Pre-strain dependent relaxation behaviour of AA6016 during automotive paint drying processes

Abstract

Heat treatable aluminium alloys are widely used in modern car bodies to achieve lightweight cars. Compared to steel, aluminium alloys are more sensitive to creep behaviour under high elastic stresses, since their melting point of about 600 \'03C is much lower. In order to predict permanent deformations of outer shell panels due to thermal stresses in drying processes and ensure highest quality standards regarding appearance to the customer, interdependencies with production process history must be evaluated. In this work, stress relaxation of the aluminium alloy AA6016 in initially T4 temper has been examined within the elastic regime with focus on the dependency of plastic pre-strain during primary creep phase. As a result, pre-strain becomes more important the higher the temperature was set. Hence, the former production process history of aluminium sheet metal parts must be taken into account when viscoelastic deformations are investigated. Based on the test results a pre-strain dependent material model for finite element simulations is pointed out, which can be used for novel deformation analyses of car bodies in automotive paint drying processes that contain aluminium parts out of AA6016

Experimental investigation of multi-step stress-relaxation-ageing of 7050 aluminium alloy for different pre-strained conditions

“…3 (c) presents the total stress reduction in percent during SRA, which gives a direct %SR value under different initial stresses. Generally, the cold-stretching prior to SRA leads to a considerable enhancement in stress relaxation response, which is in agreement with the results reported by Regensburger [22]. The total stress reduction increases from 25.9% to 38.8% was observed when the pre-strain level increases from 0% to 3%.…”

“…The pre-tension generally leads to an increase in the creep resistance for stainless steels while a decrease in that for 2.25Cr-1Mo ferritic steel. Regensburger [22] examined aluminium alloy 6016 and obtained an increased creep strain rate by a factor of ( , where described the equivalent plastic strains, C and k are material constants. Although pre-strain effects have been widely studied, the influence of pre-strains were found to vary between different stress ranges for different alloys, as the microstructure of the alloys vary remarkably due to the increased dislocations in different conditions (e.g.…”

Effect of Prestrain and Tempering on the Residual Stress of Low‐Carbon Microalloyed Steel

“…It is generally believed that among the three factors influencing creep strain, changes in the dislocation density promote stress relaxation, but an increase in the amount of precipitates will pin the dislocations and inhibit stress relaxation . However, Figure a shows that the three instances of the dislocation density increase in the PST specimens correspond to cementite precipitation (300 °C), alloy carbonization precipitation (450 °C), and Mn partitioning (550 °C), and there is a close association between increased dislocation density and carbide precipitation.…”

“…The effects of prestrain on creep are generally described in terms of three aspects: the changes in the dislocation density, size of the precipitates, and volume percentage of the precipitated phase; considering the influence of dislocation density, it can be described bywhere is the normalized dislocation density, ; is the maximum dislocation density that can be formed in the material at 273 K; σ s is yield strength; and A s and k are temperature‐dependent constants, which can be described by an Arrhenius law.…”

“…Zheng et al studied the effect of introducing 1% and 3% prestrain on the stress relaxation of a 7075 aluminum alloy during the aging process and found that for 177 °C and above, the prestrain promoted the interaction between the creep and dislocation and dominated the stress relaxation behavior of the material. Regensburger et al studied the effect of prestrain on the stress relaxation behavior of a 6016 aluminum alloy and found that in the high‐temperature range, prestrain can increase the creep strain and promote residual stress relaxation. In the aging process, the second precipitated phase is the key factor that affects the evolution of residual stress .…”