Investigation of Laser Beam Welding Process of Az61 Magnesium Based Alloy 2006 Acta Metallurgica Sinica (English Letters)

7/28/2019 Investigation of Laser Beam Welding Process of Az61 Magnesium Based Alloy 2006 Acta Metallurgica Sinica (English Letters)

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288 .alloy^[^-^^. However, because arc welding has high heat input and m agnesium h as high thermal heat con-ductivity, a wide heat-affected zone (HAZ) and coarse grain structures would b e formed, which causes aweld with poor properties. It also results in large weld distortions and high residual stresses. However,laser beam welding (LBW) is an advanced method to produce sound welds o f magnesium alloys and th eabove disadvantage of arc welding could be Meanwhile, LB W possesses high weldingspeed and is an easily realized form of automatic w elding. As a result, LB W is a preferred method in themanufacture of the structural parts such as a utom otive bo dy parts, relative to conventional arc welding].

The aim of this paper is to discuss the welding properties of the AZ61 magnesium-aluminum-zincalloys with LBW. The effect of process parameters on the weld app earance and joint properties w as inves-tigated. The results showed that a weld with a good app earance and mechan ical properties could be pro-duced by selecting the process parameters properly.2. Experimental

The sheets ofAZ6l magnesium alloy (comp osition in wt%: A1 5.5-7.5, Zn 0.5- 1.5, Fe 0.01, Si 0.1,Ni 0.005, the balance is Mg) with 1.5mm thickness w ere studied in these experimen ts. The sheets werecut into 8Om mx245m m specimens. Prior to welding, the samp les were cleaned with aceton e to removethe oil film that existed on the material surface. Th e experimen ts were carried out in a COz aser weldingsystem shown in Fig.1, which includes a compact CO , laser generator with an output of upto 2.0kW lasercapacity, a special welding torch with extra gas protection, a special purpose clamping apparatus, andmechanical automatic traveling equipm ent.

During the welding of AZ61 magnesium alloys,argon gas was added from the welding torch (top) andclamping apparatus (bottom) to protect the m olten poolon the welding side and reverse side of the sheets.The butt weld between two specimens was formedwithout filler metal. The experiments were arrangedby perpendicular methods. A total of four process pa-rameters (laser power, welding speed, and two pro-tection gas flow rates, were added to the top and bot-tom sides) were varied in the w elding experiments. Thelaser focus point was fixed on the surface of the speci-mens. The experimental conditions are summarized inTable 1. A total of eight groups of L9-3-4 orthogonal Fig. 1 Experimental system of C0 2 aser welding.

Table 1Welding experimental conditions of a 6 1 magnesium alloysLaser power, W Welding speed, m d m i n T op ga s flow rate, Um in Bottom gas flow rate, Um in

400 400 5 5600800lo001200

60080010001200

15251020

15251020

1500 1400 25 25


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tables were designed to investigate the effect of the four parameters. The laser beam was produced underlaser power in the range of 0.4-1.5kW with a C.W. m ode. The welding torch was traveling with a speed(welding speed) of 400 to 1 5 0 0 d m i n . Argon was utilized a s the shielding gas at the welding (top) sideand reverse (bottom) side with a flow rate in the range o f 5-25L/min.After welding, the weld beads under the different conditions were measured u sing a JT12A projec-tometer to de termine the w idths of molten metal at the top and bottom sides. On the basis of the results ofweld widths and weld appearances, relatively optimized parameter ranges could be obtained. Three typi-cal welds were chosen, with good appearance, to analyze the properties of welds. An HRS-150 digitalmicrohardness tester was operated with a load of 98N to measure the hardness of the weld beads. T ensilestrength was determined using a CM T-4304 tensile tester with maximum load of 30kg. The microstruc-ture of the weld zone was analyzed using an ultra high magnification zoom microscope. X-ray difiac-tion (XRD) as used to observe the phase structure of the joints.3. ResultsandDiscussion

2 22 12 0 -g 1. 9

3 1 7 -?0 1 8 -

1 6 -

Fig.2 gives the suitable laser power and welding speed range according to the results of weld w idths

--

-

and the appearance of welds. In the shadow zone,the laser power and welding speed were a goodmatch to obtain an ideal weld. When the laser pow-er was 400W, no good weld appearance could beformed for a welding speed between 400 and1400mm /min. The weld me tal could not be meltedcompletely at this low power level for sheets of1S m m thickness, so the weld bead showed incom-plete penetration and an uncontinuous appearance.The same case was found when laser power was600-800W and the w elding speed was larger than1200mm /min. On the other hand, if welding speedwas too low for a relatively larger laser power, themetal would be burned through and the weld ap-pearance would be very bad.

Fig.3 displays the effect of laser power on theweld width. It is observed that the top and bottomweld width becom es obviously larger when higherlaser power is used. This indicates that the size ofthe weld pool is increased with increasing laserpower. The weld widths under different weldingspeeds are depicted in Fig.4. It is shown that thetop and bottom weld widths decrease when weld-ing speeding increases. This re sult is consistent withthe effect of laser power. When higher laser poweror lower welding speed is used, the heat input perunit weld length will increase and it results in

Iuitab le parameter range-6oo t40 0400 600 800 1000 1200 1400

Laser power, WFig.2 Suitable laser power and welding speed range.

2 3 1

1.51 ' I80 0 1000 1200 1400 1600Laser power, W

Fig.3 The effect of laser power on the weldwidth, with the experimental conditions(welding speed: 1400mm/min, top argongas flow: 25Wmin, and bottom argon gasflow: 20L/min).


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. 291 .Table 2 Weld width of three typical laser welding conditions for AZ61 magnesium alloys

Sample Laser power Welding speed Top gas flow Bottom gas Top weld Bottom weldNo. W mm/min rate, Umin flo w rate, Llmin width, mm width, mmA 800 1000 25 20 2.133 1.723B 1200 1000 25 20 2.292 1.955C 1200 1400 25 20 2.06 1.492

Fig.7 Typical laser weld appearance of AZ61 mag-nesium alloys: (a) sample A; (b) sample B; and(c) sample C .

Fig.8 Micrographs of the transverse section of threetypical samples: (a) sample A; (b) sample B; and(c) sample C.

cross section of sam ple B has the largest top and bottom weld width, and a concave top w eld surface isproduced. For sample B, the laser power is relatively higher than for sample A, and the w elding speed islower than for sample C, somore base metal is melted per unit welding length and a wider weld is formedfor sample B. The weld of sample C has the lowest top and bottom w eld width, and the sm allest moltensection area, that's because the relatively large welding speed is used for sample C.

Fig.9 depicts the microhardness profiles of the w eld zone, heat affected zone ( H A Z ) and base metalfor the three typical samples. In three cases, the hardness profile show s a gradual increase in hardnessfiom the base metal value, through the heat-affected zone, to the peak in hardness that occurs just insidethe fbsion boundary. The hardness of weld zone is higher than the base m etal.

The tensile strength of the three typical samples was measured using a tensile tester. The resultsshowed that all three samples were broken in the base metal zone with a maximum force of around3.18kN, and the calculated tensile strength was about 265MPa (which is almost the tensile strength ofAZ61 magnesium alloy). This confirmed that the tensile strength of weld zone was higher than the basemetal for the three samples. Therefore, the welds had good mechanical properties when A261 magne-sium alloys were w elded by LB W method.


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292

70-cfE. 60-YIE 5 0 -p 4 0 -Ec

The microstructures of weld zone and the zone near the fbsion line for sample B are shown in Figs.1Oa and b respectively. In Fig.1Ob, the base metal is on the right and the weld metal is on the left. A typi-cal microstructure of coarse equiaxed grain is observed in the base metal zone for wrought magnesiumalloys. However, in the weld zone an evidently fine-grained microstructure is formed. It is believed thatthe rapid cooling and resolidification speed is responsible for the fine grain of the weld when a laserbeam method is utilized. In addition, it is also found that the heat-affected zone is not evident and no ob-vious coarse-grained region is formed near the fusion boundary. The laser beam has very concentratedenergy and A261 magnesium alloys have high thermal heat conductivity,so the heat-affected zone is verysmall. The existence of alloying elements such as aluminum also helps to refine the grain structure inthis region. The XRD spectrum obtained from the weld zone of sample B is shown in Fig. 1 1. The phasecomposition in the weld zone mainly includes a-Mg, AlMg ,AIi2Mgi,, cx-Mg+All2Mg1,), nd (a-Mg +

Sample A70

I Sample B

Weld1 HAZlbase metal

Sample C/

E - m y .4030 0 0 0 3 0 6 09 1 2 15 1 8 2 1

Distance from the weld center rnm

Fig.9 Microhardness profiles of the weld zon e, heat affected zone (W),nd base metal for thethree typical samples.

Fig.10 Microstucture of weld of AZ6 1 magnesium alloys: (a) weld zone and (b) zone near fusion boundary.


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. 293I

30 40 50 60 70 80 90 100 11028, deg.

Fig.11 XR D spectrum for weld of AZ61 magnesium alloys.

AlMg) phases. D uring the cooling process after laser welding, some intermediate phases like AlMg andA1,,Mg17separate and then combine with a primary a-Mg phase and many eutectic phases are formed.Because of the very quick cooling speed for laser welding, the grains of a-M g phase and eutectic phasesare obviously fined. So a joint mainly composed of small-grained a-M g phase and eu tectic phases is ob-tained when AZ61 magnesium alloy is welded by C02 aser welding. The tensile strength is in reverseproportion to the square root of the size of grains according to Hall-Petch. Therefore, the fine-grainedstructure is believed to play a key role in the improvement of the tensile strength of weld and the in-crease in hardness observed in the weld zone and hs io n boundary.4. Conclusions

(1) Welding of AZ61 magnesium alloys were successhlly carried out by the laser beam weldingmethod using the C 02 aser welding system. When the process parameters were properly chosen, an ide-al weld w ould be produced using laser welding.

(2) Laser power and welding speed are the two main process param eters to determ ine the weld ap-pearance and weld width. The protection gas flow rates from the top and bottom sides are also im portantto obtain a good weld because the gas flow mainly influences the protecting effect of the melted metal.

(3 ) Test results of the three typical welds indicate that the microhardness and tensile strength of theweld are higher than that of the base m etal. This imp lies that the weld produced by the laser weldingmethod has excellent mechanical properties.

(4)The microstructure results show a fine-grained weld zone and no obvious coarse-grained regionis found in the heat-affected zone. The weld zone mainly consists of small-grained a-M g phase and eu-tectic phases. T he refined grain structure is expected to contribute to the excellent mechanical propertiesof A Z6 1weld beads.

REFERENCES1 M. Avedesian and H. Baker, Magnesium and Magnesium A Lloys (Ohio: ASM International Materials Park, 1999)

p.4.


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I 294 .2 Y.Y. Li, W.W. Z hang, Y. Liu, W.P. Chen an d D.H. Ni, Special Casting & Nonferrouy Alloys 1 (2004) 14

(in Chinese).3 Y.M. Zhang and S.B. Zhang, Welding Research 6 (1999) 202 (suppl.).4 M. Marya, G.R. Edwards and S. Liu, Welding Journal 83(7) (2004) 203.5 W.B. Lee, Y.M. Yeon and S.B. Jung, Materials Science and Technology 19(6) (2003) 785.6 H. Zhang, S.B. Lin, L. Wu, J.C. Feng and H.P. Guo, Chin..I.onferrous M e t . 13(6) (2003) 1510 (in Chinese).7 S.H. Wu, J.C. Huang and Y.N. Wang, Metallurgical and Materials Transactions 35(8) (2004) 2455.8 A . Weisheit, R. Galum and B.L. Mobdike, Welding J o u d 77(4) (1 998) 149.9 J.H. Zhu, L. Li and Z. Liu, Applied Surjime Science 247 (2005) 300.10 L.M. Liu, J.F. Wang and G. Song, Mate r i a l s Science and EngineeringA381 (2004) 129.11 J.F. Wang, L.M. Liu and G. Song, Transactions of 7'he China Wel ding Institution 25(3 ) (2004) 15 (in Chinese).12 M. Dhahri, J.E. Masse and J.F. Mathieu, Adzlanced Engineering Materials 3(7) (2001)504.

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Investigation of Laser Beam Welding Process of Az61 Magnesium Based Alloy 2006 Acta Metallurgica Sinica (English Letters)