African Fusion March-April 2024

CMT and cryogenic weld bead geometry

This paper from the 76 th IIW Annual Assembly and International Conference on Welding and Joining (IIW 2023) in Singapore is by Yashwant Koli and S Aravindan who investigate the bead geometry of ER4043 (AlSi5%) filler wire deposited on the AA6061-T6 (3.18 mm) sheet in cryogenic conditions using three fusion welding techniques: cold metal transfer (CMT) welding, metal inert gas pulse (MIG-P) welding, and metal inert gas standard synergic (MIG‑SS) welding. A comparison of weld bead geometry for CMT, pulse MIG and standard synergic MIG under cryogenic conditions

I ndustries are shifting towards light weight materials, which can consider ably reduce transportation costs and energy consumption. Aluminium alloys are replacing steel in the majority of ap plications such as aircraft components, external and internal vehicle body parts, railway industries, etc, due to their high strength, low weight and exceptional cor rosion resistance [1–4] . Gas metal arc welding (GMAW) is com monly used for joining aluminium alloys having higher thickness due to its higher heat input characteristics, greater penetra tion depths, and confined heat-affected zone (HAZ) [5] . Despite these benefits, residual stresses and distortion are un avoidable in GMAW welds. These occur during welding due to non-uniform thermal expansion and contraction induced by localised transient heating and non-linear temperature profiles. These problems can be reduced by us ing a low heat input welding process such cold metal transfer (CMT). This process is an upgraded technology based on the GMAW process patented by Fronius of Austria. It works on the short-circuiting mode of metal transfer where, at the time of short circuiting, the current drops to a very low non-zero value, which reduces the heat input and avoids spatter generation [6] . CMT

Experimental Procedure Materials:

can, therefore, be used to join sheets as thin as 0.3 mm thick. Various research studies have been done in cryogenic conditions during fric tion stir welding or processing (FSW/FSP). But only limited work has been done with CMT welding. Sudden cooling or quenching will refine the microstructure. During the process of quenching, the rate of cooling can have a significant effect on the degree of supersaturation, which in turn affects precipitation hardening. The FSP studies incorporate cryogenic cooling to enhance the mechanical properties of aluminium alloys by refining grain size [7] . Weld parameters directly affect bead dimensions, determining weld quality [8,9] . From a comprehensive review of the lit erature and preliminary experiments, it is evident that current and welding speed are the two most influential factors on bead dimensions and shape relationships [4,10] . Weld bead geometry consists of weld pen etration (WP), weld reinforcement (WR), weld width (WW), weld dilution (WD), and contact angle (CA). This paper focuses on the variation of weld bead geometry with and without the use of liquid nitrogen cooling (LN 2 ). Three processes (CMT, MIG-P, and MIG-SS) are compared using the same parameters for thin AA6061-T6 sheets.

For this research work, AA6061-T6, was used as a substrate sheet with dimensions of 100x60x3 mm. ER4043 (AlSi5%), having a 1.2 mm diameter, was used as a filler wire. Due to high compatibility with 6000 series aluminium alloys as per literature and trial tests. The chemical composition of AA6061‑T6 was experimentally obtained by chemical spectroscopy as per ASTM E 1251:2011. The substrate AA6061-T6 material used was found to consists of Si‑0.665%, Mg-0.840% and Al-97.75%, while ER4043 filler wire consists of Si-5.6% and Al>94% as the major elements [4] . Figure 1 (a) shows the microstructure obtained using an optical microscope of the substrate sheet, while the elements (Al, Mg, Si) confirmation is given by the EDX graph depicted in Figure 1 (b). Methodology: TPS 400i CMT welding machine setup was used with a liquid nitrogen hose pipe at tachment, as shown in Figure 2(a). Exten sive trial experiments were conducted, and the optimised process parameters were achieved using a 90 A current level and at welding speed of 7.5 mm/sec with CMT us ing response surface methodology (central composite design) [11] .

Figure 1: AA6061-T6: (a) Optical microscopy (OM) image; (b) EDX graph.

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March-April 2024

AFRICAN FUSION