ISSUE 114 AUTUMN 2024 LASER WELDING


THE LASER USER


REMOTE LASER WELDING OF STEEL TO


Al: DOES Zn COATING PLAY A ROLE? ALI BAGHBANI BARENJI ET AL.*


Remote laser welding (RLW) of Zn- coated steel to aluminium presents two key challenges: (1) Zn vaporisation causes spattering and poor seam quality in zero-gap lap joints, and (2) brittle intermetallic compounds (IMCs) form at the weld interface, resulting in cracks and weakened welds. This study investigates the interplay between Zn vapour, molten pool dynamics, and IMC formation during RLW, using metallurgical and mechanical analysis. The results show that Zn coating leads to an uneven connection between steel and aluminium, with increased mixing and IMCs formation. Locally removing Zn improved joint efficiency to 72% of the parent aluminium, compared to 65% with Zn present.


In the drive towards carbon neutrality, manufacturing must optimise product performance while minimising costs, complexity, and energy use. This is particularly important in electric vehicle production, where lightweight, efficient materials are essential, such as in dissimilar material structures. A prime example is the joining of steel to aluminium, which offers great potential for enhancing these features.


Dissimilar connections of steel to aluminium have been extensively researched for applications like battery packs and automotive structures, using methods such as mechanical joining, chemical bonding, bimetallic inserts, solid-state welding, and fusion welding [1].


Among fusion welding techniques, remote laser welding (RLW) stands out for its lower heat input, faster processing, and precise control. RLW’s ability to adjust heat distribution, through beam shaping and oscillation, is crucial for controlling Fe-Al IMCs when welding steel to aluminium. These brittle phases are well known as the primary obstacle in joining steel to aluminium [2].


IMCs are not the only problem, and another challenge arises when welding Zn-coated steel, due to Zn vaporisation in the lap configuration. This occurs because Zn has a lower vaporisation temperature (approx. 907°C) compared to steel’s melting point (approx. 1500°C), leading to spattering and joint discontinuities.


This study examines the impact of Zn coating during RLW of Zn-coated steel to aluminium in a zero-gap lap joint, investigating how the Zn coating affects joint geometry, material mixing,


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Figure 2: Topographic analysis of the top surface (steel side) and the interface (aluminium side) after tensile test.


IMC thickness, and mechanical strength.


Experimental procedure Commercial low-carbon steel DC01 and 5251- H22 aluminium alloy sheets, both 1.5 mm thick, were used for welding trials. An adjustable ring-mode Coherent fibre laser (Coherent ARM FL10000) with a maximum power of 10 kW (5 kW for the core and ring beam) was employed, along with the Precitec WeldMaster YW52 head. Figure 1 shows the experimental setup of welding in lap configuration (steel on top). The laser beam oscillated in the Y direction, focusing on the sample’s top surface without filler wire or shielding gas. Furthermore, the connection area is defined as the fusion zone on the aluminium side.


To study the impact of zinc (Zn) coating on joint strength and molten pool stability, samples with and without Zn coating were tested. In half the samples, the Zn coating was ablated at low power on both sides of the DC01 steel sheets over an area 5 mm wide and 48 mm long along the welding direction where welding happens in


the next step. Welding was carried out using two oscillation frequencies of 100 (ID 1 with Zn and ID 2 without Zn) and 150 Hz (ID 3 with Zn and ID 4 without Zn) and welding parameters aimed for a penetration depth of approximately 200 μm in the aluminium, ensuring conduction-mode welds to prevent excessive mixing and cracking.


Results and discussion


Observation of the weld surface and interface provides insights into the characteristics of samples welded with and without a Zn coating. Figure 2 displays images of the top surface (steel side) and interface (aluminium side) after tensile testing. Weld instabilities correlate with a higher total area of humps and holes. The connection area on the aluminium side is outlined by black lines in the topography images.


Significant molten pool instabilities were seen in Zn-coated samples, particularly at 150 Hz (ID 3), resulting in a larger total area of humps and holes. Lower welding instabilities in uncoated samples were observed (ID 2 and 4) compared to Zn-coated ones (ID 1 and 3), especially at higher frequencies. Increased frequency in uncoated samples promotes smoother surfaces by enabling even reheating, while Zn vapours in coated samples create turbulence, leading to defects and reduced connection areas. This disruption weakens joint strength and repeatability. The detailed observations of the influence of Zn vapour can be found in [3].


Figure 1: Experimental setup of the welding process in this study.


Figure 3 illustrates the cross-sections of ID1 and ID2. Zn vaporisation results in deficient weld pools, lacking fusion in some regions and containing pores. In contrast, uncoated samples show higher seam quality and fewer defects, attributed to improved agitation in the weld pool. Elemental maps in Figure 3a indicate greater


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