Texture and mechanical properties of metal inert gaswelded 6082-T651 aluminum alloy joints
Qi Guangbin1, Dong Honggang1, Yang Jiang1, Guo Baizheng1, Hao Xiaohu1, Xu Chenling2
祁广斌，董红刚，杨江，郭柏征，郝晓虎，许晨玲
1. School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China;2. Dalian Huicheng Aluminum Co., Ltd, Dalian 116105, China
Received 30 December 2020; accepted 11 February 2021
Abstract Metal inert gas (MIG) welding was conducted with 12 mm thick 6082-T651 aluminum alloy plate to investigate the microstruc-ture and mechanical properties of welded joint. The microstructure and element distribution of weld seam were characterized by electronbackscattered diffraction (EBSD) and electron probe microanalysis (EPMA). The weld seam has typical cube texture ({001}<100>) charac-teristics. The closer to the center of weld seam, the weaker the texture feature, the higher the proportion of high-angle grain boundaries. Theaverage tensile strength of joint was 232 MPa which is up to 72% of 6082 aluminum alloy base metal, and the bending angle for the rootbend test sample reached 90° without cracks. The lack of strengthening phase and the existence of welding pores and inclusions in the weldseam caused the degradation of mechanical properties of resultant joint. The microhardness increased from the weld center to the base metal,but the overaging zone caused by welding thermal cycle was softening part of the joint, which had lower hardness than the weld seam.
Key words 6082-T651aluminum alloy, butt joint, microstructure, mechanical property
0 Introduction
Aluminum alloy is a typical lightweight structural ma-terial, widely used in rail transit, aerospace and bridge con-struction for its low density, high specific strength, excel-lent corrosion resistance and processing moldability[1 – 3].6082 aluminum alloy is a heat-treatable reinforced alloy andbelongs to Al-Mg-Si series aluminum alloy with Mg and Sias main alloying elements. Since it contains a large amountof Mg2Si as strengthening phase, 6082 aluminum alloy hasmoderate strength[4 – 5]. In recent years, with the develop-ment of the high-speed rail industry, the research on 6082aluminum alloy has become a hot topic[6 – 8]. The weldingprocess is inevitable for the manufacture of aluminum alloystructural parts. However, there are some challenges in thewelding of aluminum alloys: (1) welding pores, (2) thermal
crack caused by excessive internal stress and (3) the burn-ing loss and evaporation of alloy elements, which results inthe significant reduction in mechanical properties and corro-sion resistance of the welded joint[9 – 10].
At present, a large number of studies on welding of6082 aluminum alloy mainly focused on friction stir weld-ing (FSW)[11 – 14]. Ehab et al.[11] investigated the microstruc-ture and mechanical properties of FSWed 6082 aluminumalloy in as-welded and post-weld heat treated conditions.Post-weld heat treatment can partially restore the softeningof the stir zone and thermal-mechanically affected zone, andincrease the joint strength and hardness. Veerendra et al.[12]
conducted an acoustic analysis of the FSW process of 6082aluminum alloy. The ultimate tensile strength of the FSWedplate can be predicted by giving the regression equationbetween the peak strength and the ultimate tensile strength.In addition, a lot of literatures[15 – 18] have explored the influ-
Foundation item: Project was supported by the National Natural Science Foundation of China (51674060) and the Collaborative InnovationCenter of Major Machine Manufacturing in Liaoning.Corresponding author: Dong Honggang (1975 − ), Ph.D, Professor. Mainly engaged in dissimilar materials welding. E-mail:[email protected]: 10.12073/j.cw.20201230001
Texture and mechanical properties of metal inert gas welded 6082-T651 aluminum alloy joints 1
ence of FSW tools on the weldability of aluminum alloy.Krishna et al.[15] reported the influence of ridges shoulderwith polygonal pins on FSW characteristics and materialflow of 6082 aluminum alloy. The polygonal pins de-creased the initial force required during the plunging phasedue to the shearing action of the material acted similar to thedrilling process compared to taper cylindrical pin. However,FSW process exhibits some limitations: the low weldingspeed, high equipment cost and the shape/size restrictions ofworkpieces, which restrict the development and applicationof FSW in the extensive manufactured aluminum alloyproduct.
Fusion welding is currently one of the most importantand widely used welding methods for aluminum alloys dueto the advantages of high welding efficiency, low equip-ment cost and low requirements on the production environ-ment. Prasad et al.[19] researched the influence of differentwelding grooves on the hardness and tensile properties oftungsten inert gas welded 6082 aluminum alloy. The mech-anical property with single V and double V was better thanwith single Y, the maximum strength of joint with double Vwas greater than that with single V. Rakesh et al.[20 – 21] con-ducted in-depth research on 6082 aluminum alloy and car-ried out butt and lap experiments by using alternating cur-rent and direct current pulsed gas metal arc welding, re-spectively. The results revealed that the three methods canobtain welded joint with preferable appearance, good per-formance and no welding defects. Ruan et al.[22] reported themechanical properties and microstructures of 6082-T6 jointsby twin-wire metal inert-gas welding with SiO2 flux. Theyfound that the penetration depth of the twin-wire metal in-
ert gas (MIG) welded joints containing SiO2 flux was about26% deeper than the welded joints without SiO2 flux, whichwas caused by the arc shrinkage and higher arc temperature.Chen et al.[23 – 24] investigated the mechanical strength oflaser arc hybrid welded 6082 aluminum alloy and discussedthe relationship between pool characteristic and weld poros-ity in this condition. Compared with pure arc welding, laserarc hybrid welding could offer low heat input, low level ofoveraging effect, small grain size and high joint strength.The welding porosity can be reduced to below 0.5% by op-timizing the parameters.
Most researches on fusion welding of 6082 aluminumalloy focus on thin plates, and the studies about the weldingof thick plates above 10 mm are not enough[25]. This work isaimed to investigate MIG multi-pass welding process of12 mm thick 6082 aluminum alloy plate with Al-Mg fillerwire. The microstructure, element distribution and micro-texture characteristics are observed and analyzed. Themechanical properties, such as tensile strength, bendingstrength and microhardness of the joint at room temperat-ure are tested.
1 Materials and methods
6082-T651 aluminum alloy plate with the size of200 mm ×100 mm ×12 mm was used as base metal (BM) inthis study, and the ER5087 filler wire with a diameter of1.2 mm was employed for the welding experiment. Thechemical composition of 6082-T651 aluminum alloy andfiller wire is given in Table 1, and the mechanical proper-ties of 6082-T651 aluminum alloy are listed in Table 2.
Table 1 Chemical composition of the base metal and filler wire (wt.%)
Material Si Mg Mn Fe Cr Cu Zn Ti Zr Al
6082-T651 0.984 0.911 0.726 0.286 0.208 0.040 0.016 6 0.021 0.000 2 Balance
ER5087 0.040 4.760 0.750 0.140 0.007 0.010 0.010 0.080 0.110 Balance
Table 2 Mechanical properties of the base metal
Material Tensile strength/MPa Yield strength/MPa Elongation (%) Hardness (HV)
6082-T651 324 290 16.0 104.6
The welding experiment was performed under MIG
mode by using a CMT welder (Fronius CMT Advanced 4 000).Fig. 1 shows the geometrical shape and dimensions of thebutt joint with single-V groove of 70°. The sequence ofwelding pass and the dimensional details of the groove areshown in Fig. 1. Two kinds of welded joints were preparedwith three-layer four-pass and four-layer five-pass weldingprocess, and the corresponding welded joints were marked
as sample 1 and sample 2, respectively. In order to ensuregood weld appearance, a copper plate was placed under thebase metal. The welding direction was perpendicular to therolling direction of the base metal. Before welding, the ox-ide films of the faying area were removed with the 800 gritSiC sandpaper, and then the alcohol was employed to elim-inate the surface impurities and contamination. The shield-ing gas is high-purity argon with a purity of 99.99%. The
2 CHINA WELDING Vol. 30 No. 1 March 2021
detailed welding parameters for each pass are listed inTable 3.
(a)
(b)
70°
70°
s3
s2
s1
s4
s3
s2
s1
s4 s5
2 3
2 3 Unit: mm
Fig. 1 Weld pass schematic (a) Sample 1 (b) Sample 2
In order to examine the microstructure features of wel-ded joints, the sample was cut perpendicular to the weldseam. The sample was ground with up to 2 000 grit SiCsandpaper and then polished using 1.5 μm diamond polish-ing paste. Keller Reagent (2 mL HF + 1 mL HCl + 1 mLHNO3 + 76 mL H2O) was finally used to etch the samplefor 30 s. The microstructure of the welded joint was ob-served by optical microscopy (OM). In addition, electronbackscattered diffraction (EBSD) was employed to analyze
the microstructure and grain misorientation distribution, andthe EBSD data was processed by Chanel 5 software. Ele-ment distribution of welded joint was characterized withelectron probe microanalysis (EPMA).
The tensile test and bending test of welded joints werecarried out by a DSN-100 universal testing machine at roomtemperature. The cross speed of tensile test was 4 mm/minand the tensile strength of welded joint was averaged withthree identical tensile samples. The specific dimensions ofthe tensile and bending samples are shown in Fig. 2. Micro-hardness of the welded joint was measured by usingHUAYIN HV-1000B microhardness tester with an intervalof 1 mm under 100 g load and dwell time of 15 s. Mean-while, the fracture surfaces of tensile samples were ob-served by scanning electron microscope (SEM).
(a)
(b)
Weld seam
100
25
37
20
R25
200
200
Fig. 2 The dimensions of Samples (a) Tensile sample (b) Bending specimen (mm)
2 Results and discussion
2.1 Macrostructure and microstructureThe appearance of two welded joints is shown in Fig. 3.
From the surface morphologies of two welded joints, nowelding defects are found on the front and back of the weldseam, such as cracks and undercut. Si can raise the fluidityof the molten pool and hot crack resistance of the resultant
Table 3 Welding parameters
NumberWeldlayer
Voltage/V Current/A
Wire feedspeed/
(m·min–1)
Weldingspeed/
(mm·min–1)
Gas flow/(L·min–1)
1
1 20 215–220 11 500 25
2 22 275–280 13 500 25
3 22 275–280 13 500 25
2
1 20 215–220 11 500 25
2 22 245–250 12 500 25
3 22 245–250 12 500 25
4 22 245–250 12 500 25
Texture and mechanical properties of metal inert gas welded 6082-T651 aluminum alloy joints 3
weld seam, and Mg can improve the specific strength ofwelded joint[26]. In addition, it can be seen from the crosssection of the weld seam that there are a small number ofsubcutaneous pores on the surface of the weld metal, whichis caused by the fast solidification rate of the molten pool.However, these subcutaneous pores are mainly concen-trated in the weld reinforcement, which has little effect onthe mechanical properties by removing the weld reinforce-ment.
Fig. 4 shows the microstructure in BM and weld seamof 6082 aluminum alloy butt joints. It can be seen that themicrostructure of BM is mainly the fibrous structure withobvious rolling streamlines and the precipitation phase isdispersed in the matrix. Fig. 4b shows the microstructurenear the fusion zone (FZ). The FZ is a transition zonebetween weld seam and heat-affected zone (HAZ), and itsmicrostructure and chemical composition are more complic-ated. On the side of the fusion line near weld seam, thecoarse columnar grains grow in the direction perpendicularto the fusion line. The microstructure in HAZ undergoes asignificant change during welding, which is caused by thedissolution and precipitation of the second phase. The pre-cipitation process of the second phase in 6082 aluminum al-loy includes GP zone, β'' (Mg5Si6), β' (Mg9Si5) and β(Mg2Si)
[27]. It can be seen from Fig. 4c that the microstruc-ture of the weld center is mainly composed of equiaxeddendrite.
The microstructure morphology of the weld metal is re-lated to the constitutional supercooling. As aluminum alloyhas high thermal conductivity, the rapid cooling of moltenpool results in a large temperature gradient near the fusionline and a low degree of the constitutional supercooling,which promotes the growth of weld metal with cell crystals.As the cell crystal grows, the concentration of solutes (Mg,
Si, Mn, etc.) in the liquid phase at the front of crystalliza-tion increases, and the degree of constitutional supercool-ing increases, leading to the transformation of the weld met-al crystal form from cell crystal to cell dendrite[28]. In thecenter of the weld, the solute concentration in the liquidphase increased significantly, the temperature gradient de-creased significantly, and the constitutional supercoolingcontinued to increase, resulting in the formation of equiaxeddendrites in weld seam.
Fig. 4d shows the microstructure of the weld passesjunction of the welded joint. It can be seen that part of thegrain boundary of the fore-pass at the junction remelts be-cause of the heating effect of the rear-pass. The rear-pass isepitaxially grown on the basis of the melted grain of thefore-pass. The direction of growth is opposite to the direc-tion of heat flow, and the grains are in the form of colum-nar crystals.
In order to further explore the microstructure character-istics of weld seam, the texture and grain boundary misori-entation distribution of weld seam are observed by EBSD.The inverse pole figure (IPF) of Fig. 4d is shown in Fig. 5.It can be seen that the IPF results in this area are consistentwith the metallographic results. There is a clear interfacetransition zone between the weld passes. The edge part ofthe fore-pass away from the BM has equiaxed crystal struc-ture after undergoing second thermal cycle. The rear-passgrows on this basis, and the direction of grain growth isconsistent with the direction of temperature gradient, whichforms a columnar crystal structure. Meanwhile, the temper-ature gradient at the front of the solid-liquid interface gradu-ally becomes smaller, and the crystal form graduallychanges into equiaxed crystal[29]. It can be concluded that inmulti-pass welding, each weld seam forms a small caststructure, which together affects the performance of the en-
20 mm
20 mm
20 mm
20 mm 6 mm
6 mm
Front
Back
Back
Front
1
2
Fig. 3 Appearance of the welded joints
4 CHINA WELDING Vol. 30 No. 1 March 2021
tire joint.In order to specifically investigate the texture dis-
tribution in this area, the pole figure (PF) of rear-pass iscalculated in Fig. 6. It can be seen that cube texture({001}<110>) is obtained in this zone. The maximum poledensity is 10.40 times more than the random background.This indicates that the weighted distribution of polar projec-tion points on the polar equatorial plane is relatively con-centrated, and the microtexture is relatively strong.
The distribution of grain boundary misorientation in the
rear-pass is illustrated in Fig. 7. The proportions of grainswith different grain boundary angle are: 58% (θ<10°), 3.6%(10°<θ<15°) and 38.4% (θ>15°), respectively. In this re-gion, there is a relatively high proportion of low-angle grainboundaries. This is because the grain boundary misorientedangle determines the magnitude of the grain boundary en-ergy. In order to minimize the interface reaction energy, it iseasy to generate grains with similar orientation (small mis-oriented angle). This result is also consistent with the res-ults observed by other researchers in 6 000 series of alumin-um alloys[30].
The EBSD result of weld center was shown in Fig. 8. Asshown in Fig. 8a, the grains in the weld center change fromcolumnar to equiaxed. PF of this region is calculated andthe result is shown in Fig. 8b. The density peak of the weldcenter is dispersing and the highest pole density is only 6.07times more than the random background. It indicates thatfrom the edge of the weld seam to the center, as the struc-ture changes, the microtexture also weakens. Fig. 8c showsthe grain boundary misorientation distribution of this re-gion. Compared with Fig. 7, the proportion of high-anglegrain boundaries increases significantly and the ratio is
ac
d b
50 μm
(a)
100 μm
(c)
100 μm
(d)
50 μm
(b)
WS HAZ
Fusion line
WS HAZ
Fusion line
Fig. 4 Microstructures of joint 1 (a) BM (b) FZ (c) Weld center (d) Weld passes junction
Rear-pass
Fore-pass
250 μm
Fig. 5 EBSD IPF between the weld passes
Texture and mechanical properties of metal inert gas welded 6082-T651 aluminum alloy joints 5
70%. It is mainly because as the grain grows, some low-angle grain boundaries will merge, which will eventually beconverted into high-angle grain boundaries. For high-anglegrain boundaries, the angle between the slip planes is verylarge, which will cause dislocations to accumulate at thegrain boundaries. When the crack propagates to the high-angle grain boundaries, it needs to overcome and consume alot of energy, so the strength of the material is improved. Itshould be noted that even inside the weld seam, the textureis not uniform due to the mode the molten pool solidified,especially in multi-pass welding. If there is a preferred ori-entation in the microstructure, the properties will show an-isotropy, which has a certain influence on the processingand performance of materials.
2.2 Distribution of alloying elementsFig. 9 shows the major element distribution of the BM.
Y0
X0
{100} {110} {111}
Max=10.40
2468
Fig. 6 The pole figure of rear-pass
0.5
0.4
0.3
0.2
0.1
00 10 20 30 40 50 60
Misorientation angle/(°)
Fre
quen
cy (%
)
Angle range
Percentage (%)
θ<10°
58
10°<θ<15°
3.6
θ>15°
38.4
Fig. 7 The grain boundary misorientation distribution ofrear-pass
(a)
(c)
Y0
X0
{100} {110} {111}
Max=6.07
12345
(b)0.12
0.10
0.08
0.06
0.04
0.02
00 10 20 30 40 50 60
Misorientation angle/(°)
Fre
quen
cy (%
) Angle range
Percentage (%)
θ<10°
25
10°<θ<15°
5
θ>15°
70
250 μm
Fig. 8 The EBSD result of weld center (a) IPF in weld center (b) The grain boundary misorientation distribution (c) PF
6 CHINA WELDING Vol. 30 No. 1 March 2021
From the backscattered electron image, it can be clearlyseen that there are white second phases distributing on thegray Al matrix and these second phases appear as short rodsor spherical. Moreover, the distribution of these secondphases has obvious directivity along the rolling direction.Combined with the distribution of major element, the se-gregation of Si, Fe and Mn are in the same position. Ele-ment quantitative analysis of the second phases at locationsA and B is shown in Table 4 and the result also proved thatthe main elements are Al, Si, Fe and Mn. So, it can be de-termined that the second phases are Al-Si-Fe-Mn phases,which are common impurity phases in aluminum alloy[7].However, there is also a small amount of segregation of Mgand Si in the same position. It has been inferred that the pre-cipitates are likely Mg2Si phase, which is the main strength-
ening phase of 6082 aluminum alloy[23].Fig. 10 shows the distribution of major alloying ele-
ments near the fusion line. The microstructure in weld seamis composed of gray matrix and white grain boundary pre-cipitates. A large number of grain boundary precipitates dis-tribute intermittently at the grain boundaries of the solid
(a) (b)
(c) (d)
(e) (f)
Conc. %Al
100.0
93.8
87.5
81.3
75.0
68.8
62.5
56.3
50.0
43.8
37.5
31.3
25.0
18.8
12.5
6.3
0.0
Ave 91.9
Conc. %Mg
35.0
32.6
30.3
27.9
25.5
23.1
20.8
18.4
16.0
13.6
11.3
8.9
6.5
4.2
1.8
0.0
0.0
Ave 0.9
Conc. %Mn
20.0
18.6
17.3
15.9
14.6
13.2
11.9
10.5
9.2
7.8
6.4
5.1
3.7
2.4
1.0
0.0
0.0
Ave 0.8
LvCOMPO
4 095.0
3 858.1
3 611.1
3 369.2
3 127.3
2 885.3
2 843.4
2 401.4
2 159.5
1 917.6
1 675.6
1 433.7
1 191.8
949.8
707.9
465.9
224.0
Ave 1959.2
Conc. %Si
18.0
16.8
15.6
14.4
13.2
11.9
10.7
9.5
8.3
7.1
5.9
4.7
3.5
2.3
1.1
0.0
0.0
Ave 0.6
Conc. %Fe
20.0
18.6
17.3
15.9
14.6
13.2
11.9
10.5
9.2
7.8
6.4
5.1
3.7
2.4
1.0
0.0
0.0
Ave 0.5
A
50 μm
50 μm
50 μm
50 μm
50 μm
50 μm
B
Fig. 9 Backscattered electron image and elemental distribution of BM (a) BSE (b) A1 (c) Si (d) Mg (e) Fe (f) Mn
Table 4 Element quantitative analysis of the specific pointin Fig. 9
PointAtomic percentage (%)
Al Si Fe Mn
A 72.81 12.50 4.31 10.38
B 73.12 12.07 5.42 9.38
Texture and mechanical properties of metal inert gas welded 6082-T651 aluminum alloy joints 7
solution and a small amount of impurity phases also precip-itate in weld seam and BM. It can be seen that there are alarge amount of segregation of Mg and Si at the grainboundary. This is because Mg and Si diffuse to the front ofthe solid-liquid interface when α-Al solid solution crystal-lizes. In addition, the white second phases are mainly com-posed of Fe, Mn and Si at zone A and zone B, which are thesame as the second phases contained in the BM. This alsoproves that the edge of the weld seam is formed by the mixand resolidification of the liquefied BM and filler wire.
2.3 Mechanical propertiesThe macro morphology of tensile fracture samples is
shown in Fig. 11. The fracture of sample 1 occurs in theweld seam and HAZ, while the fracture of sample 2 occurs
in the HAZ. The weld reinforcement of sample 1 is relat-ively small, and the subcutaneous pores can easily becomethe source of cracks, causing cracks to initiate at the weldseam. Besides, the initial fracture position locates at thejunction of the weld passes, which may be caused by inclu-sions from incomplete welding slag cleaning. Then thecrack propagates in the HAZ due to the softening of thisarea. The average tensile strength of sample 1 welded jointis 220 MPa, which is 68% of the BM strength. The averagetensile strength of sample 2 is 232 MPa, which reaches 72%of the BM strength.
The fracture morphology of tensile samples is shown inFig. 12. It can be clearly seen from Fig. 12a that there aretwo different morphology on the fracture surface of sample1. Fig. 12b is an enlarged view of zone A in HAZ. The frac-
(a) (b)
(c) (d)
(e) (f)
Conc. %Al
100.0
95.9
91.9
87.8
83.7
79.7
75.6
71.5
67.5
63.4
59.3
55.3
51.2
47.1
43.1
40.0
40.0
Ave 714.2
Conc. %Mg
15.0
14.0
13.0
11.9
10.9
9.9
8.9
7.9
6.9
5.8
4.8
3.8
2.8
1.8
0.8
0.0
0.0Ave 10.5
Conc. %Mn
20.0
18.6
17.3
15.9
14.6
13.2
11.9
10.5
9.2
7.8
6.4
5.1
3.7
2.4
1.0
0.0
0.0Ave 0.7
LvCOMPO
4 095.0
3 839.1
3 583.1
3 327.2
3 071.3
2 815.3
2 559.4
2 303.4
2 047.5
1 791.6
1 535.6
1 279.7
1 023.8
767.8
511.9
255.9
0.0
Ave 2143.1
Conc. %Si
20.0
18.6
17.3
15.9
14.6
13.2
11.9
10.5
9.2
7.8
6.4
5.1
3.7
2.4
1.0
0.0
0.0
Ave 10.5
Conc. %Fe
18.0
16.8
15.6
14.3
13.1
11.9
10.7
9.5
8.2
7.0
5.8
4.6
3.4
2.1
0.9
0.0
0.0
Ave 0.7
50 μm 50 μm
50 μm 50 μm
50 μm 50 μm
A
B
Fig. 10 Backscattered electron image and elemental distribution of FZ
8 CHINA WELDING Vol. 30 No. 1 March 2021
ture surface has numerous of dimples, which indicated theductile fracture mode. Moreover, secondary phase particlescan also be seen in the center of the dimple. The EDS ana-lysis shows that these secondary phase particles are Al-Si-Fe-Mn phases mentioned above. Fig. 12c is an enlargedview of zone B in weld seam. This region is also composedof many dimples, showing a clear ductile fracture mode.However, compared with zone A, the size of the dimple issmaller. In addition, obvious pores can be seen on the frac-ture surface. During tensile testing, large stress concentra-tion occurs around the pores, which making the pores asource of cracks. Besides, the pores would reduce the ef-fective bearing area of the joint. Consequently, the strengthof the welded joint is reduced.
Fig. 12d shows the tensile fracture surface of sample 2.Different sizes of dimples can be observed on the fracturesurface, which indicated the ductile fracture feature. It canbe seen from the inserted enlarged view that many second-ary phase particles are found on the fracture surface. Theseparticles can improve the strength of welded joint by inhib-iting the dislocation slip during tensile test. The EDS ana-lysis results of point D (Table 5) proves that the main ele-ments of these particles are Ai, Si, Fe, Mn and its content issimilar to point C. Therefore, these secondary phaseparticles are still the Al-Si-Fe-Mn phases.
Sample 1 and sample 2 after bending test are displayedin Fig. 13. The joint surface on one side of the groove isdefined as the front of the joint, and the other side, the gapside, is the back of the joint. According to China StandardGB/T 2 653-2008, the sample undergoes face bending whenthe tension face is on the front of the welded joint, and it un-dergoes root bending while the tension face is on the back
(b)
HAZ
(a)
HAZ
Weld seam
Fig. 11 Fracture positions of joints (a) Sample 1 (b) Sa-mple 2
2 μm
(c)
20 μm
(a)
A
B
Pore
2 μm
(b)
C
10 μm
(d)
D
Fig. 12 Fracture morphology of (a) Sample 1 (b) Enlar-ged view of zone A (c) Enlarged view of zone B and(d) Sample 2
Table 5 EDS analysis results of the specific point in Fig. 12
PositionAtomic percentage (%)
Al Si Fe Mn
C 74.33 6.65 6.76 9.17
D 72.39 11.84 5.76 8.00
Texture and mechanical properties of metal inert gas welded 6082-T651 aluminum alloy joints 9
of the welded joint. The bending test results show that rootbending of two samples can reach 90° without cracks, asshown in Fig. 13a and Fig. 13c. However, when the facebending was about 50°, cracks appeared, as shown inFig. 13b and Fig. 13d. The fracture position located in theweld and FZ. Compared with the back of joint, the weldzone on the front of joint is wider with relatively more de-fects such as welding pores and inclusions. When the frontof joint is under tensile stress, welding pores will reduce theeffective bearing area, and inclusions will decrease thebonding force of structure. Bending cracks easily initiatedand propagated in the dense area of defects.
Fig. 14 shows the distribution of microhardness in thewelded joints. The distribution trend of microhardness in
the two welded joints is similar and roughly symmetrical. InFig. 14b, the microhardness of the weld metal of layer 4 isabout 70 HV, which is lower than that of the base metal.Generally, the microhardness is closely related to the micro-structure. The weld metal is mainly α-Al solid solution andlacks strengthening phases, which makes the microhardnessvalue relatively low. With the distance from the weld centerincreasing, the microhardness value gradually increases toabout 82 HV. However, there is a softening zone in HAZ,due to the welding heat cycle and the dissolution and pre-cipitation of the precipitated phase resulting in overagingphenomenon. As it moves away from the weld center, thesoftening effect generated by this thermal effect decreases,and the hardness increases until the hardness of the BM isrestored. Therefore, there are two weak areas in the 6082-T651 aluminum alloy welded joints. One is the weld seam,and the other is the overaging HAZ.
Compared with layer 4, the width of weld seam of layer2 is only about 7 mm, but its microhardness value is thesame as that of layer 4. However, due to the secondary heat-ing effect of the rear-pass on the fore-pass, the overagingphenomenon is more serious, so it also has a wider over-aging zone. The minimum microhardness at 7 mm from theweld center reaches 57.9 HV. As the distance from the weldcenter increases, the microhardness gradually returns to thatof the BM. Zhang et al.[31] used MIG to weld 6082-T6 alu-minum alloy and got the same microhardness results. Andthey also studied the microhardness variation caused by thephase transformation after paint baking, and found that themicrohardness of the weld seam and overaging zone wasgreatly improved after three baking cycles.
The weld seam is a typical casting structure, and itsproperty depends on the chemical composition and crystal-lization process of the filler wire[32]. The precipitation ofsecond phases is inhibited upon solidification due to therapid cooling rate of the weld metal, so the weld seam iswith mainly supersaturated α-Al solid solution. The lack ofprecipitated phases strengthening is the main reason for thelower tensile strength and hardness of the weld seam. Inmulti-pass welding, each welding pass will heat-treat thejoint. The dissolution and precipitation of the second phasescaused by cyclic heat treatment reduces the mechanicalproperties of the HAZ. Additionally, the pores form in theweld seam due to the oxygen and hydrogen coming fromthe air, BM and welding consumables, and incompletewelding slag cleaning causes the existence of inclusions.Welding pores and inclusions also reduce the strength ofresultant joint[9].
(a)
(c)
Front
Back
(b) 48°
Cracking
(d) 52°
Cracking
Fig. 13 Appearance of the bending samples (a) Root bend-ing of sample 1 (b) Face bending of sample 1 (c) Root bend-ing of sample 2 and (d) Face bending of sample 2
10 CHINA WELDING Vol. 30 No. 1 March 2021
3 Conclusions
(1) The weld seam of multi-pass welding is relativelycomplex with different microstructure and texture feature indifferent zones. The weld seam has a typical cube texture,and the grain boundary misoriented angle is mainly low-angle grain boundary. The closer to the center of the weldseam, the weaker the texture feature, the higher the propor-tion of high-angle grain boundaries.
(2) The maximum tensile strength of welded joint is232 MPa and the bending angle of the root bend samplereaches 90° without cracks, which can meet the require-ment of engineering application.
(3) The microhardness of the welded joint is symmetric-al along the weld center, and there are two softening zones:the weld seam and the overaging zone. The lack of strength-ening phase in the weld seam and the dissolution and pre-cipitation of second phases in the overaging zone are themain causes of softening. In addition, welding pores and in-clusions also remarkably reduce the mechanical propertiesof the resultant joint.
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Layer 2Layer 4
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