Muhammad Amir

Improvement in Ductility in Commercially Pure Titanium Alloys by Stress Relaxation at Room Temperature

Present work focusses on the effect of stress relaxation on the tensile behaviour of two commercially pure titanium alloys of different strength levels (Grade 1 and Grade 4) subjected to tensile tests at room temperature. The stress relaxation tests were performed by interrupting the tensile tests at regular strain intervals of 5% in the plastic region of the tensile curve and compared to the monotonic tensile tests at different strain rates ranging from 10-4 to 10-1s-1. To understand the effect of anisotropy, samples were taken along 0° and 90° to rolling direction (RD) for both the alloys. Improvement in ductility of different levels at all the strain rates was observed in both the alloys when stress relaxation steps were introduced as compared to monotonic tests. However there is not much change in the flow stress as well as in strain hardening behaviour of the alloys. The true stress-true strain curves of Grade 4 samples taken in 90° to RD exhibited discontinuous yielding phenomenon after the yield point, which is termed as a yield-point elongation (YPE). The improvement in ductility of the Cp-Ti alloys can be linked to recovery process occurring during the stress relaxation steps which resulted in the improvement in ductility after repeated interrupted tensile tests. The paper presents and summarise the results based on the stress relaxation for the two different alloys.

Design and validation of a fixture for positive incremental sheet forming

Incremental sheet forming is an emerging manufacturing technique in which sheet metal is formed into desired shape through the application of localized force using a hemispherical tool. Potential advantages of the process are its relatively low cost and small lead times, and these have to be balanced against the disadvantages of low dimensional accuracy and a limited understanding of the process’ internal mechanics. Incremental sheet forming system can be classified as positive, or negative, depending on whether the sheet material is progressively deformed onto a protrusion or a cavity. In negative systems, the work piece is held on a static fixture; whereas, in positive incremental sheet forming, the fixture must be incrementally lowered onto a protruding die. Although the vertical movement of positive incremental sheet forming fixtures is easily illustrated schematically, its implementation is challenging; if the descent is actuated, the motion has to be carefully coordinated with those of the forming tool; if free sliding on vertical columns, the rig must move without jamming or rocking. This article reports the development and testing of a positive incremental sheet forming fixture design that uses nylon sleeve bushes. Symmetric and asymmetric components were formed using the designed fixture, modular wooden dies and a rotating tool with multiple diameters and the results are discussed.