CALCULATION OF 3-D TRANSIENT TEMPERATURE FIELD DURING LASER TRANSFORMATION HARDENING PROCESS

A three-dimensional transient heat transfer model for laser transformation hardening process has been developed in this paper. The finite size of the laser treated sample, the surface heat loss of the sample, the latent heat of phase transformation and the temperature dependence of thermal properties of materials were considered. The heat source was considered as a moving Gaussian heat flux with a constant velocity. Three-dimension unequally spatial grid explicit finite difference equations, alternating direction implicit finite difference equations and implicit finite difference equations were deduced respectively. Three programs to calculate the temperature field were developed using Fortran language. The transient temperature fields of C22, 42CrMo, C60 steel samples during laser transformation hardening process were calculated using these programs, and the widths and depths of laser transformation hardening zones were also predicted. C22, 42CrMo, C60 steel samples were treated by CO_2 laser,the widths and depths of laser transformation hardening zones of these samples were also measured experimentally. The calculated widths and depths of laser transformation hardening zones are in good agreement with the experimental results.

Calculation of Temperature Field During Laser Transformation Hardening of Cylindrical Bodies

A Two-dimensional transient heat transfer model for laser transformation hardening process of cylindrical bodies has been developed. The surface heat loss of the sample, the latent heat of phase transformation, and the temperature dependence of thermal properties of material were considered. The laser heat source was considered as a moving Gaussian heat flux with a constant velocity. Finite element method was used to calculate the transient temperature field. A program to calculate the temperature field was developed using FORTRAN language. The transient temperature field, the width and depth of laser transformation hardening zone of 42CrMo steel sample during laser transformation hardening process was calculated. The widths and depths of laser transformation hardening zones of 42CrMo steel samples were also measured experimentally. The calculated widths and depths of laser transformation hardening zones are in good agreement with the experimental results.

Study on Laser Transformation Hardening of HT250 by High Speed Axis Flow CO_2 Laser

In this article, laser transformation hardening of HT250 material by high speed axis flow CO2 laser was investigated for first time in China. Appropriate laser hardening parameters, such as laser energy power P(W), laser scanning rate V(m/min), were optimized through a number of experiments. The effect of the mentioned parameters on the hardened zone, including its case depth, microhardness distributions etc., were analyzed. Through the factual experiments, it is proved that axial flow CO2laser, which commonly outputs low mode laser beam, can also treat materials as long as the treating parameters used are rational. During the experiments, the surface qualities of some specimens treated by some parameters were found to be enhanced, which does not coincide with the former results. Furthermore in the article, the abnormal phenomenon observed in the experiments is discussed. According to the experimental results, the relationship between laser power density q and scanning rate V is shown in a curve and the corresponding formulation, which have been proved to be valuable for choosing the parameters of laser transformation hardening by axial flow CO2 lasers, was also given.

Calculation of Transient Stress Field During Laser Transformation Hardening Process

A thermal elastic-plastic Two-D finite element model to calculate the transient stress field during laser transformation hardening process was developed in this paper. The mechanical properties of material, Young’s module E, Poisson’s ratio v, yield limit s2 and thermal expansion coefficient a are all considered to change with temperature. The equivalent expansion method was used to deal with the problem with phase transformation. A program to calculate the transient stress field was developed using FORTRAN language. The transient and residual stress fields during CO2 laser transformation hardening process of MoCu nodular iron were calculated. The residual stress fields were measured using X-ray diffraction method. The calculated residual stress field of laser transformation hardened MoCu nodular iron is in good agreement with the experimental result.

Effects of laser phase transformation hardening parameters on heat input and hardened-bead profile quality of unalloyed titanium

Laser transformation hardening(LTH)of unalloyed titanium of 1.6 mm-thick sheet,nearer to ASTM Grade 3 of chemical composition was investigated using 2 kW CW Nd:YAG laser.The effects of laser power(750-1 250 W),scanning speed(1 000-3 000 mm/min)and focal point position(from-10 to-30 mm)on the heat input,and hardened-bead geometry(i.e.hardened bead width(HBW),hardened depth(HD)and angle of entry of hardened bead profile with the surface(AEHB))were investigated using response surface methodology(RSM).The experimental plan is based on Box-Behnken design matrix method.Linear and quadratic polynomial equations for predicting the heat input and the hardened bead geometry were developed.The results indicate that the proposed models predict the responses adequately within the limits of hardening parameters being used.It is suggested that regression equations can be used to find optimum hardening conditions for desired criteria.

Laser transformation hardening on rod-shaped carbon steel by Gaussian beam

Laser transformation hardening(LTH) is one of the laser surface modification processes.The surface hardening of rod-shaped carbon steel(SM45C) was performed by lathe-based laser composite processor with Gaussian-beam optical head.The LTH characteristics by dominant processes,longitudinal and depth directional hardness distributions and behaviors of phase transformation in hardened zones were examined.Especially,two concepts of circumferential speed and theoretical overlap rate were applied.When laser power increased or circumferential speed decreased,the surface hardening depth gradually increases due to the increased heat input.Moreover,the longitudinal hardness distribution particularly shows periodicity of repetitive increase and decrease,which results from tempering effect by overlap.Finally,the feasibility of laser transformation hardening is verified by using the beam with Gaussian intensity distribution.

Controlled Laser Transformation Hardening of Martensitic Stainless Steel by Pulsed Nd:YAG Laser

Laser transformation hardening （LTH） was applied to the surface of the AISI 420 martensitic stainless steel by a pulsed Nd：YAG laser to obtain optimum hardness. The influences of process parameters （laser pulse energy, duration time, and travel speed） on the depth and hardness of laser treated area were investigated. Image analysis of SEM microstructure of AISI 420 showed that plate-like carbide have almost fully and （30-40）% of globular carbide particles dissolved into the matrix after laser transformation hardening by pulsed laser and the microstructure was refined to obtain controlled tempered martensite microstructure with 450 VHN hardness.

CALCULATION OF RESIDUAL STRESS FIELD IN LASER TRANSFORMATION HARDENED MoCu NODULAR IRON

In this paper,a thermal elastic-plastic 2-D finite element model of stress generation during laser transformation hardening process was developed. In this model, the mechanical properties of the material, Young's modules E, Poisson's ratio v, yield limit s, and thermal expansion coefficient α, are all change with temperature. The equivalent expansion method was used to deal with the problem with phase transformation. Based on this model, a program to calculate the residual stress field was developed using FORTRAN language. The residual stress fields in CO2 laser transformation hardened MoCu nodular iron were calculated. The calculated results showed that in the transformation hardened zone, the residual stress state is compress,whereas adjacent to this zone the residual stress state is tensile, and there is a tensile stress peak close to the transformation hardened zone. To verify this model, the residual stress fields were measured using X-ray diffraction method. The calculated results of residual stress fields are in good agreement with the experimental results.

VIRTUAL PROCESSING OF LASER SURFACE HARDENING ON AUTOBODY DIES

A new method of collision-free path plan integrated in virtual processing is developed to improve the efficiency of laser surface hardening on dies. The path plan is based on the premise of no collision and the optimization object is the shortest path. The optimization model of collision-free path is built from traveling salesman problem （TSP）. Collision-free path between two machining points is calculated in configuration space （C-Space）. Ant colony optimization （ACO） algorithm is applied to TSP of all the machining points to find the shortest path, which is simulated in virtual environment set up by IGRIP software. Virtual machining time, no-collision report, etc, are put out atter the simulation. An example on autobody die is processed in the virtual platform, the simulation results display that ACO has perfect optimization effect, and the method of virtual processing with integration of collision-free optimal path is practical.

高能束表面改性技术在航空领域的应用

高能束表面改性适用于各种金属和合金,能够显著提升材料表面硬度、耐磨、耐蚀等性能指标,是航空部件实现性能提升的有效手段之一。本文总结了6种高能束表面改性技术的基本原理、设备构成和改性应用,其中激光相变硬化通过马氏体相变强化金属材料表面;激光熔覆通过选择不同粉末实现表面修复和表面性能提升,重点在于控制裂纹缺陷;激光冲击强化可有效解决航空发动机部件高周疲劳断裂问题;强流脉冲电子束和强流脉冲离子束一方面需要提高设备的性能和运行稳定性,另一方面要针对航空部件应用开展深入研究;而离子束辅助沉积则可以通过制备固体润滑涂层实现对微动磨损的有效防护。最后,提出对高能束表面改性机理深入研究、发展专业化智能化装备和实现多种束源复合与集成的发展方向。

The Laser Transformation Hardening Mechanism of High Speed Steel W_6Mo_5Cr_4V_2

The application of laser surface melting in the laser heat treatment of high speedsteel causes low hardness, so the subsequent quenching and tempering processesare needed. And by means of transformation hardening techniques, ultrahighhardness 】HV1000 can be obtained.In this case, the subsequent quenching pro-cess can be saved, and the wear and scuffing resistances can be considerably im-proved. However, the laser transformation hardening mechanism has not beenunderstood clearly. The laser transformation hardening mechanism of high speedsteel W6Mo5Cr4V2 has been studied in the present note by TEM and X-ray diffractionmethod in order to provide a theoretical basis for application.

Finite Element Method Analysis of Stress Field Considering Thermal Coupled Force During Laser Process

According to the prominent characteristic of rapid heating and cooling velocity in process of laser phase transformation hardening, it is important and necessary to consider the couple relation among temperature, phase transformation and stress. Based on the above, a mathematical model to analyze transformation and to reflect its characteristic is presented. Also temperature field control equation relative to stress and phase transformation is set up and thermal elastoplastic constitutive equation containing phase transformation was derived, In a given yield condition, temperature and stress field are numerically calculated by means of Finite Element Method. The result indicates the calculation value coincides with result of test Stress especially thermal stress varied violently in process of laser phase transformation hardening, which leaded to the residual stress on the surface and accorded to the fact. This process can improve the characteristic of material, so it is significant to study on thermal stress of laser quenching, to discuss the effect of phase transformation and stress on temperature, to optimize the laser quenching process and to improve the synthetic mechanic properties of quenching material.

Hardness Profile Prediction for a 4340 Steel Spline Shaft Heat Treated by Laser Using a 3D Modeling and Experimental Validation

Laser surface transformation hardening becomes one of the most effective processes used to improve wear and fatigue resistance of mechanical parts. In this process, the material physicochemical properties and the heating system parameters have significant effects on the characteristics of the hardened surface. To appropriately exploit the benefits presented by the laser surface hardening, it is necessary to develop a comprehensive strategy to control the process variables in order to produce desired hardened surface attributes without being forced to use the traditional and fastidious trial and error procedures. The paper presents a study of hardness profile predictive modeling and experimental validation for spline shafts using a 3D model. The proposed approach is based on thermal and metallurgical simulations, experimental investigations and statistical analysis to build the prediction model. The simulation of the hardening process is carried out using 3D finite element model on commercial software. The model is used to estimate the temperature distribution and the hardness profile attributes for various hardening parameters, such as laser power, shaft rotation speed and scanning speed. The experimental calibration and validation of the model are performed on a 3 kW Nd:Yag laser system using a structured experimental design and confirmed statistical analysis tools. The results reveal that the model can provide not only a consistent and accurate prediction of temperature distribution and hardness profile characteristics under variable hardening parameters and conditions but also a comprehensive and quantitative analysis of process parameters effects. The modelling results show a great concordance between predicted and measured values for the dimensions of hardened zones.

激光表面改性对轧辊用高速钢组织与硬度的影响

用横流激光器对高速钢轧辊材料进行了激光重熔和激光相变硬化表面处理,利用金相显微镜对激光重熔层与硬化层的形貌进行了研究,用带能谱的扫描电镜与X射线衍射仪(XRD)对微区成分以及相结构进行了分析,用硬度计测量了试样表面硬度。结果表明,高速钢轧辊材料经激光重熔处理后,表面硬度明显降低,碳化物呈网状分布;经激光相变硬化后的试样表面硬度显著提高,最高值达到68.5HRC,碳化物呈球状孤立分布。

H13热作模具钢激光表面改性处理技术

介绍了 H13热作模具钢的激光表面改性处理技术,分析了激光相变硬化、激光表面熔凝、激光合金化、激光冲击硬化等表面处理的特点及应用,研究了表面激光处理工艺的影响因素,以及激光在模具表面处理中的应用。讨论了表面激光改性处理存在的问题,提出了该领域的研究方向。

激光表面改性

激光表面改性技术在改善材料表面性能,提高材料使用寿命方面具有突出的优越性。随着研究的深入,激光表面改性技术在工业中的应用逐步扩大。对工业应用中比较常见的激光熔覆、激光表面熔凝、激光相变硬化、激光冲击强化、激光表面合金化等激光表面改性技术进行了综述,并在此基础上展望了激光表面改性技术的发展前景。

激光相变硬化处理对轮轨钢磨损性能影响

利用滚动磨损试验机研究了激光相变硬化处理前后轮轨钢试样的磨损与损伤性能。结果表明:激光相变硬化处理主要得到马氏体组织,显著提高轮轨试样的表面硬度,钢轨试样和车轮试样分别提高43.06%、44.39%;轮轨试样经激光相变硬化处理后滚动摩擦系数与未处理试样相差不大,单一处理轮轨试样摩擦系数有所增大;轮轨试样经激光相变硬化处理能明显提高轮轨试样表面抗磨损和变形能力,车轮试样磨损率减少约44.02%,钢轨试样减少约13.6%,单一处理轮轨试样虽能降低处理试样的磨损,但显著加剧对摩副的磨损;激光相变硬化处理后,钢轨试样表现为轻微疲劳磨损,车轮试样则是粘着磨损与疲劳磨损共存,以疲劳磨损为主;重载工况下轮轨钢试样均激光相变硬化处理能有效改善材料抗磨损和损伤能力。

18Cr2Ni4W钢渗碳激光强化复合处理组织的研究

对１８Ｃｒ２Ｎｉ４Ｗ钢进行了渗碳激光强化复合处理研究。利用扫描电镜、透射电镜和图象分析仪，对其组织进行较详细的研究。结果表明，在复合工艺作用下，随着表面硬化区层深的变化，组织结构发生明显的变化。由于钢中存在大量合金元素，致使表面硬化层产生大量的残留奥氏体，降低了表面硬度。

球墨铸铁QT600-3激光相变硬化数值模拟研究

采用商业有限元软件ProCAST,建立了激光相变硬化温度场的3维数值模拟模型,对球墨铸铁QT600-3进行激光相变硬化的数值模拟研究,计算了表面淬火的温度场,根据温度场预测了硬化层的深度和宽度。激光功率在800～1 000 W之间,激光扫描速度在2.000～2.667 mm/s之间,光斑直径在4～5 mm之间时,计算得到的硬化层深度在0.20～0.64 mm之间,硬化层宽度在2.0～3.7 mm之间。球墨铸铁数值模拟结果与试验基本吻合,两者变化趋势一致。

低碳钢表面激光相变硬化研究

为了研究不同激光功率及不同的冷却条件下,激光相变硬化处理对低碳钢表面性能和组织的影响,采用激光表面相变硬化方法,在低碳钢表面获得了比原先母材硬度高100HV～150HV 的硬化层,采用金相显微镜分析了激光处理区的组织,且用显微硬度计测量了单道扫描时的纵向和横向的硬度分布。研究发现,激光作用区主要是低碳板条马氏体与未转变的索氏体甚至屈氏体、回火索氏体组织。搭接区组织均为细小的马氏体及中间分布着索氏体组织;由于10CrNiMo 钢含碳量较低和碳扩散系数不同的原因,其最高硬度层并未在表面形成,而是形成在次表层。在软化区,前一道扫描形成的马氏体受到回火作用,原先固溶在马氏体中的碳析出,形成了回火索氏体,降低了硬度。结果表明,激光相变硬化工艺可以将10CrNiMo 钢的表面硬度提高100HV～150HV 左右,且表面保持很好的韧性,若想进一步提高其表面硬度,还需采取熔覆等其它工艺。