Industrial Processing of an Al-Zn-Mg-Cu Powder Metallurgy Alloy
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The industrial processing of a commercial 7xxx series aluminum powder metallurgy (PM) alloy was studied in this work. Key aspects considered included direct comparisons of laboratory and industrially processed specimens as well as the implementation of post-sinter operations in an effort to increase the mechanical properties of the material. These included sizing, heat-treatment, and shot peening. For the latter, an automated system was developed capable of applying various peening intensities in a controlled manner. A nominal peening intensity of 0.4 mmN was found to be appropriate. Characterization of industrially processed pucks (100 x 75 x 17 mm) emphasized chemical analyses (bulk and localized measurements), sinter density measurements, tensile testing, fatigue testing, Rockwell and nanoindentation hardness, optical microscopy and SEM. Residual stresses were quantified by x-ray diffraction (XRD) and neutron diffraction (ND) when assessing the near-surface and sub-surface gradients of residual stress respectively. Industrial pucks experienced appreciable losses of Zn via evaporation in sintering. Ultimately, the Zn concentration dropped to 3.1 wt% near surface, before increasing and stabilizing at the bulk composition of 5.6 wt% approximately 3 mm deep into the product. The corresponding through thickness nanoindentation hardness ranged from ≈1.65 GPa at the surface stabilizing to ≈2.50 GPa at a depth comparable to that at which the Zn concentration stabilized. Nominal values for the sintered density (2.74 g/cm3), Young’s modulus (65 GPa), yield strength (459 MPa), ultimate tensile strength (465 MPa) and elongation to fracture (1.0%) were all in-line with previously published results for laboratory processed specimens, attesting to the scalability of the alloy for industrial applications. Peening to an intensity of 0.4 mmN resulted in strain hardening within a surface layer of the material, inducing a maximum compressive residual stress at the surface of 230 MPa, extending to a depth of 60-100 µm prior to transitioning to tensile stresses. Sizing was incorporated within the post-sinter processing sequence to better represent industrial production of geometrically complex parts from the alloy. The metallurgical effects were principally studied through a combination of differential scanning calorimetry (DSC), transmission electron microscopy (TEM), XRD, and 3-point bending fatigue. In certain instances, sizing was applied directly after sintering and prior to the solutionization and aging stages of T6 heat-treatment. In others, sizing was applied as an intermediate step within heat treatment operations, after solutionizing but prior to artificial aging. Respectively, these two sequences were denoted as “Size-Sol-Age” and “Sol-Size-Age” processes. Application of the former yielded a product with a hardness of 85 HRB and fatigue strength of 228 MPa. As both values were well aligned with the properties of unsized T6 samples, it was concluded that sizing had a neutral impact on these particular attributes when applied in this manner. Interestingly, when the “Sol-Size-Age” process was applied, the apparent hardness (78 HRB) and fatigue strength (168 MPa) were reduced to a statistically significant extent. These declines were ascribed to the partial annihilation of quenched-in vacancies that subsequently altered the nature of precipitates within the finished product as supported by DSC and TEM findings. Research also confirmed that the alloy responded well to shot peening, as fatigue strength was increased to 294 MPa. However, thermal exposure at 80°C and 160°C reduced the fatigue performance to 260 MPa and 173 MPa respectively as a result of residual stress relaxation, and in the case of 160°C exposure, in-situ over-aging.