotational speed. At the end the tube to tube-sheet joint is obtained when the tube is removed from the FSW


tool. Narrow TMAZ and HAZ are the advantages along with the satisfactory joint strength and limited IMCs. CONCLUSIONS AND SUMMARY The salient features of dissimilar materials Cu to Al FSW system have been summarized. The effect of different process parameters on the properties of dissimilar materials are also discussed in detail. In addition to this, microstructures, welding defects, and variants of FSW for dissimilar system of Cu to Al materials have been surveyed. It can be noted that, dissimilar joining of Cu to Al by FSW is still not widely employed because of low mechanical properties and formation of IMCs in large amount. Imperfections such as fragmental defects, voids, pores, and cracks are commonly found in dissimilar Cu–Al FSW system which are formed due to improper process parameters that consequently forms different IMCs and lead to the low mechanical properties. These IMCs also increases the hardness of joint area and that also makes the joint area brittle which is driving parameter for brittle fracture and low elongation. There is no analytical relation for optimum process parameters through which desired properties can be achieved. However, different variants such as HFSW, FSBBW, FSB, UFSW, FSDB, the use of intermediate layer in FSW and mFSW have been employed to improve the properties of this system. But, improvement in these processes can be considered as a strenuous area for research. In addition to this, there is a lack of comprehensive data of tool design and tool material for different thicknesses and alloys of this system. It is also reported that, most of the work for this system is on characterization and properties of the joint. But, it is necessary to develop specific industrial applications of dissimilar Cu to Al joint welded by FSW technology. FUNDING The authors would like to thank board of research in fusion science and technology (BRFST), Institute for plasma research (IPR), Gandhinagar Pandit Deendayal Petroleum University (PDPU), Gandhinagar, for motivating through funding under projects NFP=MAT=A 10=04 and ORSP-SRP-Project. REFERENCES 1. Mubiayi, M.P.; Akinlabi, E.T. Friction stir welding of materials between aluminum alloys and copper-An overview. In Proceedings of the World Congress on Engineering, London, UK, July 2013. 978–988. 2. Bergmann, J.P.; Petzoldt, F.; Schu¨rer, R.; Schneider, S. Solid state welding of aluminum to copper-case studies. Welding in the World 2013, 57 (4), 541–550. doi: 10.1007=s40194013-0049-z. 3. Okamura, H.; Aota, K. Joining of dissimilar materials with friction stir welding. Welding International 2004, 18 (11), 852–860. doi: 10.1533=wint.2004.3344.

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