基于厚向组织性能考量的7B50铝合金中厚板回归再时效热处理

Retrogression and re-aging 7B50 Al alloy plates based on examining the through-thickness microstructures and mechanical properties

为解决T6态高强铝合金强度高而耐蚀性难以满足使用需求,采用三级时效工艺来改善析出强化相特别是晶界析出相的形貌、尺寸、分布等,并通过研究不同回归处理制度对组织、性能的影响而获得适宜7B50铝合金中厚板的三级时效工艺.研究发现提高回归温度或延长回归时间均会使中厚板心部及表层组织的晶内和晶界析出相发生粗化并析出稳定η-Mg Zn2相,导致强度下降、电导率上升,其中回归温度对强度和电导率的影响显著.三级时效处理虽使晶内析出相尺寸有所增加,但却使T6态连续分布的晶界析出相呈断续分布,结合心部和表层强度及电导率测量结果认为合适的回归处理制度为165℃/6h.然而,热轧引起中厚板表层较心部更为严重的变形使表层含有更多的亚晶或亚结构且其分布更均匀,从而使表层更快到达峰时效,进一步的回归再时效处理则使表层析出更多稳定η相,而η相的形成与晶内析出相的粗化长大是造成表层和心部强度差异的关键.虽然淬火/三级时效态表层和心部的晶粒结构存在差异,且局部出现亚晶合并长大,但其对强度的提升效果远低于表层析出稳定η相所引起的强度下降.可见,三级时效工艺并不能缓解7B50铝合金中厚板心部和表层的性能差异,但可使表层和心部的强度、电导率满足某实际工况要求.

For enhancing the corrosion resistance of the T6-aged high-strength Al alloys with higher strength,retrogression and reaging（ RRA） treatments were used to optimize the morphologies,sizes,distribution of precipitates,especially grain boundary precipitates（ GBPs）. The effects of different retrogression treatments on the microstructures and mechanical properties were studied so as to gain suitable RRA process for 7B50 Al alloy plates. It is found that increasing the retrogression temperature or time will promote the coarsening of transgranular and intergranular precipitates in the center and surface layers of 7B50 Al alloy plates as well as the precipitation of stable η-MgZn2 phase,which will decrease the strength and raise the conductivity. The retrogression temperature will greatly affect the strength and conductivity. The continuously distributed GBPs induced by T6 aging become discontinuous after RRA treatment,accompanying with slightly increasing sizes of transgranular precipitates. Based on the strength and conductivity of the center and surface layers,165 ℃/6 h is the suitable retrogression process for 7B50 Al alloy plates. However,the severe deformation of the surface grains compared to that of the central grains caused by hot rolling leads to a higher content of subgrains or substructures in thesurficial grains,which promotes the surface layer to quickly reach the peak aging,and the subsequent retrogression treatment results in much more stable η phase in the surface layer. The formation of stable η phase as well as the coarsening or growth of transgranular precipitates could be mainly responsible for the strength difference between the surface and center layers. Although there are some differences about the grain structures between the surface and center layers after quenching/RRA treatments with some local subgrain growth,the positive impact of RRA treatment to the strength is apparently unable to compare with the obvious strength reduction caused by early precipitation of stable η phase in the surface layer. Thus,the RRA treatment cannot relieve the property difference between the center and surface layers of 7B50 Al alloy plates,but it can make the strength and conductivity of the center and surface layers to concurrently meet some working requirements.