| The large consumption of energy resources, as a by-product of the increase of car ownership, has made safety, energy conservation and environment protection the theme and orientation of the current automotive industry. As scientifically demonstrated, lightweight technology proves to be an effective solution of the problems mentioned above while the application of lightweight materials in place of steels in the manufacture of car body panels can both reduce car body weight and ensure the safety and comfort of the passengers inside the cabin. Aluminum alloy, with a relatively low density and high strength-to-weight ratio, is an ideal material for the manufacture of car body panels. Nevertheless, the wide application of such material is limited due to its poor formability and the incurring difficulty in the forming of complex component in room temperature. With the rise of temperature, however, the fluxility of aluminum alloy increases and so does its formability. Based on such characteristics, warm forming technology is put forward. Basically speaking, the aluminum alloy blank is heated to a certain temperature, under which circumstance the formability of the blank increases and it is therefore easier to be formed into complex components. Effective as this technology appears to be, there remains one problem as components formed with traditional warm forming technology is subject to resilience. A newly emerged aluminum alloy forming technology, namely the Solution Heat Treatment-Forming-Cold Die Quenching (HFQ), which is put forward on the basis of high strength steel hot stamping technology, can avoid resilience while improve the formability of aluminum alloy and therefore enables the manufacture of complex auto parts.In this paper,7075-T6, a typical ultra high strength aluminum alloy, has been chosen as the research object, with its warm formability under the HFQ technological conditions investigated both experimentally and numerically. Firstly, experiments on the strengthening and heat treatment process of 7075-T6 aluminum alloy are conducted and the influence of heat treatment conditions on its mechanical properties and microstructure is analyzed. The optimum heat treatment technology for 7075-T6 is obtained, which paves the way for the subsequent optimization of the HFQ technology. Based on the HFQ temperature variation route, a thermal-mechanical tensile test is designed and carried out (the temperature is within the range of 25℃-475℃and the strain rate is between 0.001 and 0.1s-1). Based on the stress-strain curve obtained from the experiment and the modified Field-Backofen model, a constitutive model which applies to high-temperature material such as 7075-T6 aluminum alloy is established.Secondly, based on the deep drawing box test, an investigation on the warm formability of 7075-T6 aluminum alloy is conducted with the combination of experiment and numerical simulation. The influence of technical parameters such as initial temperature, blank holding force (BHF) and lubrication conditions on formability is analyzed. As indicated by the experiment and simulation, the formability of 7075-T6 aluminum alloy improves with the increase of the initial forming temperature (25℃-475℃) and hits the peak at 450℃. On the other hand, with the increase of BHF, the formability of 7075-T6 aluminum alloy degrades under the condition of identical temperature. Also, BHF and the blank size are mutually influential, as the maximum drawing depth varies with different combination of BHF and blank size. For the sake of obtaining the optimum formability, BHF and the blank size should be selected with under a comprehensive consideration. This paper provides a theoretical and experimental guidance for the following research of 7000 series aluminum alloy forming at HFQ technology. |
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