Effects of Pre-Strain on the Aging Behavior of Al 7075 Alloy for Hot-Stamping Capability

Abstract

Abstract:The present study investigates the significance of pre-strain on the T6 aging behavior of an Al 7075 alloy for evaluating the applicability of hot stamping. In practice, the alloy was pre-strained up to 15% during solution heat treatment at 480 • C prior to quenching, and artificial aging was conducted at 120 • C. The peak aging time and precipitation behavior were compared with the alloy with pre-straining at room temperature after quenching but immediately before the artificial aging. The results showed that increasing amounts of pre-strain tend to reduce the aging time up to 50% for achieving peak hardness, which is consistent with the alloy at the T6 condition. There is a limitation for the maximum attainable amount of pre-strain of 10% for the homogeneous distribution of strain when the alloy is strained at room temperature (RT) due to the low formability. The pre-strained alloy as hot stamping exhibited lowering of the peak reaction temperatures for dissolution and formation of Guinier-Preston (GP)-Zones and precipitated with increasing amounts of pre-strain towards 15% through the differential scanning calorimetry analysis, thereby confirming the shortening of the peak aging time. The present study confirms the excellent potential of the hot-stamping process to extend the capability of an Al 7075 alloy.

Influence of Hot Deformation on the Precipitation Hardening of High-Strength Aluminum AA7075 during Thermo-Mechanical Processing

“…The plastic deformation is accompanied by a high dislocation density and crystal lattice defects inside the material, which can induce changes in the precipitation sequence and canaffect the final material strength after the aging treatment [27,28]. Jung et al investigated the effect of plastic deformation at different temperatures on the aging behavior of AA7075 [29]. The experimental results in this study showed an increase in the mechanical properties of the material formed at elevated temperatures with the increase in the deformation level.…”

“…Several imperfections in the crystal lattice structure, e.g., high concentration of vacancies, as well as a high dislocation density, can activate the formation of precipitates. Both can act as a nucleation source and sink, enhancing the precipitation nucleation rate by an extensively high diffusion rate of solutes within the grain interior [23,29,48,49]. However, by comparing all emulated conditions, the obtained material hardness of the hot-formed microstructures after an artificial aging at 120 • C for 20 h indicated no dramatic or significant differences.…”

Optimizing Heat Treatment Parameters for the W-Temper Forming of 7xxx Series Aluminum Alloys

“…On the contrary, pre-straining the samples introduces a high dislocation density [10], leading to heterogeneous precipitation. The time to peak hardness in the pre-deformed samples as compared to the undeformed material, which is ascribed to an increased number of nucleation sites and an enhanced diffusivity, leading to faster precipitation [27]. Peak hardness values of 121 HV0.5 after 42 h and 118 HV0.5 after 27 h were achieved for the undeformed and pre-strained samples, respectively.…”

“…It was possible to restore the as-received T6 hardness of 190 HV0.5 for all samples, without and with pre-deformation. The time to peak hardness is 24 h without pre-deformation and it is reduced to 15 h by pre-straining the material, indicating faster precipitation due to an increased number of nucleation sites and an enhanced diffusivity [27].…”

Hot Sheet Metal Forming Strategies for High‐Strength Aluminum Alloys: A Review—Fundamentals and Applications

“…In another study, [ 179 ] the microstructural evolution in AA7075 after hot deformation at different degrees of deformation is presented ( Figure a,b). The material was soaked at 480 °C and strained from 0% to 15%, followed by water quenching to room temperature.…”