LASER WELDING OF AM60 MAGNESIUM ALLOY

Transcription

1 Magnesium Technology 2004 Edited by Alan A. Luo TMS (The Minerals, Metals & Materials Society), 2004 LASER WELDING OF AM60 MAGNESIUM ALLOY Ashish K. Dasgupta 1 and Jyoti Mazumder 1 1 CLAIM, University of Michigan; 2041 G. G. Brown, 2350 Hayward St., Ann Arbor, MI USA Keywords: Laser, Welding, Magnesium, Alloys, etc. Abstract Due to limited fossil fuel reserves and stringent emission regulations, automotive companies today are keen on finding out ways to reduce fuel consumption and emission of vehicles. As a result, there has been an increased focus on lighter metals like aluminum and magnesium for auto fabrication. While much research has been done on the use of aluminum, there are lots of unanswered questions as far as use of magnesium is concerned. In this paper we have discussed the feasibility of laser welding of die-cast magnesium alloy AM60 for automotive applications. Porosity, mechanical strength and material characteristics of welds are presented. Techniques for reducing porosity are also discussed. Introduction Magnesium and aluminum alloys are finding increased use in the automotive industry because of their excellent specific strength, good elongation and toughness. The specific strength of die-cast magnesium alloy AM60B is 122 MPa which far exceeds the 45 MPa of low-carbon steel. The use of these light weight alloys can help automakers improve fuel economy and reduce greenhouse gas emissions. To make the most weight and cost savings in the use of automotive alloys, tailor-welded blanks are used in the auto body. Laser beam welding is a preferred method in the manufacture of tailor-welded blanks due to its high speed, low heat input and low weld distortion. Increasing the use of magnesium alloys in tailor-welded blanks will require improved technology for fabricating structurally sound, defect free and reproducible welds. AM60 magnesium alloy is manufactured by high pressure diecasting (HPDC) process. Generally HPDC components cannot undertake heat treatment due to their high gas porosity content. Their solidification microstructure is of vital importance to their mechanical properties. Cai et al. [1] studied the solidification microstructure of AM60 magnesium die-castings and found it to be of a divorced eutectic [Mg( ) + Mg 17 Al 12 ( )] type. This structure formation was due to the high solidification rate involved in magnesium die-casting process. They noticed that the alloy has a finer microstructure near the surface as compared to the interior. They also reported that porosity was prominent in central portion of castings. Weisheit et al. [2] reported that laser welding of magnesium alloys is not restricted by major problems according to metallurgy. In their study, they tried joining 2.5, 3, 5 and 8 mm thick cast and wrought magnesium alloys with and without filler metal, similar and dissimilar materials using a 2.5 kw CO 2 laser. They found that most alloys can be easily welded achieving small fusion zones and heat affected zones. Best surface quality was obtained with biggest penetration depths with Helium as the shielding gas. The AM60 alloy in particular showed increased fusion zone hardness and an unchanged HAZ hardness. The porosity results for this alloy were less promising and they did not explore this issue further. Zhao et al. [3] studied pore formation in AM60B magnesium alloy. They found that smaller pores in the base material (formed during the die-casting process) undergo thermal expansion and coalesce to form bigger pores during welding. A mathematical model is also presented to estimate the size of such bigger pores and the resulting area-percent porosity. They reported that stability of keyhole was not a major factor in fusion zone pore formation during laser welding. In addition, they found that the amount of porosity in the fusion zone decreased with a decrease in the heat input i.e. with a lower laser power or a higher welding speed. As a remedy, they suggested controlled remelting of the fusion zone for reducing porosity. Based on this background, we started investigating the feasibility of laser welding AM60 magnesium alloy samples provided by DiamlerChrysler Corporation. Experimental Setup A RF excited Trumpf TLF kw CO 2 laser (TEM 01 mode) coupled with an Allen Bradley NC Controller was used for performing the welds. These systems are shown in figures 1a and 1b. A two stage concentric nozzle as shown in figures 2a and 2b, was designed for delivering laser beam and shielding gas. Helium was used as the shielding gas. The material used was AM60 magnesium alloy in the form of 1 inch x 5 inch x 2 mm coupons, provided by DiamlerChrysler Corporation, MI. A fixture, shown in figure 3 was used to hold the magnesium alloy samples in lap configuration during welding. The coupons were cleaned with acetone and dried before welding. A novel II-VI/Spiricon laser beam analyzer was also used in some experiments to monitor the beam characteristics during welding. The objective was to find out if there was any change in beam properties due to beam-material interaction or beam reflection. This system can be seen in figure 4. Number of welds were then performed with varying parameters. The parameters were then optimized based on observed results. The following NC code was used for welding experiments 43

2 G99# M99# G90# G01 Xxstart Yystart# (Start Point) G92 X0 Y0 Z0# G72 X1.0 Y1.0# (Set Scale) M8# (Open Shutter) G01 Fipm# (Weld Speed) M27# (Beam On) Xweldlength# (Weld length along X direction) M28# (Beam Off) X0 Y0 Z0# M9# (Close Shutter) M2# Figure 2b: Two stage nozzle. Figure 1a: Trumpf TLF 5000 RF Excited 6 kw CO2 Laser. Figure 3: Welding fixture with clamped AM60 Mg alloy samples. Figure 1b: Allen Bradley NC Controller. Figure 4: Spiricon/II-VI industrial laser beam analyzer. Figure 2a: Two stage nozzle schematic. 44

3 Effect of Beam Focus Position Results and Discussion All welds showed porosity in varying amounts. This was due to the dynamics of rapid vaporization of magnesium and coalescence of trapped gases in the base material. The pores were mostly observed to be circular in shape. Since the coupons were 2 mm thick, a study was done to identify the optimal focal position for welding. It was believed that a particular position slightly below the top coupon surface was optimal for welding. This position was experimentally observed to be at 1/3 rd thickness of the top coupon. When the focal point went above or below this particular position a bulgy top bead was observed. Porosity was also observed to be lesser and finer at this position. Typical results can be seen in figures 5a, b and c. Effect of Filler Material At this focal position, a combination of 1750 W laser power (before 23% transmission losses) at 65 khz keying frequency, 75 ipm traverse speed and 45 SCFH Helium flow rate produced good Figure 5c: Top section of a weld with focus ½ thickness below top surface. results as compared to other combinations. A study was performed using these optimal parameters to identify the effect of addition of a filler material. Aluminum was chosen as a filler material owing to its good alloyability with magnesium inch thick aluminum foils were sandwiched between the coupons arranged in lap configuration. Experiments were then performed with either no foil, one foil or three foils between the coupons. The acoustic emission from the weld was observed to be significantly less in case of aluminum foils. However, the weld micrographs showed that porosity increased with addition of aluminum. Typical results can be seen in figures 6a, b and c. Further study of the filler weld was done to identify the distribution of aluminum in weld section. A Philips Scanning Electron Microscope and X-ray elemental mapping device was used for the purpose. The results are shown in figures 7a and b. Figure 5a: Top section of a weld with focus at top surface. Though aluminum composition increased in the fusion zone, no typical distribution trend was observed. Figure 5b: Top section of a weld with focus 1/3 rd thickness below top surface. Figure 6a: Top section of a weld made with no Al foil (filler). 45

4 Figure 6b: Top section of a weld with one Al foil sandwiched between the coupons. Figure 7b: XEDS mapping of weld section with Al distribution highlighted. Weld made with three sandwiched Al foils. The thick line indicates the position of the Al foil within the weld. Mechanical Test Mechanical test samples were made as per ASTM E8 standards, by lap welding 5 inch x 1 inch coupons with determined optimal parameters and without any filler material. The tests were carried out at a heat rate to failure of 0.05 in/min. For comparison, samples were made with beam focus at top surface and at 1/3 rd thickness below the top surface. The shield gas (helium) flow rate was also varied to study its effect on the strength of the weld. Typical results are shown in figures 8a, b, c and d. The highest mechanical strength was observed in welds made without any filler material, and with focus at 1/3 rd thickness below the top coupon surface. The strength was of the order of 650 lbs. Figure 6c: Top section of a weld with three Al foils sandwiched between the coupons. Helium flow rate also affected the weld strength. Maximum strength was obtained around 45 SCFH flow rate. Higher flow rates resulted in poorer strength as low as around 290 lbs. Lower flow rates also reduced the weld strength but not that much. Hardness Test Hardness test was performed on weld samples without any filler material. A Buehler High Quality Hardness Tester Model: was used for the test. A load of 0.5 kgf was used for 10 seconds. Hardness was tested in the base material, the heat affected zone (HAZ) and the fusion zone. A typical result can be seen in figure 9. Not much of a change in hardness was observed after laser welding. It meant that even after welding in open atmosphere using helium as shielding gas, oxygen did not react harmfully to form harder oxides in the weld zone. Therefore welding in open atmosphere with a good helium shield was feasible. Laser Beam Monitoring Figure 7a: XEDS mapping of weld section with Mg distribution highlighted. Weld made with three sandwiched Al foils. Parallel experiments were carried out to study the variations in beam diameter during laser welding process. A Spiricon / II-VI 46

5 Figure 8a: Beam focus 1/3 rd thickness below the top surface, 45 SCFH Helium flow rate. Maximum load = 687 lbs. Figure 8c: 55 SCFH Helium flow rate, beam focus at top surface. Maximum load = 289 lbs. Figure 8b: Beam focus at the top surface, 45 SCFH Helium flow rate. Maximum load = 449 lbs. Figure 8d: 35 SCFH Helium flow rate, beam focus at top surface. Maximum load = 523 lbs. 47

6 Vickers Hardness Fusion Zone HAZ Base Material Figure 9: Hardness in various zones. industrial beam analyzer was placed in the beam path at nearfield. Data was recorded for both raw and in-process beams and compared for any variation. The effect of beam reflection from the weld was also studied by making horizontal and inclined (5 degree with horizontal) welds and comparing their results. It was observed that the beam diameter fluctuated periodically during the weld. This was attributed to the variations in the laser gas flow caused by root blowers. The beam size also gradually decreased over the length of the weld. It was also worth noting that the average beam diameter in horizontal weld was higher than that of the inclined weld. It meant that beam reflection from the weld pool definitely affected the beam properties. Figure 10 shows the discussed observations. Autogenous welds show the highest mechanical strength with maximum load of the order of 650 lbs. Hardness of open atmosphere welds did not increase significantly. Laser beam properties can get affected by reflections from the melt pool. Acknowledgements The authors wish to thank Dr. Mariana Forrest, Sr. Manager Adv. Mfg. Technology Division, Liberty and Technical Affairs, DiamlerChrysler Corporation for providing the necessary funds and material for this work. Thanks are also due to the members of Center for Laser Aided Intelligent Manufacturing, University of Michigan for their cooperation in carrying out the experiments. References 1. Y. Cai, W. Zhou, M. J. Tan, B. H. Hu, and I. Pinwill, Solidification Microstructures of AM60 Magnesium Die Castings, Processing and Fabrication of Advanced Materials VIII, Edited by K. A. Khor, T.S. Srivatsan, M. Wong, W. Zhou, and F. Boey, World Scientific Publishing Company Pte. Ltd., pp A. Weisheit, R. Galun and B. L. Mordike, CO 2 Laser Beam Welding of Magnesium-Based Alloys, Welding Journal, April 1998, pp. 149s-154s. 3. H. Zhao and T. DebRoy, Pore Formation during Laser Beam Welding of Die-Cast Magnesium Alloy AM60B Mechanism and Remedy, Welding Journal, August 2001, pp 204s-210s. Beam Diameter (mm) Horizontal Weld kw 73 ipm 6.6 psi Helium Mean Beam Diameter = mm 4. A. Dasgupta, J. Mazumder and M. Bembenek, Alloying based Laser Welding of Galvanized Steel, Proceedings of ICALEO, Laser Institute of America, Dearborn MI, October L. Green, H. Qi, A. Dasgupta, P. Li and J. Mazumder, New High Power In-line Industrial CO 2 Laser Beam Instrumentation for Real-time Monitoring, Proceedings of ICALEO, Laser Institute of America, Jacksonville FL, October Degree Inclined Weld kw 73 ipm 6.6 psi Helium Mean Beam Diameter = 25.2 mm Time (sec) Figure 10: Beam diameter variations over time in horizontal and inclined weld settings Conclusions Laser welding of AM60 magnesium alloy in open atmosphere with optimal helium shield is feasible. For thicker material in lap configuration, laser beam should be focused at a specific location below the top surface. Any other focus position results in incomplete penetration and bulgy top bead. Aluminum was tested as a filler material, however it introduced more porosity in the welds. Other filler materials need to be tested. 48