| ABSTRACT:Aluminum alloy6016is age-hardenable with good formability, strong corrosion resistance, high strength and excellent combination of properties, which are suitable for car body sheets with great significance for "lightweight of automobile". But due to the effect of natural aging, the strengthening performance with a low stamping formability (paint-bake response) cannot meet the industrial requirements. In order to improve PBR of6016aluminum alloy to reach the application standard of automobile sheets, in this article, the effect of microalloying (Mn, Cr and Sr joint added or Mn, Cr and Zr joint added) and special heat treatment (interrupted-quenching, pre-aging and restoration) on microstructure and mechanical properties of6016aluminum alloy were investigated, with metallographic observation, differential scanning calorimetry (DSC), scanning electron microscope (SEM), transmission electron microscopy (TEM), hardness test, tensile test and forming performance tests. The main conclusions are as follows:(1) The age hardening and precipitation behavior of aluminum alloy6016were investigated. Both natural aging hardening effect and artificial aging hardening effect were found, but the artificial aging hardening rate of natural aged samples declined. The precipitation process of aluminum alloy should be as following:SSSâ†'clustersâ†'GP zonesâ†'β"â†'β’+Q’â†'β(Mg2Si). The main strengthening phase during paint-baking is the β" phase, and the clusters formed during natural aging is responsible for the negative effect of PBR.(2) The effects of microalloying (Mn, Cr and Sr or Mn, Cr and Zr) on microstructure, age hardening properties and mechanical properties of aluminum alloy6016were investigated respectively. The results show that addition of trace Mn, Cr and Zr or Mn, Cr and Sr elements modifies the constituent particles and refines grains. For the Mn Cr and Zr added alloy, the strength before and after paint-baking were improved but elongation rate was slightly lowered with the effect of dispersed phases, but the active energy for the precipitation of β" nearly remains the same. For the Mn, Cr and Sr added alloy, on one hand, the base alloy was strengthened by Mn, Cr contained dispersed phases. On the other hand, the addition of Sr modified the Fe-rich particles and lowered the avtivation energy for precipitation of β", finally improved the mechanical properties and matained the plasticity.(3) The effect of the holding temperature and time of interrupted-quenching(I.Q), pre-aging(P.A) and restoration(R.T) treatments on paint-baking response were investigated and they were optimized as100℃×1h for I.Q,160℃×5min for P.A and220℃×5min for R.T, PBR for these treatments are:I.Q>P.A>R.T. During artificial aging for ordinary sample, the slowly disolving of clusters((â… ) resulting from natural aging responsible for the negative effect of PBR. For interrupted-quenched sample, a large number of Mg-Si clusters(â…¡) formed during interrupted-quenching were stable in natural aging and could act as nuclei of β", which decreases the activation energy of β", a high dense of β" precipitates are found after paint-baking. For pre-aged sample, a large number of similar Mg-Si clusters(â…¡) formed during pre-aging, but the density of the nuclei was lower than I.Q sample due to the growth of β" phase during artificial aging. While for reversion-treated sample, except for Mg-Si clusters(â…¡), a small amount of Pre-β" and β" precipitates were formed during reversion treatment, which decreases the density of the β" phase nuclei, thus leaded to a lower PBR.(4) Combining microalloying with special heat treatment further improved the paint-bake response. Pre-aged at160℃for5min, the mechanical properties and formability of Mn, Cr, Zr added alloy all met the industrial requirements, especially its PBR reached120MPa. |
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