Pressure welds occur in extruded aluminium profiles either as longitudinal welds (seam welds) or as transverse welds (charge welds). While longitudinal welds only occur in hollow profiles, transverse welds always occur when the billet is extruded on top of a metal rest left in the die from the previous extrusion, i.e. in extrusion of hollows as well as in billet-to-billet extrusion, and in pre-chamber extrusion of solid profiles. Longitudinal welds are formed as the metal streams of different die ports gather behind each die-bridge and join. They extend longitudinally through the profile in the location behind each die web. Transverse welds form as the front end face of each billet pressure welds to the sheared-off back-end face of the extrusion residue left inside the die from the previous extrusion. Transverse welds commonly have a tongue-like shape inside the resulting extrusion and often extend over a large length of the extrusion. Since extrusion welds are formed by solid pressure-welding, they are produced with mechanical properties corresponding to the rest of the profile. Unlike fusion welding, there are no filler metal or soft heat-affected zones. But in spite of this, problems in terms of poor weld quality sometimes occur in industrial production of aluminium profiles. Extrusion welds of insufficient quality contain no voids or cavities and could, therefore, be difficult to detect by non-destructive tests as ultrasonic or x-ray inspection. Since extrusion welds defects are often unfavourable conditions in the extrusion process, they can to a large extent be eliminated through corrective measures taken by the extruder. This paper reviews the existing knowledge, as regards formation of extrusion welds, and defects in such welds. Preventive measures that can be made by the extruder to avoid formation of such defects are also discussed.

Extrusion welding in aluminium extrusion

An equiatomic FeCoCrNi high-entropy alloy with a face-centered cubic structure was fabricated by a powder metallurgy route, and then processed by high-pressure torsion. Detailed microscopy investigations revealed that grain refinement from coarse grains to nanocrystalline grains occurred mainly via concurrent nanoband (NB) subdivision and deformation twinning. NB-NB, twin-NB and twin-twin interactions contributed to the deformation process. The twin-twin interactions resulted in severe lattice distortion and accumulation of high densities of dislocations in the interaction areas. With increasing strain, NB subdivision and interactions between primary twins and inclined secondary stacking faults (SFs)/nanotwins occurred. Secondary nanotwins divided the primary twins into many equiaxed parts, leading to further grain refinement. The interactions between secondary SFs/nanotwins associated with the presence of Shockley partials and primary twins also transformed the primary twin boundaries into incoherent high-angle grain boundaries.

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Dual mechanisms of grain refinement in a FeCoCrNi high-entropy alloy processed by high-pressure torsion.

An upper bound solution of ECAE process with outer curved corner

Microband formation in cyclic extrusion compression of aluminum

The effects of cyclic extrusion and compression (CEC) processing on the microstructure and mechanical properties of ZK60 Mg alloy were investigated at room temperature. ZK60 alloy was dramatically refined by CEC processing at 350°C. In addition, the initial fiber texture became disintegrated and changed into a {1013}(3032) + {1011}(1543) texture. Due to microstructure refining and texture variation, the compressive yield stress of ZK60 alloy increased remarkably while the tensile yield stress decreased slightly. As a result, the intensity of strength-differential effect (SDE) of ZK60 alloy was decreased noticeably. The ductility of ZK60 alloy also enhanced, especially under compression test condition.

J. Zasadziński

Papers

Effect of large deformations on the microstructure of aluminium alloys

Weld quality in extruded aluminium hollow sections

Dobór warunków zgrzewania trudnoodkształcalnych stopów aluminium przeznaczonych do wyciskania kształtowników na matrycach mostkowo-komorowych

Prognozowanie parametrów siłowych wyciskania na gorąco aluminium i jego stopów.

Analiza numeryczna procesu wyciskania trudno odkształcalnych stopów aluminium w zakresie małych współczynników wydłużenia