This research was aimed to reveal atomic diffusion across the bonding interface through the extrusion-welding experiments of dissimilar materials, namely pure magnesium and Mg–Al–Zn–RE alloy. A special tooling setup used to simulate weld seam formation during extrusion through a porthole die was designed for this model study. To deform the metal streams symmetrically and create a sound weld seam, the extrusion-welding experiments from sandwich-structured billets were carried out. Chemical analysis of diffused atoms in the welding region was performed by electron probe micro-analysis. The results confirmed that atomic diffusion indeed occurred across the interface during extrusion. The gradients in element concentration, local stresses, and hydrostatic pressure were considered to be the necessary conditions for extensive atomic diffusion to occur. Atomic diffusion was significantly enhanced by raising extrusion temperature. The analysis of the stress and hydrostatic pressure distributions in the welding region provided new insights into the mechanisms of weld seam formation during the extrusion of light metals.

Conventional extrusion limit diagram (ELD) involves only two extrusion process variables and as such it does not account for the combined effects of multiple process parameters on the extrusion process with respect to pressure requirement and extrudate temperature. Attempts were made in the present research to construct three-dimensional (3D) ELD for a magnesium alloy in the space of initial billet temperature, extrusion ratio and extrusion speed. A method to build 3D ELD by integrating finite element (FE) simulations, extrusion experiments and artificial neural networks (ANN) was developed. In addition to initial billet temperature, extrusion ratio and extrusion speed, the temperature difference between the extrusion tooling and billet, the size of the billet and the shape complexity of the extrudate were taken as the additional process variables and integrated into the equivalent initial billet temperature, extrusion ratio and extrusion speed. The FE simulations, verified by performing extrusion experiments to produce magnesium alloy rods, were used to generate datasets for training the ANN. The ANN then predicted the peak values of extrusion pressure and extrudate temperature over a wider range of extrusion conditions, based on which a 3D ELD for the magnesium alloy was constructed. The 3D ELD was finally validated by performing extrusion experiments to produce magnesium alloy tubes. The results demonstrated that the constructed 3D ELD was reliable and able to provide guidelines for the selection of appropriate extrusion conditions.

Solid-state bonding takes place during the extrusion process to produce a hollow metal profile through a porthole die, known as extrusion welding. Defective weld seams degrade extruded products in mechanical properties. The present research was aimed to determine the effect of extrusion condition on the longitudinal weld seam quality of a magnesium alloy, Mg-8Al-0.5Zn-0.5RE, using an integrated physical and numerical simulation method. A special die set-up for physical simulation was designed, through which two magnesium alloy rods were welded in the solid-state under high hydrostatic pressure. Extrusion welding experiments under different conditions were performed. It was demonstrated that, with this die set-up, the formation of weld seams during extrusion to produce hollow profiles could be physically simulated. The extrusion welding experiments were then numerically simulated to reveal strains, stresses and hydrostatic pressures that could not be experimentally measured. Finally, the tensile strength and elongation of the extrusion-welded magnesium alloy were determined and its microstructure was examined. The results showed that the bonding strength increased with decreasing extrusion speed and rising extrusion temperature. For well-bonded rods, weld seam was invisible under optical microscope. Attributed to high temperature and large equivalent strain, complete dynamic recrystallization occurred across the interface, leading to a reduced average grain size and disappearance of weld seam. By applying the integrated physical and numerical simulation method, extrusion process parameters for a particular magnesium alloy can be optimized to ensure weld seam quality of extruded hollow profiles.

The present study was aimed at determining the extrudability of a newly developed Mg-Al-Zn-RE magnesium alloy with large amounts of alloying elements. The experimental and numerical investigation clearly showed that the extrudate temperature was a crucial factor in deciding if a critical temperature between 754 K and 768 K (481 °C and 495 °C) was reached during extrusion, above which hot shortness occurred. Under the extrusion conditions applied, dynamic recrystallization (DRX) occurred, leading to grain refinement from a mean grain size of 165 μm in the as-solid-solution-treated billet to 8.0 to 10.9 μm in the extruded rods. Second-phase particles, such as Mg₁₇Al₁₂ and Al₁₁La₃, were found to distribute on grain boundaries and aid in grain refinement. The mechanical properties of the extrudate were greatly influenced by the as-extruded microstructure and extrusion condition. As the initial billet temperature decreased, the ultimate tensile strength (UTS) and elongation of the alloy increased, while yield strength (YS) remained almost unchanged. At an initial billet temperature of 523 K (250 °C), a stem speed of 3.93 mm/s, and a reduction ratio of 29.8, the extruded magnesium alloy had a mean grain size of 8 μm. Its YS, UTS, and elongation reached 217 ± 3 MPa, 397 ± 7 MPa, and 20 ± 1.3 pct, respectively.

The research was aimed to determine the effects of extrusion process condition on the weld seam quality of the aluminum alloy AA6082 by using a novel physical simulation method. A weld seam between two bars was formed under hydrostatic pressure in a specially designed die setup to simulate the longitudinal weld seam formation during extrusion through porthole die. With this die setup, extrusion process variables, i.e., temperature, extrusion speed and strain, could be varied so that their individual effects on weld seam quality could be discriminated. With the help of finite element (FE) simulation, the distributions of strains, strain rates and hydrostatic pressures inside the welding chamber were quantified. Tension tests were performed to evaluate the bonding strengths of solid-state welded samples. It was found that the amount of deformation imposed inside the welding chamber had a dominant effect on the bonding strength. A high deformation temperature and a high extrusion speed enhanced the bonding strength. The microstructures across the weld zone were examined by using a polarized light microscope and electron back-scatter diffraction (EBSD). The microstructure evolutions inside and around the welding zone were found to be influenced by the deformation condition. High temperature, high extrusion speed and large deformation promoted the occurrence of local dynamic recrystallization, leading to reduced mean grain sizes inside the welding zone, corresponding to an enhanced strength at the weld seam.