Investigation of extrusion welding in sideways extrusion processes

Abstract

Differential velocity sideways extrusion (DVSE) is novel process for fabricating cross-sectional shapes of curved profiles in one step and has great potential for industrial applications. However, little attention has been paid to the welding behaviour of DVSE of aluminium alloy, and the fundamental studies on extrusion welding has yet to be carried out. The aim of this research is to develop the proposed profile extrusion technique and investigate the microstructure and welding quality of different extrusion profiles produced by DVSE. In this research, four sets of extrusion die with different cross-sectional profiles have been designed and employed to produce four types of aluminium alloy AA1070 extrudates including solid and hollow profiles using two-billet DVSE. Different extrusion temperature and ram speed are selected to study their effect on microstructure and welding quality. The microstructure and the weld seam of the extruded profiles are examined using optical microscopy (OM) and electron backscatter diffraction (EBSD). Mechanical testing including hardness test, tensile test, shear test and pressure bearing capacity test has been carried out to assess the change in material property and welding quality. The material flow pattern, microstructural evolution, formation of longitudinal and transverse weld seams, and the effect of extrusion temperature and ram speed on these features have been studied. Finite element (FE) simulation of the extrusion process has been carried to predict the stress and strain distributions and to aid the understanding of material flow behaviour and microstructural evolution as well as the effect of extrusion parameters.

The test set 1 aims to investigate the material flow in the chamber and characterise the microstructure, texture and mechanical properties of the sideways extrusion profile in the chamber and the extrudate. The feasibility of the process is demonstrated by fabricating solid bars of the aluminium alloy AA1070 through the solid-state welding of two billets at elevated temperatures. The microstructural examination shows that the weld formed between the two billets consists of an unsound area in the dead metal zone within the die and a sound bonding area in the welding zone. Along the welding path, the effective and normal strains gradually increase while the shear strain decreases, leading to the transformation of grains from equiaxed to bamboo-like structures and increases in the hardness, average grain size, and fraction of low angle grain boundaries. Increasing the temperature or speed of extrusion decreases the unsound bonding area length and the shearing angle. The hardness of the extrudate decreases with decreasing extrusion speed or increasing temperature.

The test set 2 examines the effect of different processing parameters on the microstructure and mechanical properties of AA1070 tube profile extrudates. A sound welded interface of AA1070 tube without crack or defects has been obtained. The different grain structures of tube profiles during the DVSE process are revealed by EBSD, showing grain refinement through continuous dynamic recrystallisation (CDRX) or geometric dynamic recrystallisation (GDRX). The hydrostatic pressure bearing capacity increases with continuous extrusion and becomes stable after about 35 mm, and failure occurs consistently in the region outside the weld seam.

The test set 3 is to investigate the transverse and longitudinal weld during sideways extrusion with a solid profile. The weld formed between billets includes longitudinal and transverse welds. Signs of both CDRX and GDRX have been observed in the extrudate. The transverse weld has poor mechanical properties, especially at the front of the weld. Voids and gaps can be found in these transverse welds. The longitudinal weld has the best ultimate tensile strength (80 MPa) and total elongation (67%) among the tested regions. It also had a higher degree of recrystallisation and a continuous grain structure across the bonding interface, beneficial to welding quality.

The test set 4 investigates the effect of extrusion ram speed on the welding quality of the sideways extruded thin-wall profile. Increasing the ram speed appears to have a better bonding interface and there is no sign of weld seam at 1.0 mm/s sample under optical microscope. The microstructure along the weld line is strongly dependent on the ram speed. As the ram speed increases from 0.05 mm/s to 1.0 mm/s, dislocation density and low-angle grain boundaries decrease, but high-angle grain boundaries increase (i.e. grain size decreases), and the degree of recrystallisation increases. As a results, the UTS decreases from 63 MPa in a 0.05 mm/s sample to 58 MPa in a 1mm/s sample and the total elongation increases from 34.5% to 45.0%.