Processing of aluminium and titanium alloys by severe plastic deformation

Abstract

Severe plastic deformation (SPD) techniques are applied to polycrystalline materials to significantly refine their grain sizes which lead normally to an enhancement of the material properties. Equal-channel angular pressing (ECAP) and high pressure torsion (HPT) were considered as very important SPD techniques. Very high deformation is applied to the samples in ECAP and HPT which refines the grain size and enhances the properties of materials.
In the present research, the evolution of homogeneity in microhardness
measurements in the aluminium alloys was investigated after processing by ECAP and HPT. In the ECAP experiments, billets of Al-1050 alloy were processed by ECAP at room temperature using route BC for up to six passes. The results of the microhardness measurements show that the hardness increases significantly after the first pass and continues to increase by smaller amounts in the subsequent passes. There are regions of lower hardness near the top and bottom surfaces of the billet after one pass. The hardness of the alloy becomes homogeneous along the longitudinal and cross-sectional planes after six passes of ECAP except in a very small region with lower hardness near the lower surface. Better results were achieved when processing the same material by HPT. The HPT experiments were conducted at room temperature at a pressure of 6 GPa and for up to five turns. The hardness was completely homogeneous across the entire surface after five turns of HPT. A comparison between the results performed by processing Al-1050 alloy by ECAP and by HPT shows clearly that better homogeneity and higher hardness was achieved by using the HPT process. Another aluminium alloy (Al-1%Mg) was processed by HPT under similar conditions of processing as for Al-1050. The results illustrate that there is still a small region of lower hardness at the centre of Al-1%Mg disk after five turns. It was clear that the Al-1%Mg alloy needs more than five turns under the pressure of 6.0 GPa to become totally homogeneous.
A series of [114] convergent beam electron diffraction (CBED) zone axis patterns were obtained at 148.7 kV with a 20 nm diameter electron probe from dislocation-free regions close to and away from the grain boundaries in the billets processed by ECAP through two and four passes after cooling to 80 K. The results show that there were no detectable strains in the centre of the grains of both billets. Near the grain boundaries, compressive and shear strains were detected. The compressive strain was constant in both billets with a value of ~0.1% and the shear strain at the two passes sample was ~0.044% and increased at the four passes sample to reach a value of ~0.177%. Similar patterns from the billet processed through eight passes were difficult to be analyzed due to the higher dislocation density which led to a blurring of the HOLZ lines.
Processing of commercial purity titanium alloy by ECAP at room temperature
proved the feasibility to perform this when the die channel angle increased and the pressing speed decreased. The experiments were performed at an angle of 135 deg and pressing speeds of 0.5 and 0.05 mm/s. A maximum of one pass were reached by processing under the speed of 0.5 mm/s whereas two passes were successfully performed under the speed of 0.05 mm/s without experiencing any visible cracks in the billet.
Finally, HPT experiments were performed monotonically (m-HPT) and cyclically (c-HPT) on two aluminium alloys (Al-1050 and Al-1%Mg) and two titanium alloys (CP Ti and Ti-6Al-4V). The results show that the rate of the evolution towards the homogeneity of microhardness along the diameter of the disks in the aluminium alloys is higher when the disks are processed by m-HPT rather than processing by c-HPT. Opposing results were found at the titanium alloys where the rate was higher when the disks are processed by c-HPT rather than processing by m-HPT.

More information

Identifiers

Catalogue record

Export record