Mechanism and possible solution for transverse solidification cracking in laser welding of high strength aluminium alloys

Abstract

In laser and hybrid laser/arc welding of high strength aluminium alloys, a large number of transverse cracks were found in the weld fusion zone. The cracking behaviour was evaluated experimentally and scanning electron microscopy images of crack surfaces confirmed that the cracks occurred when the weld fusion zone was in the semi-solid state. Thermal histories in the workpiece under representative welding conditions were measured and constitutive modelling of thermo-mechanical behaviour in the weld was performed. It was found that the cracking is related to the elongated temperature distribution in the welding direction, which induces a transverse tensile strain in the weld fusion zone during the cooling phase. One of the possible solutions to the cracking problem is to use an additional heat source to alter the temperature distribution and thus to reduce the cracking tendency. The effect of welding with an appropriately placed secondary heat source was verified by experimental tests.

Introduction

Heat-treatable aluminium alloys 2024(T3) (AlCu4Mg1) and 7075(T6) (AlZn5.5MgCu) are two of the most common commercial high strength aluminium alloys. Because of their excellent strength to weight ratio, these alloys can be employed for a wide range of applications, especially in the transport industry. Welding of these high strength aluminium alloys, however, still remains a challenge. Apart from softening in the weld fusion zone and heat-affected zone, hot cracking in the weld can be a serious problem [1], [2], [3]. Extensive studies have been conducted in the past on cracking phenomena in arc welding of high strength aluminium alloys. Cracking susceptibility of the alloys has been evaluated by specially designed tests, and results indicate that AA2024 and AA7075 are highly crack susceptible alloys, due to their alloy compositions and weld microstructures [3].

Because of the commercial availability of high power Nd:YAG lasers in recent years, laser beam welding and hybrid laser/arc welding processes have become attractive for the welding of aluminium alloys. A laser beam has a much higher energy density than a typical arc plasma. Early experiments showed that in comparison with conventional arc welding, laser and hybrid laser/arc processes could achieve an order of magnitude higher welding speed and a much narrower weld fusion zone and heat-affected zone. Furthermore, the fast cooling rate from the laser or hybrid laser/arc welding process leads to a fine sub-grain microstructure in the weld fusion zone, which is beneficial to the strength and ductility of the weld [4], [5]. Preliminary results also indicate, unfortunately, that at a high welding speed, transverse solidification cracking occurs in the weld fusion zone. In fact the number of the cracks was found to increase with increasing welding speed [5], [6].

In this study, cracking behaviour in AA7075 and AA2024 welds was first evaluated experimentally. The welding processes employed were hybrid laser/gas metal arc welding and autogenous laser beam welding. Thermal histories in the workpiece under representative welding conditions were measured carefully using small diameter thermocouples. In order to understand the cracking mechanisms, constitutive modelling of thermal and thermo-mechanical behaviour was then undertaken. In the model, the thermal history in the welds was first calculated numerically, followed by the thermal stress and strain in the welds. Cracking occurs when the deformation or strain in the weld is above the maximum ductility level that the alloy can sustain. The calculated results of the tensile strains are compared with the experimental observation. The mechanisms of cracking and possible solutions to avoid transverse cracking in laser welding are illustrated.