African Fusion March-April 2024

CMT and cryogenic weld bead geometry

Several researchers working on the effect of cryogenics on welded aluminium alloy specimens reveal that it helps in grain refinement, has better mechanical properties, and mitigates residual stresses and distortions [5,7,13,14] . In this work, as the LN 2 hose pipe was connected adjacent to the welding torch, the LN 2 was sprayed over the solidus form of the weld bead, creating a much better effect as compared to cryogenic post-weld heat treatment (PWHT). With the help of spraying LN 2 , it was observed that the sheet distortion was minimal as compared to ambient conditions. It also helps in the sudden cooling of the weld bead, which reduces the weld width, as shown in Figure 4. Dilution % is also observed to be reduced in all three processes with the effect of LN 2 , as shown in Figure 4 graph. This states that the weld penetration decreases while increasing the weld rein forcement during solidification. While using LN 2 , the % reduction in dilution is 19.10% 13.87% and 8.93% for MIG-SS, MIG-P, and CMT, respectively. The maximum reduction in dilution is experienced in MIG-SS due to its high heat input characteristics. A Pulstec -X360n Full 2D High-Resolution x-ray Diffraction (HRXRD) machine was utilised to measure residual stresses in the CMT and MIG-SS weldments. It is based on the Cos α method (Bragg’s Law), which acquires a complete Debye–Scherrer ring (which reveals grain orientation (texture) and grain coarsening, etc.) with a single short x-ray exposure from a 2-D detector [15] . The residual stresses measured in CMT and MIG-SS weld bead without LN 2 are: σ x = –91 MPa, σ y –160 MPa; and σ x = –138 MPa, σ y = –505 MPa respec tively, as shown in Figure 5. The comparative numbers below show the significant effect of LN 2 in reducing the residual stress: σ x = –68 MPa, σ y =–89 MPa and σ x = –103 MPa, σ y = –395 MPa for CMT and MIG-SS, respectively.

Figure 4: Macroscopic images of the three different welding techniques with varying conditions at the same process parameter. substrate sheet thickness due to the high heat input welding pro cess compared to CMT. Maximum penetration (4.432 mm) is ob served with the MIG-SS welding process, while a minimum (1.848 mm) is observed in CMT due to low heat input characteristics.

Figure 5 shows the Debye Scherrer ring and distortion plots at the CMT and MIG-SS weld beads. It is observed that the maximum residual stress is observed in MIG-SS (without LN 2 ), as shown in Figure 5(c). The % reduction of residual stress is around 25.27% and 44.375% in the x and y direction, respectively, for CMT weld beads. In comparison, the % reduction of residual stress is about 25.36% and 21.78 % in the x and y direc tion, respectively, for MIG-SS weld beads with the application of LN 2 . Conclusions The effect of liquid nitrogen on weld bead geom etry in three welding processes using the same process parameters was investigated in this research. The main results are as follows:

Figure 5: Residual stress graphs comprising of Debye Scherrer ring and distortion plots (a) CMT without LN 2 (b) CMT with LN 2 (c) MIG-SS without LN 2 (d) MIG-SS with LN 2

Weld bead dimensions

Condition

Process MIG-SS

WP (mm)

WW (mm)

WR (mm)

WD (%)

CA (°) 21.073 42.731 49.145 42.757

Without LN 2

4.432 4.348 1.848 3.695 3.602 1.941

10.615

1.108 1.885 2.333 1.960 1.997 2.295

83.31 71.59 41.76 67.39 61.66 38.03

MIG-P

6.775 6.625 9.299 6.569 6.084

CMT

With LN 2

MIG-SS MIG-P

45

CMT

68.962

Table 2: Weld bead geometry and contact angles.

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March-April 2024

AFRICAN FUSION