How to Improve the Welding Quality of MIG Spot Welding of 6 Series Aluminum Alloy?

Different welding processes under MIG spot welding mode are compared for 6-series aluminum alloy lap joints.

The research is carried out from two directions of changing groove form and welding current.

After welding, the appearance inspection, tensile property inspection, macro metallographic observation and micro metallographic observation are carried out.

The results show that excessive welding parameters will lead to liquefaction cracks in the joint.

Under suitable welding parameters, the use of I-groove combined with MIG spot welding mode is easy to cause the problem of weak connection of the bottom plate, while the use of single V-groove combined with MIG spot welding mode can avoid the problem and improve the quality of the joint.

1. Preface

In recent years, automobile lightweight development has become a trend, and aluminum alloy has become one of the preferred materials for automobile lightweight because of its light weight, corrosion resistance and other advantages.

At present, MIG welding is a very common aluminum alloy welding technology with relatively low production cost.

When MIG welding the joints of 6 series aluminum alloy sheet, it is very easy to produce the phenomenon of over melting.

Studies have shown that for lap joints, the joint strength at penetration is the highest, but the research object is stainless steel.

For 6 series aluminum alloy, its thermal crack sensitivity is high, and the over melting phenomenon caused by excessive welding heat input is easy to cause liquefaction cracks and other defects.

If MIG spot welding mode is adopted, it is easier to avoid the occurrence of over melting of welding points, reduce welding deformation and ensure the quality of joints.

However, due to the characteristics of arc welding, MIG spot welding is prone to shallow penetration and false welding, which also limits its application.

Aiming at the existing problems of MIG spot welding, this post puts forward a solution and provides relevant data, which provides reference for welding designers and production technicians of related products.

2. Test materials and methods

The test base metal is 6063-T6 extruded profile with a wall thickness of 3mm.

The chemical composition and mechanical properties of the base metal meet the requirements of GB/T 3190-2020 “chemical composition of wrought aluminum and aluminum alloys” and GB/T 6892-2015 “extruded profiles of aluminum and aluminum alloys for general industrial use”.

See Table 1 and table 2 for details.

Table 1 chemical composition (mass fraction) of 6063-T6 aluminum alloy (%)

Table 2 mechanical properties of 6063-T6 aluminum alloy

The welding equipment used in this test is Fronius TPS 5000 inverter digital aluminum welder, the robot arm is KUKA KR90, and the welding wire is EN ISO 18273:s Al 5356（ φ1.2mm), and the welding method is single pulse MIG automatic spot welding.

In Fronius TPS 5000, the main adjustable parameters of MIG spot welding function are welding current and welding time, and there is no setting of arc striking and arc stopping current.

The welding posture is shown in Fig. 1.

Fig. 1 Welding posture

Note:

• α₁ is the welding gun side angle, α₂ is the caster angle of the welding gun, and b is the distance between the welding wire and the edge of the upper plate of the lap joint.

The test joint is divided into I-shaped groove lap joint and single V-shaped groove (45 °) lap joint, and the welding gun side angle α₁ all are 5 °, welding gun caster α₂ all are 90 °.

The distance between the welding wire and the edge of the upper plate of the lap joint is b=3mm, and the dry extension of the welding wire is 15mm.

See Table 3 for MIG spot welding parameters.

The shielding gas is 99.99% Ar and the gas flow is 20L / min.

Table 3 MIG spot welding parameters

This test is carried out by using the control variable method.

In Table 3, serial numbers I-1 ~ I-5 are the welding parameters of I-groove under different welding currents, and serial numbers V-1 ~ V-5 are the welding parameters of single V-groove under different welding currents.

After welding, carry out visual observation, tensile property test, macro metallography and micro metallography observation in turn, and then analyze the test result data.

3. Test results and analysis

3.1 Visual observation and joint tensile fracture results

The welding parameters numbered I-1 ~ I-5 in Table 3 are used for welding, and the post welding test results are shown in Table 4.

It can be seen from table 4 that when the welding current is 130A and 140A,and the welding parameters are too small, resulting in weak connection at the connection position of the bottom plate of the joint.

When the welding current is 150 ~ 170a, the maximum tensile load increases gradually with the increase of welding current.

When the welding current reaches 170A, the tensile fracture position is transferred from the weak connection position of the bottom plate to the filler metal, and the maximum tensile load value is the highest at this time, indicating that the weak connection problem of the bottom plate has been solved to a certain extent.

Table 4 I-groove welded joints

Welding current /A	Visual observation	Tensile fracture location	Maximum tensile load /N
130			0
140			0
150			4143
160			4871
170			8015

Use the welding parameters numbered V-1 ~ V-5 in Table 3 for welding.

See Table 5 for the test results after welding.

It can be seen from table 5 that under the single V-shaped groove, the tensile fracture position of the joint is located in the filler metal, indicating that the use of the single V-shaped groove can effectively avoid the problem of weak connection of the bottom plate.

With the increase of welding current, the volume of filler metal and the connection area between the upper and lower plates gradually increase, and the maximum tensile load of the joint also gradually rises.

Therefore, MIG spot welding mode with single V-groove can more effectively avoid the occurrence of weak connection of the bottom plate than with I-groove, and the joint quality is also more stable.

Table 5 single V-groove welded joints

Welding current /A	Visual observation	Tensile fracture location	Maximum tensile load /N
130			253
140			506
150			1922
160			5530
170			8015

3.2 Metallographic analysis

Through metallographic observation of the welded joints in Table 4 and table 5, it is found that when the welding current is ≤ 160A, there is no micro crack in the joint, while when the welding current is 170A, there is micro crack in the joint.

Now take the macro and micro morphology of i-groove joint and single V-groove joint with 160A and 170A welding current as an example, as shown in Fig. 2.

a) 160A, macro morphology of I-groove  
b) Micromorphology at α₁  (50 ×）

c) 170A, macro morphology of I-groove  
d) micro morphology of b₁  (50 ×）

e) 160A, macro morphology of single V-groove  
f) micro morphology at C1 position (50 ×）

g) 170A, macro morphology of single V-groove
h) micro morphology at d₁ position (50 ×）

Fig. 2 macro and micro morphology of joint

According to the research results of scholars in the industry, this kind of microcrack can be determined as liquefaction crack.

The liquefaction crack is caused by the rapid heating in the welding process.

In the rapid heating process of the base metal, phase β is too late to dissolve, and when the temperature exceeds the eutectic temperature, the components in phase β have not been completely dissolved into the solid solution of α, and finally a weak eutectic liquid phase appears at the interface between α and β, and finally cracks along the grain boundary liquid film under the action of stress, forming cracks.

For figures 2c and 2d, liquefaction cracks mainly exist in the coarse-grained layer of the base metal, which is due to the higher sensitivity of the coarse-grained layer to cracks.

For figure 2g and figure 2h, compared with the I-shaped groove, more welding heat input is transmitted to the bottom plate.

With the downward pressure of the self gravity of the clad metal, liquefaction cracks appear in the coarse-grained layer area on the back of the base metal and inside the base metal.

For the base plate connection position prone to weak connection problems, with the increase of welding current, the penetration at the base plate side gradually increases.

Through comparison, it can also be found that the penetration of the bottom plate side of the single V-shaped groove is significantly greater than that of the I-shaped groove, which further confirms the important role of the single V-shaped groove in increasing the penetration and ensuring the stability of the joint quality.

To sum up, the use of MIG spot welding mode is limited by the liquefaction crack of the I-groove of lap joint and the penetration of the bottom plate.

After changing the I-shaped groove to the single V-shaped groove of the lap joint, the penetration problem of the bottom plate has been effectively solved, and the appropriate welding process window is also wider, which improves the stability of the application of MIG spot welding mode.

4. Conclusion

For the production application of MIG spot welding mode in 6-series aluminum alloy lap joints, using a single V-groove instead of I-groove can effectively avoid the occurrence of insufficient penetration and weak connection of the bottom plate.

However, due to the high sensitivity of 6 series aluminum alloy to hot cracks, excessive welding heat input is easy to cause liquefaction cracks in the joint.

Therefore, welding parameters should be reasonably controlled in production to obtain high-quality MIG spot welded joints.