Experimental and Finite Element Studies of Cold Pressure Welding of Commercial Purity Aluminium by Divergent Extrusion
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This doctoral thesis is concerned with experimental and finite element studies of cold pressure welding of commercial purity aluminium by divergent extrusion. The thesis is divided into five parts. Part I gives a general introduction to welding of aluminium alloys focusing on the heat affected zone (HAZ) strength loss following traditional fusion welding, the mechanisms of solid state bonding and the most commonly used solid state joining techniques for such materials. In Part II the divergent extrusion method, originally designed for simulation of cold pressure welding, has been further developed and explored. As a starting point the combination of advanced process instrumentation and finite element (FE) modelling is used to unravel the underlying material flow and deformation pattern during cold extrusion of commercial purity aluminium. It is shown that the plastic work within the shear deformation zone of the workpiece constitutes about 60% of the total forming load, the remaining being the result of friction, upsetting and plane strain compression. Because of this shear deformation the divergent extrusion process bears a close resemblance to equal channel angular pressing (ECAP). In Part III the aptness of the divergent extrusion process in simulating cold pressure welding (CPW) is further documented and explored. As a starting point, the situation existing both prior to and after cold bonding is adequately reproduced using finite element (FE) modelling. Combined extrusion and joining experiments are then carried out to unravel the conditions under which cold bonding takes place in commercial purity aluminium. Based on three-point bend testing and fracture surface investigations in the scanning electron microscope (SEM), it is observed that bond initiation occurs gradually during the course of the extrusion process. Full metallic bonding is achieved when the surface exposure, as determined by the x-component of the strain acting in the extrusion direction, reaches a value of 0.97. This surface exposure is comparable to that reported for bond formation using conventional CPW and cold roll bonding (CRB) in the presence of oxide and lubricant films at the mating interfaces under similar contact pressure conditions. Finally, Part IV is concerned with cold pressure welding (CPW) of severely plastically deformed (SPD) aluminium. As a starting point, commercially purity aluminium is subjected to single pass equal channel angular pressing (ECAP) and subsequently joined by divergent extrusion (DIE). The mechanical integrity of the spliced components is then documented using notch tensile testing, showing that both the strength and the flow properties of the SPD processed base material are fully maintained across the joint after cold welding. This is because the DIE process implies a mixed deformation mode, i.e. homogenous shear deformation in the bulk and plain strain compression at the joint line where bonding occurs. Moreover, finite element (FE) modelling reveals that the combination of contact pressure to yield strength ratio and surface exposure needed to achieve full metallic bonding is comparable with that reported for bond formation in soft annealed aluminium. This means that SPD processing by ECAP does not alter the conditions under which bonding takes place during CPW, which makes the DIE method particularly useful for joining such materials.