【作者】 王立伟；

【导师】 朱立华；

【作者基本信息】 山东理工大学 ， 材料加工工程， 2025， 硕士

【摘要】 镁锂合金因其密度低、比强度高以及导电导热性好等优点展现出广阔的应用前景,但是其硬度低、耐磨性差以及耐腐蚀差等问题严重限制了其在实际生产中的应用。现有研究已经证实激光表面熔凝(Laser Surface Melting,LSM)和超声喷丸(Ultrasonic Shot Peening,USP)工艺均可以细化表层的组织,提高材料的性能,但是无论是单一还是两种复合工艺对在双相镁锂合金的应用还缺乏系统的研究。因此,研究单一工艺和复合工艺对双相镁锂合金的强化机理具有重要的现实意义。本文对双相镁锂合金分别进行了LSM、USP处理以及LSM-USP复合工艺。详细探究了LSM过程中离焦量、扫描速度以及搭接率,USP过程中的振幅、丸粒直径、喷丸时间以及温度,LSM-USP复合工艺中时间和温度对组织和性能的影响,具体的结论如下:(1)LSM处理的α-Mg和β-Li两相的晶粒明显细化。而且快速冷却过程中α-Mg和β-Li两相均会形成过饱和固溶体。经LSM处理后,两相的硬度均有所提升,其中硬度较高的β-Li相占比更大。由于熔凝层存在较强的细晶强化和固溶强化效果,以及β-Li比重增大这三个原因使其表面硬度和截面硬度均显著提高;这同时提高了试样的耐磨性。LSM试样相比于原始试样耐腐蚀性显著提高,其自腐蚀电位相比于原始试样提高了0.276 V,其容抗弧半径明显大于原始试样。(2)USP使试样表面硬度达到95.45 HV,比原始样品提高了68.9%。WUSP处理进一步细化了双相镁锂合金的表面晶粒。表面硬度达到最大值107.82 HV,比原始样品提高了90.8%,比室温下提高了12.9%;温度的升高会产生“温度的细化效应”和“温度的粗化效应”。这两种效应在超声温喷丸(Warm Ultrasonic Shot Peening,WUSP)同时存在并发挥作用。当温度为150℃或以下时,“温度的细化效应”占主导地位,当温度为150℃以上时,“温度的粗化效应”占主导地位。(3)LSM-USP复合工艺处理后,试样的表面硬度达到最大值124.7 HV,相比单一LSM试样硬度提高了12.8%。单一LSM工艺处理后的试样表面粗糙度为10.38μm,经过80 sUSP处理后,试样的表面粗糙度降低至3.25μm。在LSM中熔凝层与基体之间存在硬度较低的热影响区,由于超声喷丸的影响层深度较大,通过超声喷丸处理,激光熔凝产生的热影响可以被抵消,弥补了激光熔凝的不足,消除了热影响区的不利影响。同时,耐磨性和耐腐蚀性相比原始试样均有提高。更多还原

【Abstract】 Mg-Li alloy shows broad application prospects because of its low density,high specific strength and good electrical and thermal conductivity,but its application in practical production is seriously limited by its low hardness,poor wear resistance and poor corrosion resistance.The existing research has proved that laser surface melting(LSM)and ultrasonic shot peening(USP)can refine the surface structure and improve the properties of materials,but there is still a lack of systematic research on the application of single or two composite processes in dual-phase magnesium-lithium alloys.Therefore,it is of great practical significance to study the strengthening mechanism of dual-phase Mg-Li alloy by single process and composite process.In this paper,the dual-phase Mg-Li alloy is treated by laser surface melting,ultrasonic shot peening and laser surface melting-ultrasonic shot peening.The effects of defocus,scanning speed and overlap ratio in laser surface melting,amplitude,shot diameter,shot peening time and temperature in ultrasonic shot peening process,and time and temperature in laser surface melting-ultrasonic shot peening composite process on microstructure and properties are investigated in detail.The specific conclusions are as follows:(1)The grains ofα-Mg andβ-Li treated by LSM are obviously refined.Moreover,bothα-Mg andβ-Li form supersaturated solid solution during rapid cooling.After LSM treatment,the hardness of both phases is improved,and theβ-Li phase with higher hardness accounts for a larger proportion.The surface hardness and section hardness of the fused layer are significantly improved due to the strong fine grain strengthening and solid solution strengthening effects and the increase ofβ-Li specific gravity.At the same time,the wear resistance of the sample is improved.Compared with the original sample,the corrosion resistance of LSM sample is significantly improved,its self-corrosion potential is increased by 0.276 V,and its capacitive reactance arc radius is obviously larger than that of the original sample.(2)USP makes the surface hardness of the sample reach 95.45 HV,which is 68.9%higher than that of the original sample.WUSP treatment further refines the surface grains of dual-phase Mg-Li alloy.The surface hardness reaches the maximum value of 107.82 HV,which is 90.8%higher than the original sample and 12.9%higher than that at room temperature.The increase of temperature produces"temperature refinement effect"and"temperature coarsening effect".These two effects coexist and play a role in WUSP.When the temperature is below 150℃,the"refinement effect of temperature"is dominant,and when the temperature is above 150℃,the"coarsening effect of temperature"is dominant.(3)During the LSM-USP composite process,the surface hardness of the sample reaches the maximum value of 124.7 HV,which is 12.8%higher than that of the single LSM sample.The surface roughness of the sample treated by single LSM process is 10.38μm,and after80 sUSP treatment,the surface roughness of the sample is reduced to 3.25μm.In the process of laser melting,there is a heat affected zone with low hardness between the melting layer and the substrate.Because the depth of the affected layer is large,the thermal influence caused by laser melting can be offset by ultrasonic shot peening,which makes up for the deficiency of laser melting and eliminates the adverse influence of the heat affected zone.Compared with the original sample,the wear resistance and corrosion resistance are improved.更多还原