采用激光熔化沉积技术制备了4045 铝合金薄壁试样，研究了沉积试样热处理前后的显微组织演化，并测试其显微硬度。结果表明，沉积试样显微组织具有定向快速凝固特征，由Al枝晶和Al-Si共晶组成，其一次枝晶间距约22.3 μm，二次枝晶间距约7.5 μm，体积分数约42.7%，沉积试样顶部出现柱状树枝晶向等轴树枝晶转变。当前沉积层从底部到顶部冷却速度的降低导致Si相从珊瑚形貌向片状形貌转变，足够高的重熔率能保证后续沉积层重熔掉当前沉积层顶部片状Si相，以获得变质效果良好的珊瑚形貌Si相，热影响作用使紧邻熔合线过热区的珊瑚形貌Si相发生粗大等轴粒状化，导致其显微硬度低于层内区。珊瑚形貌Si相在540 ℃等温热处理保温20 min 即可完成粒状化，并且随着保温时间延长，Si相发生粗化长大，显微硬度下降。

Thin plate-like 4045 aluminum alloy sample is fabricated by laser melting deposition (LMD). Microstructure and microhardness of the laser melting deposition 4045 aluminum alloy before and after heat treatment are analyzed. Results indicate that a hypoeutectic microstructure comprising about 42.7% (volume fraction) of directionally solidified Al dendrites with the primary dendrite arm spacing of approximately 22.3 μm and the secondary dendrite arm spacing of approximately 7.5 μm is developed during LMD process. The transition from columnar Al dendrites to equiaxed Al dendrites takes place at the top of the as-deposited sample. Coral-like Si develops at the bottom of the depositing layer and changes to flake -like Si at the top of the depositing layer due to the decrease of the cooling rate. Flake-like Si should be sufficiently remelted out during the subsequent layer deposition to obtain well modified coral-like Si. Coarse equiaxed Si develops at the overheated zone just below fusion line due to the spheroidizing and coarsening of coral-like Si, and the microhardness of the overheated zone is lower than that of the inner zone. Coral-like Si can be totally spheroidized after 540 ℃ isothermal heat treatment for 20 min, and Si coarsens and grows up leading to the decrease of the microhardness as the holding time extends.