Extrusion machining: A thermo-mechanical process for producing strips of alloys having limited workability
Deformation processing of alloys with limited workability is one of the principal technological challenges for the metal manufacturing industry. Traditional processes (rolling, extrusion) are usually performed at high temperatures (0.5 Tm) for increased plasticity and crack-free products, which makes them costly, energy intensive and raw material inefficient. In this thesis, extrusion machining (EM) a shear-type deformation process is applied to alloys with limited workability for sheet and foil production from bulk forms in a single step. EM is shown to have a narrowly confined deformation zone with much wider control of deformation than in conventional deformation processing (e.g., severe plastic deformation). Controllable deformation parameters include strain, strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of two classes of limited workability alloys: 1) Alloys with intrinsic susceptibility to cracking (and segmentation) due to flow localization and shear band formation and 2) Alloys with extrinsic susceptibility to cracking due to macroscopic defects such as porosity, casting defects and weak second phases. Magnesium AZ31 alloy is selected as a model material for the first class and sintered copper, cast brass alloy 360 and cast Mg AZ31 are selected for the second class. Noteworthy features of the process are: suppression of segmentation by combinations of high hydrostatic pressures (p/2k > 1.2) and deformation temperatures (T > 0.5 Tm) realized by in-situ plastic heating, realization of a range of strains and deformation rates, engineering of microstructures ranging from conventional to ultrafine grained, shear type deformation textures (non-basal in the case of Mg AZ31) and creation of sheet from the bulk in a single step of deformation without pre-heating . The thermo-mechanical conditions and microstructural processes (e.g. continuous dynamic recrystallization for Mg AZ31) that are responsible for continuous and sound sheet products are established. Guidelines for scalability and cost-effectiveness of EM for commercial production are analyzed and discussed.
Trumble, Purdue University.
Engineering|Mechanical engineering|Materials science
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