添加碳氢化合物与硼硅对激光金属成形过程及其性能的影响

Laser remanufacturing is an important application and development direction of laser metal forming technology. The key of improving the forming quality under ambient environment is to reduce/control the oxidation, burning loss, slag inclusions and cracks during the laser metal remanufacturing process. A brand-new method for the laser remanufacturing special powder fabrication by using the common stainless steel powder with the trace elements boron and silicon addition coated by the hydrocarbon film is proposed in this project. By means of the experimental and numerical methods, the effects of the reducing atmosphere generated by the sub-micro hydrocarbon film combustion in the laser molten pool on the oxygen content and mechanical properties of the laser forming specimens are investigated. The relationship among the process parameters such as the thickness of coating, the oxygen content and mechanical properties of the laser forming specimens are established. The reaction process between the trace elements boron-silicon and the oxide, together with the movement rule of the low melting-point borosilicate slag in the laser molten pool are studied. The influences of the trace elements boron-silicon addition on the formation mechanism of defects such as inclusions and pores produced in the laser forming process as well as the mechanical properties of the specimens are clarified. The precipitation of the trace elements boron-silicon in the grain boundary of the formed specimens and the effects on the microstructure and mechanical properties are researched. A new method to prevent the element burning loss, oxidation, slag inclusions and cracking effectively in the laser metal remanufacturing process are discussed in the project, which will provide the basis for the design of the special alloy powders for laser remanufacturing.

激光再制造是激光金属成形技术的重要应用和发展方向，在大气环境下减少激光再制造过程中的合金元素氧化、烧损与夹渣并控制开裂是提高成形质量的关键。本项目提出用碳氢化合物膜包覆添加有微量硼硅元素的不锈钢基粉末作为激光再制造专用粉末的新思路，并利用实验和数值方法研究金属粉末外包覆的亚微米厚碳氢化合物膜在激光熔池中燃烧产生的还原气氛对激光成形试样含氧量和力学性能的影响，沟通包覆膜厚度等参数与成形试样含氧量、力学性能之间的关系；研究微量硼硅与激光熔池中氧化物的反应过程及其生成的低熔点硼硅系熔渣在熔池中的运动规律，阐明添加微量硼硅对激光成形过程中夹渣、孔洞等缺陷形成机制的影响规律，并沟通其与试样机械性能的关系；研究微量硼硅元素在成形试样晶界中的析出规律及其对微观组织和力学性能的影响。探讨在大气环境下减少激光金属再制造过程中元素烧损、氧化、夹渣与裂纹的新方法，为设计激光再制造专用合金粉末材料提供依据。

大气环境下减少激光再制造过程中的元素合金氧化、烧损与夹渣并控制开裂是提高成型质量的关键。利用自制的微膜包覆装置制备了有机物膜厚约160nm的包覆铁合金粉末，数值仿真表明：金属粉末外包覆的有机物膜可将激光熔池表层氧浓度降到环境氧浓度的20%；单道熔覆层的中心温度最高达到2287.7℃，固液边界线附近的温度为1738℃，熔池液相区最长存在时间约为0.5s，当扫描搭接率为50%时搭接区的熔渣有最大的运行速度、最充足的运行时间上浮至熔池表面，熔池有最好的造渣性能。当搭接率低于20%时，搭接区的熔渣上浮运动的时间小于0.25s，激光熔池凝固后会出现搭接夹渣现象。激光成型实验表明：有机物微膜包覆处理可使合金中C、B、Si元素的烧损减少50%以上；多试样抗拉强度方差下降30%以上；单位面积缺陷率下降60%以上。添加的B元素对激光熔池造渣、抗氧化、显微组织和力学性能有强烈的影响，B含量大于0.5%时，高温氧化形成的玻璃态硼硅酸盐可明显阻止熔池氧化，提高造渣性能，但显微组织中会在界面析出成片分布的硬质硼化物相，抗拉强度明显上升，塑韧性急剧下降。铁基合金激光成型熔池冷凝过程中的马氏体相变对残余应力、开裂行为与力学性能有巨大的影响，当出现马氏体相变时，激光熔覆层的残余应力为压应力，可完全避免熔覆层开裂，其硬度和抗拉强度明显上升，研究发现：C含量为0.26%至0.3%的马氏体激光成型样低温时效（210℃-300℃保温两小时，空冷）可保证其断后延伸率不低于8%，抗拉强度不低于1800MPa。基于以上的研究结果，设计了四种激光成型铁基粉末材料，一种是高硬度不开裂的铁基合金粉末，其硬度可达HRC62-64，成型时基材不预热，厚度可达10mm；一种是高强度A/F型不锈钢粉末，其激光成型试样抗拉强度940-1040MPa，延伸率20%以上；一种是高强度M/A型不锈钢粉末，其激光成型试样抗拉强度1400-1500MPa,延伸率10%以上；一种是超高强韧M/A型不锈钢粉末，其激光成型试样低温时效（210℃-300℃保温两小时，空冷）后抗拉强度1800-1900MPa,延伸率8%以上，并进行了应用示范，展示出巨大的工业应用前景。

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