Microstructure and mechanical properties of extrusion welds in continuous extrusion of AA6063 aluminium alloy with double billets

Based on the continuous extrusion machine LLJ350, experiment and Deform numerical simulation were conducted to analyze the metal flow and welding process during the continuous extrusion of AA6063 aluminium alloy with double billets. The microstructure and mechanical properties of extrusion welds were investigated through optical microscopy, scanning electron microscopy, and tensile tests. The results reveal that the oxides on the billet surface participate in the metal flow and affect the microstructure and mechanical properties of the extrusion welds. The extrusion weld exhibits bud morphological characteristics on the cross-section of the extrudate, and the welding quality of the bottom portion is superior to that of the upper portion. The welding lines are mixed with fine grains of several micrometers, and the surrounding area contains grains with a size of several hundred micrometers. When the specimens fracture at the weld, the macrofracture forms with a striped surface; by contrast, the microfracture displays a streaky structure, which includes striped protrusions and small dimples of several micron diameters. And it is also found that the extrusion welds slightly affect tensile strength, but markedly influence extrudate elongation.