【作者】 李进；

【导师】 符永宏；

【作者基本信息】 江苏大学 ， 机械工程（专业学位）， 2017， 硕士

【摘要】 激光表面织构技术用于改善摩擦副间摩擦学特性,可以对机械零部件表面起到减磨、增寿效果。在众多表面织构技术中,激光织构技术因其诸多独特优势得到更加广泛关注,在轧辊、磁盘、机械密封、模具、刀具、内燃机缸套、轴承等都有应用。但是,目前主要研究更多集中在激光织构中的微凹坑（槽）研究,对毛化（凸体）织构的理论、试验研究尚缺少系统性、创新性。不同材料、相同材料应用场合存在差异,应用差异导致对工件摩擦副要求不同,因而对毛化形貌、尺寸、毛化点分布规律都会提出不同要求。鉴于此,本文以金属塑性成型模具的工程应用为目标,对45钢、Cr12模具钢两种常用材料进行了系统、带有创新性的激光毛化数值模拟与试验研究,并对激光微凹坑加工工艺进行试验研究。主要研究内容及结论如下。（1）基于激光与金属物质相互作用机理,利用FLUENT工具软件建立激光毛化三维数学模型。探究脉宽、激光功率密度对45钢、Cr12模具钢温度场、流场的影响。结果表明:两种材料熔池流动方向均是从四周流向中心,形成凸起。45钢在脉宽500us及其更大脉宽下,熔池中心最高温度超过沸点,会有材料气化,该结论得到试验验证:脉宽小于500us形成球冠状,500us及更大脉宽形成凹顶球冠状。设定功率密度不变,45钢熔池直径、深度增加率,随着脉宽增大均呈现出先大后小变化规律。通过分析流场、温度场发现,大温度梯度可以促进熔池快速生长,温度梯度变小,熔池生长速率变缓,该结论得到试验验证:毛化点直径、高度增大率从大到小转折点为脉宽500us。Cr12与45钢因材料性质不同,导致液体对流存在些微差异,模拟出Cr12熔池深径比大于45钢,两种材料毛化点剖面金相图验证了该模拟结果;两种材料温度场结合毛化点金相进行分析,从内到外分为气化区、熔凝区、热影响区、基材区,并对各区金相组织、硬度进行检测分析,毛化点相对基材硬度大幅提高。（2）对毛化形貌进行定量表征。引入推衍参数凹陷率T、内外径比Pb。球冠状T为0;凹顶球冠状0<T<0.33;M状0.33≤T≤1;火山口状T>1,0<Pb<0.83。并给出各个形貌凸起、凹陷体积近似计算公式,V凸起体现毛化点耐磨性,V外凹、V内凹体现毛化点存储润滑油和磨粒的能力。（3）依据模拟计算给出的结果,采用光纤激光器进行毛化工艺试验研究。45钢在选用参数内,加工出球冠状,以及因材料气化并因此诱发的反冲压力,加工出凹顶球冠形貌。Cr12因铬元素沸点较低,且含量较高,因熔池中心材料气化以及因此诱发的反冲压力,在选用参数内加工出M状,因等离子体屏蔽作用在选用参数内加工出凹顶球冠状。（4）在激光毛化中发现该激光器在低功率密度1.02*104W/mm2以下打出高质量微凹坑,对该奇怪现象做出初步分析,并对微凹坑加工工艺进行系统探索。（5）探究不同辅助气体,不同压力下,毛化形貌及尺寸变化情况。以N₂作为辅助气体,45钢在所给加工参数内,依次出现M状、凹顶球冠状、球冠状;Cr12全部形成凹顶球冠状。此时两材料毛化点直径,相比未加辅助气体情况下直径变大,高度变小。以O₂作为辅助气体,0.05MPa气压下,45钢毛化形貌无规律可循,Cr12毛化点在M状和火山口状中随机出现;0.1MPa下,两种材料熔池均被吹散,无法形成毛化点。（6）从改变激光吸收率出发,在加工表面涂敷一层水玻璃,探究毛化形貌及尺寸变化规律。该涂层下,两种材料毛化形貌高径比均要大于未涂层时的高径比。（7）在工件侧面加磁铁,希望加工出偏向一侧毛化点。两种材料作为铁磁体材料,当温度超过它们居里温度成为顺磁体,此时磁场无法对液体金属产生影响,所以毛化形貌无变化。（8）因为活性元素S对熔池对流产生影响,在加工表面涂敷一层直径5um硫粉进行激光毛化。未出现预想中火山口形貌,发现形貌成为椭球型,且各个毛化点之间长短轴不一,该现象从试验上证实了S元素确实对金属液体对流有影响。更多还原

【Abstract】 Laser surface texturing technique is used to improve the friction between the friction pair which can reduce wear of mechanical parts and increase longevity.Among several surface texture technique,laser texture is applied more because of many advantages,roll,disk,mechanical seal,mold,tool,engine cylinder,bearings are all applying the technique.Previous works more concentrated in the dimple,the theory and experimental researches on bulge are short of systematicness and innovation.Different materials or same material applications are different,these differences caused requirements of friction pair are different,so the bulge morphology,size,distribution are different.In view of this,with the aim of engineering application,numerical simulation and experimental study on 45 steel,Cr12 steel were carried out.The main researches and conclusions are as follows:（1）The three-dimensional mathematical model of laser surface texturing was established by FLUENT based on the interaction mechanism of laser and metal material.The effects of pulse width and laser power density on the temperature field and flow field of 45 steel and Cr12 die steel were investigated.The results show that the flow direction of the pool are both from the edge to the center in these two materials,which leads to form bulge.For 45 steel,When the pulse width is larger than 500 us,the maximum temperature of the pool center exceeds boiling point and causes material gasification.The above conclusion was experimentally verified: when the pulse width is smaller,larger than 500 us,it tends to form spherical crown and concave crown,respectively.The rate of pool diameter and depth firstly increases with the increase of pulse width then decreases.By analyzing the flow field and the temperature field,it is found that the large temperature gradient can promote the rapid growth of the molten pool,while the small temperature gradient makes slow growth rate of the molten pool.The conclusion was also experimentally verified: the turning point of bulges diameter and height rate from large to small is pulse width of 500 us.The liquid convection between Cr12 and 45 steel due to differences in material properties is different.The profile metallographic diagram illustrates Cr12 diameter/depth is greater than 45 steel by simulation.The temperature field of the two kinds of materials was analyzed by the metallographic point,which is followed by the gasification zone,the fusing zone,the heat affected zone and the substrate zone from the inside to the outside.Themicrostructure and hardness of each zone were measured.It is believed the hardness of surface tecturing is larger than substrate.（2）The quantitative characterization of the morphology of these bulges were carried out.The parameters are as follows: T is depression rate,and Pb is the inner and outer diameter ratio.T of spherical crown is 0;T of concave crown is between 0 and 0.33;T of M-shape is between0.33 and 1;and crater’s T is larger than 1,and 0 <Pb <0.83.The approximate calculation formula of convex and concave was given,V-convex reflects the wear resistance of the point,V-concave represented abrasion resistance and V-shape represented the ability to store lubricants and abrasive particles.（3）Based on the guidance given by the simulation calculation,the fiber laser was used to study the texturing.The spherical crown was produced,and depending on the 45 steel gasification and the recoil pressure of the gasification process,the concave crown formed.Because the chromium boiling point of Cr12 is low and the content is high,as well as the molten pool center material gasification and thus induced recoil pressure,within the selection of parameters,M-shape was produced.And due to the role of plasma shielding,concave crown wad produced.（4）High quality dimple could be produced at a low power density of 1.02 * 104 W / mm2.This paper carried out a preliminary analysis of the strange phenomen,and explored the processing technology of dimple systematically.（5）To explore bulge morphology and size changes under the different auxiliary gas and different pressure.Adding N₂ as auxiliary gas,45 steel in the given processing parameters appeared M-shape、concave crown、spherical crown in turn;Cr12 formed concave crown,both materials’ bulge diameter bigger than the diameter and height is smaller than the height that without auxiliary gas.With O₂ as the auxiliary gas,45 steel bulge morphology changes without regular,Cr12 bulge randomly appear in the M-shape and crater under condition of 0.05 MPa.Two kinds of material molten pool were blown,which unable to form bulge under the condition of 0.1 MPa.（6）In the processing surface coating a layer of sodium silicate,changing laser absorption rate,to explore the change rule of bulge morphology and size.When the coating conditions,high asp ect ratio of two materials bulge morphology larger than high aspect ratio of uncoated.（7）Adding a magnet on the side of the specimen expect to work out tited bulge.Two material s as ferromagnetic material.When the temperature exceeds Curie temperature,materila will beco me a paramagnetic material.Therefore,the magnetic field can’t influence on the liquid metal,so bulge morphology without changing.（8）The active element S influence the molten pool convection,in the processing surface coa ting a layer of 5um diameter sulfur powder for laser texuring.Although crater morophology did not appear,forming ellipsoid shape.And,long and short axis is inconsistent between every bulges.Thisphenomenon confrim that the S element does have an impact on metal liquid convect ion in the experiment.更多还原