Investigations on laser-tig hybrid welding of magnesium alloys

Transcription

1 Investigations on laser-tig hybrid welding of magnesium alloys Gang Song, Liming Liu, Mingsheng Chi and Jifeng Wang State Key Laboratory of Material Surface Modification by Laser, Ion, and Electronic beams, Depart. of Material Engineering, Dalian University of Technology, Dalian 11624, China. Ph Fax Abstract: This paper presents results of recent investigations on the weldability of several wrought (AZ31, AZ61) and cast magnesium-based alloys (AZ91) by laser-tig welding process. The investigations showed that magnesium alloys can be easily welded by laser-tig welding. The grain of the fusion zone was finer than that of in base metal. The width of the heat-affected zone welded by laser-tig welding process was obviously narrower than that of welded by TIG. Besides, with the Al content of magnesium alloys increasing, the width of the heat-affected zone (HAZ) was increased,as well as the content of β phase(mg 17 Al 12 ). The hardness in the fusion zone (FZ) and in HAZ of AZ61 and AZ91 has a large change to the base metal due to the existing of β phase, while no change relative for AZ31. It results from above discussing that laser-tig welding is an excellent welding process for magnesium alloys. Key words: Magnesium alloys, Laser-TIG hybrid welding, Weld Introduction Magnesium and its alloys, as a light structure materials, have attracted more attentions especially in automotive industrial because of its low density and high aspect density. Besides, the development of new alloy types, manufacturing techniques such as welding play an important role in exploiting the new fields of applications and excellent joining is an effective solution for simplifying product design and decreasing the cost. Presently the literatures about magnesium welding have increasing a lot in a day, mainly focusing on arc welding, laser beam welding, electron beam welding and friction-stir welding. Each of them has advantages and disadvantages. For instance, the arc welding process has a much lower energy density, but causes a bigger focal spot on the surface and is characterized by a slower processing speed. The laser welding process has a very narrow heat affected zone (HAZ), a large ratio of welding depth to seam width and very high welding speeds, but the gap bridging ability is very low due to its small focus diameter and much laser energy was reflected because of materials low absorption for laser. The above discussion leads us to put forward a new welding process for magnesium alloys the laser-tig hybrid welding process. By merging both processed, useful synergies can be achieved such as high quality advantages and production engineering benefits, as well as improving cost efficiency. Based on earlier research[1,2], the objective of the present paper is to report the results on the weldability of AZ-based magnesium alloys by the laser-tig hybrid welding, and to provide an in depth understanding of the laser-tig welding process. Experimental procedure Butted joint with a single-pass weldment without fillers were produced on 2.5- to 3.-mm-thick plates of wrought magnesium alloy AZ31B, AZ61 and die-cast alloy AZ91 using an LWS-5YAG pulsed laser with an AC-TIG. A CNC 3-axis workstation capable of proving linear movement of up to 8mm/s was used to achieve the desired range of welding speeds. The experimental setup was 转载

2 shown in Fig.1 the chemical compositions of the base metal were shown in Table 1. Each specimen was cleaned with acetone to remove grease and residue, and brushed with stainless steel wire to remove oxides before welding. Fig.1 Experimental set up of Hybrid laser-tig welding Table 1 Chemical Composition of the Magnesium alloy Alloy Type Chemical Composition [wt-%] AZ31 Wrought 3Al 1Zn.3Mn AZ61 Wrought 6.5Al 1Zn.3Mn AZ31 Cast 9Al 1Zn.3Mn Results and discussion Weld Appearance With the Al content increasing, the AZ-base magnesium alloys appeared to be more readily laser-tig hybrid process welded. Fig.2 compares the representative top-surface appearances for three Mg materials. The AZ31 welds usually exhibited a regular top surface, which include many ripples just like scale; While AZ91 became very easy to weld, with a smooth top surface. Fig.3 shows the variation of weld cross section in different heat input level. 4mm a) AZ31 b) AZ61 c) AZ91 Fig.2 Optical micrographs showing the fusion zone top surface appearance 2mm Fig.3 Cross section of AZ31 joints by laser-tig welded in different heat input level (a) I=9A, V=8mm/min; (b) I=A, V=mm/min; (c) I=1A, V=15mm/min (Others such as P=4W, Dla=1.5mm, Z=-1mm are the same) Investigation of microstructure For all laser-tig welded AZ-based magnesium alloys, no grain coarsening within the HAZ was found. But it found that the width and microstructure of HAZ were evidently different with the Al content increasing. The HAZ of AZ31 welds was so narrow that it was difficult to found distinctly,

3 while AZ61 and AZ91 welds were wider, as can be seen in Fig.4. This results the different thermoconductivity of materials, which was shown in Table 2. Besides, it also found that the HAZ of AZ61 and AZ91 comprised three parts two black zone and a white zone. The typical microstructure of AZ91 magnesium alloy welded by hybrid laser-tig process was shown at Fig.5. It found that the boundary of grain within the HAZ near the base metal became coarse as shown in Fig.5b. In Fig.5c, the boundary began to disappear, and many spot educts were separated out. Near the weld fusion, the educts increased and theβphase(mg 17 Al 12 ) emerging which was zonal structure, as can be seen in Fig.5d. In Fig.5e, in weld fusion, lots of βphase (Mg 17 Al 12 ) formed in the boundary of grain. In addition, it was found that with the Al content increasing, theβ phase(mg 17 Al 12 ) content was increased in this experiment..5mm a) AZ31 b) AZ61 c) AZ91 Fig.4 the microstructure of laser-tig welded magnesium alloys Table 2 The thermoconductivity of different symbol of magnesium alloys Symbol of magnesium alloys MB2 (AZ31) MB5 (AZ61) MB7 (AZ91) Thermoconductivity [W m -1-1 ] (a) Base metal (b) HAZ1 15µm (c) HAZ2 (d) HAZ3 (e) Weld fusion Fig.5 the microstructure of laser-tig welded AZ91 magnesium alloys Evaluation of Microhardness Measurements of the microhardness were conducted to detect submicroscopic changes in the structure, especially

4 in the HAZ. The results were used to evaluate the influence of the laser-tig welding process on the mechanical properties of the joints. The hardness in the fusion zone (FZ) and in the heat-affected zone (HAZ) of AZ61 and AZ91 has a large change to the base metal due to the existing of β phase, while no change relative for AZ31, which was shown at Fig.6,7, Weld HAZ AZ31 Summary and Conclusion 1. The AZ-base magnesium alloys appeared to be more readily laser-tig hybrid process welded with the Al content increasing. 2. With the Al content increasing, the width of HAZ became larger. The spot educts began to separated out within the HAZ of AZ61 and AZ91 joints, while lots of βphase(mg17al12) formed in the boundary of grain in weld fusion. 3. The hardness in the fusion zone (FZ) and in the heat-affected zone (HAZ) of AZ61 and AZ91 has a large change to the base metal due to the existing of β phase, while no change relative for AZ31. Acknowledgements Fig.6 Hardness profile across weld in AZ Weld HAZ AZ Fig.7 Hardness profile across weld in AZ Weld HAZ AZ Fig.8 Hardness profile across weld in AZ91 The project is supported by a grant from the national Hi-Tech Research and Development Program

5 of China (863Program) (No.2AA33116) and the Excellent Young Teachers Program of MOE, P. R. C. References [1] Song Gang, Liu Liming, Wang Jifeng. Transactions of the China Welding Institution[J]. Vol.25, No.3, pp.31-34, June 4. [2] Liu Liming, Song Gang, Wang Jifeng and Liang Guoli. Trans. Nonferrous Met. Soc. China, Vol.14, No.3, June 4.