被誉为“21世纪绿色工程材料”的镁及镁合金已成为继钢铁和铝材料之后的第三大金属工程应用材料, 但较差的硬度、耐磨损性及耐腐蚀性在一定程度上制约了它的广泛应用。采用最大输出功率5 kW的HUST-JKT5170型横流CO2气体激光器对AZ31B镁合金表面进行激光熔凝处理, 同时对熔凝层的正面和背面进行液氮极速冷却, 对该条件下的冷却速度进行计算, 并分别从显微组织、物相、显微硬度和腐蚀性能等方面对比分析了原始镁合金和极速冷却熔凝层。研究结果表明: 激光+液氮的冷却条件下, 熔化层的冷却速度v＝324.9 K/s; 熔化层冷却并凝固后没有裂纹、气孔等缺陷。微观上, 极速冷却的熔凝层的晶粒大小远小于原始镁合金, 且晶界已不明显; 与母材的过渡处微观组织呈现典型的白亮的平面晶形态, 从底部到顶部, 熔凝层的晶粒尺寸逐渐减小; 熔凝层主要由α-Mg相组成, 而β-Mg17Al12相的衍射峰强度相对于原始镁合金极大地弱化。主要受细晶强化作用, 极速冷却的熔凝层的显微硬度提高到140 HV0.05, 是原始镁合金(约50 HV0.05)的2.8倍。在质量分数为3.5%的NaCl(pH＝7)溶液中的电化学腐蚀发现, 该极速冷却条件下获得的熔凝层的腐蚀电流为3.889×10-4 A, 高于原始镁合金的1.470×10-5 A, 且腐蚀电位为-1.483 V, 低于原始镁合金的-1.438 V, 说明该极速冷速条件下熔凝层的耐腐蚀性能略差于原始镁合金。

Magnesium and magnesium alloy considered as “green engineering materials in the 21st century” have become the third metallic engineering applying materials after steel and aluminum materials, however bad hardness, wear resistance and low corrosion resistance have limited the application of magnesium alloys in a way. In this paper, laser surface melting with liquid nitrogen cooling condition was used on the surface of AZ31B by HUST-JKT5170 CO2 gas laser with the maximal power of 5 kW, the cooling rate under this condition was calculated, the microstructure, phase, microhardness and corrosion property of as-received magnesium alloy and laser melted layer with extreme cooling rate were analyzed. The results showed that: under the condition of laser melting and liquid nitrogen cooling, the cooling rate was v＝324.9 K/s, and there was no crack and pore in the melted layer. The grains of melted layer were smaller than that of as-receive AZ31B, and the crystal boundary was not clear. The transitional part was typical and white plane crystal, and the grains decreased gradually from toe to top of the melted zone. The melted layer was mainly consisted of α-Mg, but the β-Mg17Al12 was reducted enormously. The microhardness of melted layer raised to 140 HV0.05 which was 2.8 times than that of as-received AZ31B mainly because of fine-grained strengthening. The corrosion current of melted layer was 3.889×10-4 A which was higher than as-received AZ31B of which the corrosion current was 1.470×10-5 A, and the corrosion voltage was-1.483 V which was lower than as-received AZ31B of which the corrosion voltage was-1.438 V, so the corrosion property of the melted layer was worse than that of as-received magnesium alloy.