【作者】 张涛；

【导师】 张英乔； 侯建兵；

【作者基本信息】 中北大学 ， 材料工程（专业学位）， 2019， 硕士

【摘要】 镁合金以其低密度、良好的导热性和良好的可回收性等优点,在航空航天、交通运输、电子工业等领域得到了广泛的应用。但由于其硬度低、耐磨性差、耐腐蚀较差等问题,阻碍了进一步应用。目前,具有快速加热、快速冷却特点的激光熔覆技术常被用于改善镁合金的表面性能。非晶合金由于其独特的长程无序的原子排列结构,具有很多不同于晶体合金的优良性能,为镁及镁合金的表面改性研究提供了新的思路。本论文利用光纤耦合半导体激光器,选用非晶形成能力较强的Zr₆₅A1_7.5Ni₁₀Cu_17.5体系,在AZ91D镁合金表面分别激光熔覆以纯锆粉为Zr元素添加方式的Zr-Al-Ni-Cu（A）粉末和以氢化锆粉末为Zr元素添加方式的Zr-Al-Ni-Cu（B）粉末制备Zr基非晶复合涂层。通过X-ray衍射仪、光学显微镜和扫描电镜对涂层的组织结构和物相组成进行分析研究。通过显微硬度计以及电化学工作站对涂层的显微硬度和耐腐蚀性进行测试分析。在激光熔覆Zr-Al-Ni-Cu（A）粉末的试验过程中,讨论分析了激光工艺参数对涂层表面成型的影响。最佳工艺参数为P=1400 W,v=8 mm/s。涂层由非晶相、α-Zr、NiZr₂和Zr₃O等相组成。其微观结构为α-Zr相弥散分布在非晶基质中,相尺寸可达5.7μm,涂层顶部的结晶程度高于涂层中部。涂层平均硬度为693.9 HV_0.2,约为镁基体的10倍。在电化学腐蚀试验中,非晶复合涂层存在钝化现象。镁基体、涂层中部和顶部的自腐蚀电位分别为-1.585 V、-0.498 V和-0.370 V,自腐蚀电流密度分别为1.444×10^-4A/cm²、8.587×10^-7A/cm²和9.700×10^-8A/cm²,涂层的腐蚀性能远高于镁基体,且耐腐蚀性能随着结晶程度的增加而提高。激光多层熔覆Zr-Al-Ni-Cu（B）粉末制备单层（299μm厚）、两层（454μm厚）、三层（771μm厚）和四层（1008μm厚）的非晶复合涂层。随着层数增加,涂层顶部的晶体相尺寸扩大、数目增加。多层涂层层间结合良好,涂层中0.7μm³μm之间的α-Zr晶体相尺寸明显比Zr-Al-Ni-Cu（A）粉末制备的涂层小。不同厚度的非晶涂层硬度相近,四层涂层平均硬度最高,为714.9 HV_0.2。在电化学腐蚀试验中,多层涂层均存在钝化现象,二层试样、三层试样和四层试样的自腐蚀电位分别为-0.815 V、-0.625 V和-0.555 V,自腐蚀电流密度分别为2.184×10^-66 A/cm²、2.705×10^-66 A/cm²和1.36×10^-66 A/cm²,多层涂层的耐腐蚀性远远高于镁基体,且随着层数增加,结晶程度增大,涂层表面耐腐蚀性提高。通过对比发现,两种Zr基非晶复合涂层物相组成相同,但是Zr-Al-Ni-Cu（A）粉末制备的涂层结晶程度更高,涂层耐腐蚀性更好。更多还原

【Abstract】 Magnesium alloys have been widely applied in aerospace,transportation,electronics industry due to their excellent performances such as the low density,good thermal conductivity and good recyclability.However,some problems,including low hardness,poor wear and corrosion resistance hamper their further applications.At present,laser cladding technology with high power density and rapid cooling rate has been often used to improve the surface properties of magnesium alloy.Amorphous alloys have many excellent properties different from crystalline alloys due to their unique long-range disordered atomic arrangement structure,which provides a new idea for the surface modification of magnesium and magnesium alloys.In this paper,Zr-based amorphous composite coatings were prepared by laser cladding of Zr-Al-Ni-Cu（A）powder with pure Zr powder as Zr element addition mode and Zr-Al-Ni-Cu（B）powder with ZrH₂ powder as Zr element addition mode on AZ91D magnesium alloy surface using fiber-coupled semiconductor lasers.The phase compositions and microstructures of the coatings were analysed and studied by scanning electron microscope and X-ray diffractometer.Microhardness and corrosion resistance of the coatings were measured by microhardness tester and electrochemical workstation.During the experiment of laser cladding Zr-Al-Ni-Cu（A）powder,the influence of laser processing parameters on the formation of coating surface was discussed and analyzed.The optimum process parameters are P=1400 W,v=8 mm/s.The coating consists of amorphous phase,α-Zr,NiZr₂ and Zr₃O.The microstructure of the coating is that the phase ofα-Zr is dispersed in the amorphous matrix,and the phase size can reach 5.7 um.The crystallinity of the top of the coating is higher than that of the middle of the coating.The average hardness of the coating is 693.9 HV_0.2,which is about 10 times higher than the hardness of the magnesium matrix.In the electrochemical corrosion test,the amorphous composite coating has passivation phenomenon.The self-corrosion potential of the magnesium matrix,the middle partof the coating and the top part of the coating are-1.585 V,-0.498V and-0.370 V,and the self-corrosion current densities are 1.444 x 10^-44 A/cm²,8.587 x 10^-77 A/cm² and9.700 x 10^-8A/cm²,respectively.The corrosion resistance of the coating is much higher than that of the magnesium matrix,and the corrosion resistance increases with the increase of crystallinity.Laser multi-layer cladding Zr-Al-Ni-Cu（B）powder was used to prepare amorphous composite coatings with single layer（299μm thick）,two layers（454μm thick）,three layers（771μm thick）and four layers（1008μm thick）.With the increase of layers,the size and number of crystalline phases increase.The interlayer bonding of the multi-layer coatings is good,and the crystal phase size ofα-Zr between 0.7μm and 3μm in the coatings is obviously smaller than that of Zr-Al-Ni-Cu（A）coatings.The hardness of amorphous coatings with different thicknesses is similar.The average hardness of the four coatings is 714.9 HV_0.2.In the electrochemical corrosion test,passivation exists in the multi-layer coatings.The self-corrosion potential of the two-layer,three-layer and four-layer coatings is-0.815 V,-0.625 V and-0.555 V respectively.The self-corrosion current densities are 2.184×10^-66 A/cm²,2.705×10^-66 A/cm² and 1.36×10^-66 A/cm²,respectively.The corrosion resistance of the multilayer coatings is much higher than that of the magnesium matrix,and the crystallinity increases with the increase of the number of layers.The corrosion resistance of the coating surface is improved.It is found that the phase composition of the two Zr-based amorphous composite coatings is the same,but the coating prepared by Zr-Al-Ni-Cu（A）powder has higher crystallinity and better corrosion resistance.更多还原