No abstract

In hot extrusion, tearing plays an important role on surface quality of profiles. In this study, extrudability of 7075 aluminum alloy without tearing was quantified by extrusion limit diagram, which is a relationship between temperatures and speeds. Tearing appearance and distribution of elements were investigated to assess tearing mechanism. According to the diagram, tearing easily occurrs at high temperature and high speed conditions. Micro-solid bridge was observed at tearing surface to indicate the localization of melting of soluble Al 2 CuMg and MgZn 2 compounds because temperature increased by friction coupled with tension stress at a die region. During hot extrusion, recrystallization affects the grain growth, intermetallic compounds were formed at grain boundaries reducing bonding strength between grains promoting tearing sensitivity. Zinc and magnesium concentration is high at the vicinity of grain boundaries because of diffusion into aluminum-matrix whereas insoluble Al 7 Cu 2 Fe particles were large and remained at grain boundaries resulting in tearing. Grain refinement can decrease a size of insoluble compounds and promote scattering of soluble compounds. Therefore, to reduce tearing sensitivity, grain refinement has to be controlled during hot extrusion process by controlling chemical composition of billet and friction at bearing region. This causes temperature to increases and thus, induce recrystallization.

The precision and accuracy of the final geometry in micro-parts is crucial, particularly for high-value-added metallic products. Micro-extrusion is one of the most promising processes for delivering high-precision micro-parts. The curving tendency observed in micro-extrusion parts is a major concern, significantly affecting the final part geometry. The purpose of this paper was to investigate the driving mechanism behind the curvature in micro-extrusion at room temperature. A finite element (FE) simulation was carried out to observe the influential primary factors: (1) grain size, (2) grain boundary, (3) grain orientation, and (4) bearing length of a 6063 aluminum alloy. The Extrusion Curvature Index (ECI) was also established to indicate the level of curvature in micro-extruded parts. The results showed that the grain boundary at the high strain and die opening area was the dominant factor for single-grain conditions. The interactive effects of the grain boundary and grain orientation also affected the curvature under single-grain conditions. If the number of grains across the specimen increased up to 2.7 (poly-grains), the curvature effect was dramatically reduced (the pins were straightened). For all conditions, the curvature in micro-extrusion could be eliminated by extending the bearing length up to the exit diameter length.

Experiments have been conducted on AZ31B magnesium alloy cutting chips in order to investigate the feasibility of producing a compaction with homogeneous mechanical properties at room temperature using equal-channel angular ECA pressing. Repetitive pressings, consisting of a total of 4 passages through a die at 573 K with and without MoS 2 as a lubricant, were conducted on each sample. After the initial pressing of each sample was completed the samples were rotated 90 degrees and pressed again. This pressing procedure was repeated two more times for all four samples. It was shown that ECA pressing without lubricant was capable of obtaining the compaction with crack-free smooth surfaces after a single pressing. This was most likely because after pressing, a thin, continuous surface layer is created by the heavy shear strain caused by the resulting friction constraint from the die surface. The FEM analysis confirmed the heavy shear strain distribution in the surface layer of the compaction after the single pressing without lubricant. Grain refining after the pressing was observed via the TEM. However, the mechanical properties at room temperature were not homogeneous even after repetitive pressings up to a total of 4 passages through a die. Contrary to the non-lubricated samples, samples that were ECA pressed using lubricant did not display the thin shear strained surface layer. The lack of the distributed shear strain surface layer was most likely caused by a lower coefficient of friction created by the MoS 2 lubricated die. Experimental procedures also concluded that the pressing force required for the lubricated samples was remarkably low in comparison with that for the non-lubricated pressings. However, surface crack formation could not be avoided during the pressings that utilized the lubricated ECA pressing process. It was shown that a combined ECA pressing process consisting of an initial pressing without lubricant followed by repetitive pressings up to a total of 3 passages with lubricant was an effective method for producing a compaction with homogeneous mechanical properties at room temperature under low pressing force.

The recent trend towards miniaturization of products and technology has boosted a strong demand for such metallic micro-parts with micro features and high tolerances. Conventional forming technologies, such as extrusion and drawing, have encountered new challenges at the micro-scale level due to the ‘size effects’ that tends to be predominant at this scale level. Friction is one of the predominant factors exercising strong effects in micro-forming. Previous studies varied grain size of the test pieces in order to examine size effects in micro-extrusion. In addition, the effects on the extrusion load, forming shape, as well as hardness of different grain sizes, die coatings and lubricants were compared. DLC coating has been proven effective as a die coating. Increasing grain size was effective with lubricants having high viscosity. In this study, the effect of different die Properties is compared and examined.