Influence of the Initial Temper Conditions and Post-Deposition Heat Treatments on the Friction Stir Deposition of AA7075.

Abstract

Due to the importance of friction stir deposition in many applications such as the repair of surface defects, rib stiffeners, and functional gradient materials for aerospace, biomedical and automotive industries. The present study aims to examine the influence of two different tempered conditions of (T6) and (O) for the as-received material and the post-deposition heat treatment (PDHT) on the mechanical properties and the microstructure of the AA7075 aluminum alloy deposited via additive friction stir deposition technique. To achieve these goals, the FSD experiment was conducted using a 40 mm consumable rod of AA7075-T6 aluminum alloy on a substrate of AA2024-T4 at a constant rotation speed of 400 rev/min and deposition rate of 3 mm/min as a first step to produce continuously multi-layers of AA7075-T6 and AA7075-O. The PDHT was the second step to study the enhancement of the mechanical properties after the deposition process, using two different techniques retrogression re-aging (RRA) and Re-ageing (RA). For the deposited materials the hardness and compression tests as mechanical properties were conducted. In addition, Optical microscopy (OM), scanning electron microscopy (SEM) using an advanced EDS system, and XRD technique were used to analyze the microstructure features. The result showed that after FSD process the hardness of the deposited AA7075-O increased by 25% but the deposited AA7075-T6 attains 90% of the as-received consumable rod. The optimal levels of hardness and compression strength were attained using post-deposition heat treatment of the deposited AA7075-T6 alloy. The resulting average hardness value was measured at 231 HV, while the compressive stress at a strain of 35% was approximately 1142 MPa. The compressive strength of the deposited material was higher than the initial material in both tempered conditions. The microstructure shows a recrystallized grains refinement throughout the deposited AA7075, in which the gain size decreases from 25 µm ± 4 for the as-received material to be 2.06 µm ± 0.51, 1.67 µm ± 0.47 and 4.80 µm ± 1.01 for the DPs AA7075-T6, AA7075-O, and the PDHT, respectively.