Investigation Of The Billet Skin Flow Behavior And Butt Discard Microstructure In The Hot Extrusion Process Of Aluminum Alloy

Aluminum alloy profiles have been widely used in aeroplane manufacture, rail transportation, automobile industry and architecture etc. for their superiority in specific strength, corrosion resistance, oxidation resistance, processability, thermal conduction, recycling and other good performance. Hot extrusion process is the main manufacturing technique of the aluminum alloy profiles. And low utilization rate of materials is one of the major problems bothering aluminum alloy extrusion industrial.At the end of extrusion of each billet in continuous extrusion process, billet skin such as lubricants, oxide skin and coarse grain is prone to be extruded into profiles forming back-end defects, which serious damage the metal continuity and the quality of the aluminum profile. To avoid back-end defects, there are two common methods are now adopted. First one is to remove billet skin by turn-milling technology or paring dies, both of which are time and energy consuming, expensive and more applicable to profiles requiring higher quality. The second one is to retain a certain length of the extruded butt discard, which is time-saving, low-cost and applicable to all extrusion products. Therefore, the second method is widely applied in actual production. Apparently, the longer the butt discard is, the lower production efficiency and material utilization ratio are. Minimum length of extruded butt is closely related to billet skin metal flow behavior in extrusion process. Thus the study of billet skin flow behavior, as well as evolution principles, microstructure and composition distribution of butt discards, are significantly beneficial for accurately predicting the minimum length of butt discard, improving production efficiency, controlling quality of profiles and reducing the material loss in aluminum alloy profile extrusion process.The objective of this paper is to reveal the mechanism of metal flow behavior of billet skin and formation of butt discard in aluminum alloy hot extrusion. Hot compression experiments of 2A12 aluminum alloy was firstly conducted for the establishment of Arrhenius constitutive model and hot processing map. Microstructure and crystallization of 2A12 aluminum alloy ingot before and after extrusion was analyzed as well. The billet skin tracking 3D numerical model of 2A12 aluminum alloy rod extrusion process was built to study the influence of billet size, billet skin thickness, billet temperature, container temperature, die temperature, pad velocity and pocket size on billet skin flow behavior of. Two billet skin flow model during extrusion process of aluminum alloy solid profile were defined. The influence of above factors on the minimum butt discard length was revealed.Comparing with metal flow behavior in extrusion process of aluminum alloy solid profile, the metal behavior of the hollowed one is more complex. Thus the billet skin tracking 3D numerical model of 2A12 aluminum alloy square pipe extrusion process was built afterwards to study the influence of porthole axial angle, welding chamber height and bridge width on minimum butt discard length, maximum extrusion load and standard deviation of velocity in bearing exist. Two metal flow model of billet skin during aluminum alloy hollow profile extrusion process were defined as well. The influence of above three factors on minimum butt discard length, maximum extrusion load and standard deviation of velocity in bearing exist was finally summarized.Three sets of pothole extrusion dies with different structure were designed and manufactured, for corresponding extrusion experiment producing 6063 aluminum alloy plate-shaped profile. Three butt discards with different lengths were abstained. And macroscopic defects, microstructure and chemical composition on the vertical section of the butt discards were detected and analyzed. Metal flow pattern, microstructure evolution and recrystallization were studied. In order to better explain the experiment result with thermal force field, strain field and velocity distribution data, three corresponding billet skin tracking 3D numerical simulation model were built and calculated. With comparison between experimental result and simulation result, the influence of physical field quantities in butt discard on microstructure evolution and principles were revealed. A reasonable method to predict the minimum butt discard length is presented.