A COMPARATIVE STUDY OF LASER, CMT, LASER-PULSE MIG HYBRID AND LASER-CMT HYBRID WELDED ALUMINIUM ALLOY Paper 1304 Chen Zhang, Ming Gao, Geng Li, Xiaoyan Zeng Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China Abstract Using cold metal transfer (CMT) welding which was a special pulse metal inert gas (MIG) welding method, a new hybrid welding, laser-cmt hybrid welding, was introduced to join aluminum alloy thin plate. In order to evaluate laser-cmt hybrid welding, a comparison research on the microstructures and mechanical properties of aluminum alloy joints made by laser welding (LW), CMT welding, laser-cmt hybrid welding (LCHW) and laser-pulse-mig hybrid welding (LPHW) was carried out. Because of lower heat input, the LW and LCHW joints had smaller grain sizes and narrower widths of welds than these of CMT and LPHW joints. The ultimate tensile strengths of LW, CMT, LCHW and LPHW joints reached up to 220MPa, 200MPa, 219MPa, and 198MPa, respectively. Meanwhile, the LW and LCHW joints brought less deformation in comparison with CMT and LMHW joint. These results showed LCHW had obvious advantages in joint quality, indicating it would be a suitable technique for joining aluminum alloys. Introduction Welding distortion is a widespread problem in the welding of aluminum and aluminum alloy thin plate due to big thermal expansion coefficient of aluminum alloys. Finding a low heat input welding process is important for getting good joining of aluminum components. Laser welding with high energy density is a fine choice. Usually, laser welded joint has characteristics of small fusion area and high aspect ratios which are in favor of little welding distortion [1]. But there are some disadvantages in laser welding of aluminum alloys such as: high laser beam reflectivity, welding material evaporation and high precision welding fixtures, which restricts industrial application of laser welding for aluminum alloys [2]. Laser-arc hybrid welding is developed by integrating laser welding and arc welding [3]. Recently, Fronius GMBH in Austria developed a novel CMT arc welding with low heat input which is characterized by mechanically controlled material deposition during short circuit of wire electrode to the workpiece. By this arc technique, the plate with the thickness of 0.3 mm can be well welded [4, 5]. If CMT and laser are integrated to LCHW, the heat input during welding would be reduced compared to LPHW, which is more suitable to join thin plate of aluminum alloys. However, no prior work exists on LCHW of aluminum alloys. This study aims to evaluate LCHW of aluminum alloys by the comparison of LW, CMT, LCHW and LPHW of 6061-T6 aluminum alloys. Experimental Method IPG YLR-6000 fiber laser and Fronius TPS4000-CMT welder were employed in current experiment. The welder both had the functions of CMT and PMIG. The maximum current of CMT and PMIG were 97 A and 400 A, respectively. The wave length of laser beam was 1070 nm, beam parameters product was 6.9 mm mrad and the diameter of beam focus spot was 0.4 mm. The laser beam was transferred through a fiber with a 200 μm core diameter. Laser beam was focalized via optical lens with focal length of 250 mm. The arrangements of the laser beam and the welding torch are shown in Fig.1. Where, the distance between laser beam focal spot and wire tip was 3mm. Figure 1 Schematic diagram of welding arrangement 430
The base metals were AA6061-T6 sheets with the size of 100mm 50mm 2mm and the filler wire was ER5356 with the diameter of 1.2mm. The mass chemical compositions of AA6061-T6 base metal and ER5356 filler wire are shown in table 1. Table 2 shows the optimized welding parameter. Before welding, the groove was cleared by copper brush and acetone. After welding, the distortion of welded joints was examined by Panasonic laser range finder. The measurement method is shown in Fig.2. The metallographic specimens were etched by Keller reagent (95 ml H 2 O, 1 ml HCI, 1.5 ml HF, and 2.5 ml HNO 3 ) before it was examined under optical microscope. The tensile strength of welded joints were tested according to the standard of ISO 4136-2001 with tensile speed of 2 mm/min, and the result was the average value of three specimens. Table 1 Chemical compositions in mass (wt-%) of base metal and filler wire Composition Si Fe Cu Mn Mg Cr Zn Ti Al 6061-T6 0.4-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.04-0.35 0.25 0.15 Balance ER5356 0.10 0.30 0.05 0.05-0.2 4.5-5.5 0.10-0.15 0.09 0.02-0.08 Figure 2 Schematic diagram of Deformation measuring method Table 2 Welding parameters Value Welding parameters LW CMT LCHW LPHW Laser power (kw) 3.2-3.2 3.2 Defocused distance 0-0 0 (mm) Welding speed (m min -1 ) 5.5 1 5.5 5.5 Argon flow of weld torch (l min -1 15 20 20 20 ) Argon flow of root nozzle (l min -1 ) 10 10 10 10 Wire filling speed (m min -1-5 6 6 ) Arc current (A) - 93 93 105 Arc voltage (V) - 13.3 13.3 18.2 heat input (J/mm) 34 123 48 58 Cross sections of joints are shown in Fig.4 and the bead width is identified by white lines. Because there is no filler wires added, sagging imperfection appears on the surface of LW joint. Because the welding speed of CMT is slow, the width and reinforcement of CMT joint is far larger than those of other joints due to the significant increase of filler materials. Moreover, the width of LCHW joint is narrower than that of LPHW joint due to lower heat input of CMT compared to PMIG. Bead Shape Results and Discussion As shown in Fig.3, good surface appearances are obtained by hybrid welding. It indicates that although the welding speed is high to 5.5m/min, the arc can be stabilized by laser-arc interaction homogeneous hybrid. Figure 3 Surface morphologies of welded joints (a) LW, (b) CMT, (c) LCHW, (d) LPHW 431
Fig.7 shows the transverse tensile strength of welded joints. The tensile strength of LW and LCHW joints is 220 and 219 MPa, respectively, which are up to 70% of base metal. Although the grain size of LCHW joint is larger than that of LW, the tensile strength of them are similar, which may be due to the decrease of columnar dendrite. Similarly, the decrease of columnar dendrite in CMT joint makes its tensile strength is up to the degree of LPHW although the heat input of CMT is far higher than that of LPHW. Figure 4 Cross sections of welded joints (a) LW, (b) CMT, (c) LCHW, (d) LPHW Microstructure Characteristic Fig.5 shows the center equiaxial dendrite in different joints. Grain size of LW joint is 3-10 μm, which is the smallest one of the four joints. CMT joint has the biggest grain size which is 20-40 μm. In spite of the same welding parameters are used in LCHW and LPHW, grain size of LCHW (5-15 μm) is smaller than that of LPHW (10-20 μm). The variation of the grain size is well corresponding to the heat input shown in table 2. The bigger the heat input, the coarser the grain size in weld center. The microstructure near fusion line is shown in Fig.6. Apparent columnar dendrite zones appear in LW and LPHW joints. The width of columnar dendrite zone of LW joint is 241 μm, which is narrower than that of LPHW joint (541 μm). However, the growing direction of columnar dendrites in CMT and LCHW joints are disturbed, which leads a transition from columnar dendrites to equiaxed dendrites. That is, there are no apparent columnar dendrite zones in CMT and LCHW joints. Hunt s research [6] shows the transition from columnar to equiaxed grains can be expected providing that: (i) there is a supply of nucleation sites from which new grains might develop, and (ii) low thermal gradient which favors the nucleation and growth of new grains is presented. Owning to the low heat input and high welding speed, LCHW has a larger thermal gradient than CMT. According to above theory, the LCHW joints should contain more columnar dendrites than CMT, but in fact the columnar dendrite content of LCHW joints is less than that of CMT. Obviously, the low heat input is not the primary reason for the formation of the transition from columnar to equiaxed grains in CMT and LCHW joints. It is presumed that the specific arc characteristic of CMT may cause the fragmentation of dendrites, and then increase the nucleation and refined grains during CMT and LCHW. Figure 5 Equiaxial dendrite of joints (a) LW, (b) CMT, (c) LCHW, (d) LPHW Welding Deformation All welded joints have angular deformation. Because angular deformation is difficult to be measured, transverse weld displacement which is relative to horizon line is used to evaluate the distortion of different welding. Fig.8 shows the test results. LW and LCHW joints have the smallest displacement of 0.13 mm. The displacement of LPHW is 0.19mm. CMT has the biggest displacement which is 0.22mm. Usually, the heterogeneously distributed solidification shrinkage and thermal contraction in the throughthickness direction of joints make the joints tend to be wider at the upper part which results in the angular deformation of welded joints [7]. As shown in Fig.4a, the LW joint is narrow both at the upper and the lower part because it is a deep keyhole welding. It indicates that both the upper and lower parts of LW joint are heated evenly, and then the joint has little angular deformation. During hybrid welding, the upper part of the joint is both heated by laser beam and arc, so solidification shrinkage and thermal contraction of the upper are larger than those of the lower part, resulting in a larger angular deformation. Accordingly, the deformation of LPHW joint is less than that of LCHW joint owing to the lower heat input of CMT. 432
Figure 6 Appearance of columnar dendrite zone, (a) LW, (b) CMT, (c) LCHW, (d) LPHW Figure 7 Transverse tensile strength of welded joints Figure 8 Deformation of joints, (a) LW, (b) CMT, (c) LCHW, (d) LPHW Conclusion Through comparisons of AA6061-T6 aluminum alloy joints made by laser, CMT, laser-cmt hybrid and laser-pmig hybrid welding, the research has demonstrated: 1. The tensile strength of laser-cmt hybrid welded joint is 219MPa, which is close to that of laser joint but higher than that of CMT and laser-pmig hybrid welded joints. 433
2. Although the grain size of laser-cmt hybrid welded joint is larger than that of LW due to the higher heat input, the columnar dendrites near to fusion line in laser-cmt hybrid weld decreases because the specific arc characteristic of CMT cause the fragmentation of column dendrites. 3. The angular deformation of laser-cmt hybrid welds is close to that of laser weld, but smaller than that of CMT and laser-pmig welds due to more even heating of the upper and lower parts within the welded joint. University of Science & Technology. His research interest is laser and laser-arc hybrid welding. Professor Xiaoyan Zeng is a professor of Wuhan National Laboratory for Optoelectronics, Huazhong University of Science & Technology. His research field includes laser materials processing, advanced laser manufacturing. He has published over 180 journal papers. References [1] Cao, X., Wallace, W., Immarigeon, J. P. & Poon, C. (2003) Research and Progress in Laser Welding of Wrought Aluminum Alloys. II. Metallurgical Microstructures, Defects, and Mechanical Properties, Materials and Manufacturing Processes 18, 23-49. [2] Kou, S. (2002) Welding metallurgy, Wiley, 95-110. [3] Ribic, B., Palmer, & T.A., Debroy, T. (2009) Problems and issues in laser-arc hybrid welding, International Material Review 54, 223-247. [4] Pickin, C.G., & Young, K. (2006) Evaluation of cold metal transfer (CMT) process for welding aluminium alloy, Science and Technology of Welding and Joining 11, 583-585. [5] Pinto, H., Pyzalla, A., Hackl, H., & Bruckner, J. (2006) A Comparative Study of Microstructure and Residual Stresses of CMT-, MIG- and Laser-Hybrid Welds. Mater Sci Forum; 524-525: 627-632. [6] Norman, A.F. (1999) Effect of welding parameters on the solidification microstructure of autogenous TIG welds in an Al-Cu-Mg-Mn alloy, Material Science Engineering A 259, 53 64. [7] Suder, W. (2011) Comparison of joining efficiency and residual stresses in laser and laser hybrid welding, Science and Technology of Welding and Joining 16, 244-249. Meet the authors Mr. Chen Zhang is a PhD student of Wuhan National Laboratory for Optoelectronics, Huazhong University of Science & Technology. His research interest is laser hybrid welding of aluminum alloys. Dr Ming Gao is an associate professor of Wuhan National Laboratory for Optoelectronics, Huazhong 434