预热温度对激光熔凝RuT300气门座残余应力场的影响研究

目的 为了突破激光熔凝蠕墨铸铁RuT300气门座裂纹抑制技术的瓶颈,研究了预热温度对激光熔凝RuT300气门座残余应力场的影响,从而为工程上抑制裂纹的参数优选提供支撑。方法 基于热弹塑性理论,建立了激光熔凝RuT300气门座残余应力场分析的数学物理模型,模型中考虑了预热温度、激光熔凝参数、材料性能参数的变化对残余应力的影响。结果 预热温度对激光熔凝RuT300气门座残余应力场的影响与熔池冷却速度、峰值温度等密切相关：当预热温度在25~150 ℃时,随着预热温度的升高,冷却速度下降对激光熔凝RuT300气门座热膨胀变形引起的应力降低起主要作用,导致气门座环向残余应力值随预热温度的升高而减小;当预热温度在150~250 ℃时,随着预热温度的升高,峰值温度上升诱发的热膨胀变形加剧所引起的应力增加起主要作用,导致气门座环向残余应力值随预热温度的升高而增大。结论 预热温度的变化影响气门座的冷却速度和峰值温度,而残余应力值的变化是气门座冷却速度和峰值温度等综合影响的结果,通过合理的调控预热温度,可以使激光熔凝RuT300气门座的残余应力值降低到最低,从而减小气门座激光熔凝形成裂纹的倾向。     

RuT300气门座激光熔凝温度场数值模拟

建立了激光熔凝气门座的数学物理模型,模型中考虑了材料的热物性参数随温度的变化及材料的相变潜热。结果表明:随激光功率的增加,激光熔凝气门座熔池最高温度升高,且激光熔凝区的深度和宽度增加;随激光扫描速度的提高,激光熔凝气门座的最高温度下降,且熔凝区的深度和宽度减小;随激光光斑半径的增加,激光熔凝气门座的最高温度降低,激光熔凝区深度减小。激光熔凝气门座过程是新熔池形成和已形成的熔池凝固二者同步的过程,由于不同区域温度分布和冷却速度等差异,导致气门座表面熔池形态为彗星拖尾状;且彗星拖尾状随激光功率的增大而增加,彗星拖尾状随扫描速度和激光半径的增加而降低。随激光半径的增加,熔池彗星拖尾的尖形曲率减小。实验与数值模拟结果基本吻合,这说明了模型的有效性。     

激光选区熔化WC 12Co复合材料温度场模拟研究北大核心CSCD

为提高激光选区熔化WC 12Co硬质复合材料的成形质量,采用有限元仿真软件Ansys 2021R1对SLM成形WC 12Co硬质复合材料过程的温度场进行数值模拟仿真研究,研究成形温度场的温度分布和成形工艺参数(激光功率、扫描速度、扫描间距和基板预热温度)对温度场的影响,为优化WC 12Co硬质复合材料成形提供试验依据。结果表明:激光功率增大,成形区域温度增大,位置点3的峰值温度从3507.47℃增大至3837.52℃;激光扫描速度增大,成形区域温度降低,位置点5的峰值温度从3592℃下降至2897℃,峰值温度下降695℃;扫描间距的增加使各扫描区域的温度有所降低,位置点3的峰值温度从3330℃逐渐降低至3123℃。在同一成形工艺参数下,激光扫描前一路径对后一路径有预热作用,随着扫描路径的增加,成形区域的温度呈现逐渐上升趋势。基板预热至120℃能够提高熔池的内部温度,减小成形件之间的温度差异,缩小温度梯度差。当激光功率增大时,熔池的宽度和深度随之增大;当激光扫描速度增大时,熔池的宽度先增大后减小,熔池的深度线性反向减小;当扫描间距增大时,熔池的宽度和深度均减小。模拟获得的温度场仿真结论能够大致反映成形试样的表面质量和合金粉末的熔化状态随成形工艺参数变化的趋势。     

光纤激光-渗氮处理对Cr12MoV钢组织和性能的影响

采用IPG的YLS-3000型光纤激光器对Cr12Mo V钢表面进行原位激光-渗氮处理。通过光镜、扫描电镜、X射线衍射及金相显微硬度计,分析研究不同激光处理参数对渗氮层组织及性能的影响。结果表明,在扫描速度和离焦量一定的条件下,Cr12Mo V激光熔凝层深度随激光功率的增加而增大,激光渗氮处理可使材料表面显微硬度提高。Cr12Mo V钢激光渗氮后的组织由熔凝区、热影响区及基体三部分组成。随着激光熔凝速度的增大,熔凝区树枝晶逐渐变得细小。随着激光熔凝功率的增加,熔凝区树枝晶逐渐变得粗大。熔凝层的硬度峰值出现在距材料表面1.0 mm附近,两侧呈对称降低,硬度峰值则随激光功率的增加而增加。     

激光熔凝对304奥氏体不锈钢拉伸力学性能的影响

采用5 kW横流CO2激光器,分别在常温空气冷却、冰水快速冷却条件下对304奥氏体不锈钢的上下表面进行熔凝试验,并对比分析其显微组织和拉伸力学性能.结果表明:经激光熔凝处理后,熔凝层组织形态由里及表依次为平面晶、胞状晶、树枝晶和等轴晶;组织明显得到细化,常温空气冷却和冰水快速冷却的熔凝层组织分别比基体组织细化约14倍和18倍;经两种不同的处理方式得到的试件,其拉伸力学性能均得到增强,且随熔凝层冷却速度的增加,相应的拉伸力学性能指标提高的幅度也增加.其中,经冰水快速冷却激光熔凝处理后的试件屈服强度、抗拉强度和伸长率分别可提高约22.4％、16.4％和15.7％     

激光熔凝过程的SPH法数值模拟

运用光滑粒子流体动力学(smoothed particles hydrodynamics,SPH)法建立了激光熔凝过程的数值模型。并应用该模型对AISI304不锈钢的激光熔凝过程进行数值模拟,分析在不同激光功率和扫描速度下对熔池速度场、温度场及其宽度和深度的影响。结果表明:熔池的最高温度相对激光光斑中心偏移一定的距离,且偏移距离随扫描速度的增加而增大,但随激光功率的增加则保持不变。同时也为研究激光熔覆的自由表面形貌的形成过程提供SPH理论基础和数值分析方法。     

AM50镁合金搅拌摩擦焊接温度场研究

对AM50镁合金进行了搅拌摩擦焊接实验,研究了各特征点的焊接温度场变化规律.结果表明:在搅拌摩擦焊接过程中,各特征点都经历了"加热-峰值-冷却"的过程,且各特征点的加热速度、冷却速度、峰值温度以及到达峰值温度的时间各不相同.在一定的旋转速度下,各特征点的峰值温度随焊接速度的减小而升高;而在一定的焊接速度下,各特征点的峰值温度随旋转速度的增大而升高.     

铝合金6061高转速无倾角微搅拌摩擦焊温度分布研究

对0.8 mm厚6061铝合金超薄板微搅拌摩擦焊过程进行测温实验,并对微搅拌摩擦焊过程工件温度控制技术进行研究,分别研究了工艺参数如焊接速度、主轴转速、下压量以及焊接过程外加压缩空气冷却等因素对微搅拌摩擦焊过程工件温度的影响。测温实验结果表明,随焊接速度的升高,焊接热输入降低,温度峰值下降,冷却速度值减小,主轴转速越高,热输入越大,温度峰值上升,冷却速度值增大。当下压量增大,产热剧烈升高,下压量从0.05 mm增大至0.1 mm,距离焊缝5 mm处的温度峰值从72.28℃上升至115.06℃,增加了54.65%。在保证焊接质量的前提下,尽量降低下压量对控制温度峰值具有良好的效果。此外,外加气冷是控制温度峰值的有效手段,对冷却速度的影响最大。加气冷时冷却速度值增大约为未加气冷时的4倍,有利于降低高温停留时间,改善焊缝冷却过程。     

激光熔凝强化RuT300收尾凹坑缺陷抑制方法研究

目的 突破气门座激光熔凝收尾凹坑缺陷技术瓶颈。方法 建立激光熔凝强化RuT300的三维瞬态温度场仿真模型,模型中考虑激光吸收率、材料相变潜热及热物性参数的影响,并结合Niyama判据,分析激光熔凝强化RuT300收尾过程激光参数线性变化对收尾凹坑缺陷的影响规律。结果 在激光熔凝收尾过程中,对比未采用激光参数线性变化的收尾方式,采用激光功率线性下降（斜率为–400）的参数变化方式时, 的最大值由最初的0.0085增大至0.0097;当斜率由–100减小至–400时, 的最大值由最初的0.0087增大至0.0097;采用激光扫描速度线性上升（斜率为20）的参数变化方式时, 的最大值由最初的0.0087增大至0.0142;当斜率由5增加至20时, 的最大值由最初的0.0112增大至0.0142。伴随着激光功率线性下降或激光扫描速度线性上升,激光熔凝气门座收尾凹坑有缩减趋势,且伴随着激光功率线性下降过程斜率绝对值或激光扫描速度线性上升过程斜率的增大,收尾凹坑有进一步缩减趋势。实验分析与数值模拟结果基本吻合,说明了模型的有效性。结论 在激光熔凝收尾过程中,采用激光功率线性下降或者激光扫描速度线性上升方法可以抑制收尾的凹坑缺陷,提升气门座的可靠性。     

6061铝合金淬火冷却过程中的表面换热系数

通过6061铝合金末端淬火测得的冷却曲线,结合有限差分法和反传热求解法,研究了6061合金固溶处理在不同冷却方式下的冷速及表面换热系数与温度的变化规律。结果表明,6061铝合金在水雾冷和喷水冷却过程中,端面冷速先增大后减小,在400℃左右达到峰值,峰值冷速约为30℃/s。6061铝合金的表面换热系数与温度呈非线性关系,其大小随着温度的降低先逐渐增大,在150～100℃范围内达最大值,然后下降;在风冷过程中,表面换热系数值先急剧增大,当温度下降到500℃后增速明显减慢。     

Effects of Post-weld Heat Treatment on Microstructure,Mechanical Properties and the Role of Weld Reinforcement in 2219 Aluminum Alloy TIG-Welded Joints

In as-welded state, each region of 2219 aluminum alloy TIG-welded joint shows diff erent microstructure and microhardness due to the diff erent welding heat cycles and the resulting evolution of second phases. After the post-weld heat treatment, both the amount and the size of the eutectic structure or θ phases decreased. Correspondingly, both the Cu content in α-Al matrix and the microhardness increased to a similar level in each region of the joint, and the tensile strength of the entire joint was greatly improved. Post-weld heat treatment played the role of solid solution strengthening and aging strengthening. After the post-weld heat treatment, the weld performance became similar to other regions, but weld reinforcements lost their reinforcing eff ect on the weld and their existence was more of an adverse eff ect. The joint without weld reinforcements after the post-weld heat treatment had the optimal tensile properties, and the specimens randomly crack in the weld zone.     

The First Phase of the SRIF Industrialized Base in ＂West Area＂ Has Been Put into Operation

After nearly two years＇ tense construction, the first phase of industrialized base of Shenyang Research Institute of Foundry （SRIF）, located at the Tiexi Casting and Forging Industrial Park in the west of Tiexi District, has now been completed and formally put into operation.     

Thermodynamics Study on the Decomposition of Chromite with KOH

Institute of Process Engineering, Chinese Academy of Sciences, China, has proposed a method for oxidative leaching of chromite with potassium hydroxide. Understanding the mechanism of chromite decomposition, especially in the potassium hydroxide fusion, is important for the optimization of the operating parameters of the oxidative leaching process. A traditional thermodynamic method is proposed and the thermal decomposition and the reaction decomposition during the oxidative leaching of chromite with KOH and oxygen is discussed, which suggests that chromite is mainly destroyed by reactions with KOH and oxygen. Meanwhile, equilibrium of the main reactions of the above process was calculated at different temperatures and oxygen partial pressures. The stable zones of productions, namely, K2CrO4 and Fe2O3, increase with the decrease of temperature, which indicates that higher temperature is not beneficial to thermodynamic reactions. In addition, a comparison of the general alkali methods is carried out, and it is concluded that the KOH leaching process is thermodynamically superior to the conventional chromate production process.     

Influence of Heat Treatment on Damping Behaviour of the Magnesium Wrought Alloy AZ61

The effect of isochronal heat treatments for 1h on variation of damping, hardness and microstructural change of the magnesium wrought alloy AZ61 was investigated. Damping and hardness behaviour could be attributed to the evolution of precipitation process. The influence of precipitation on damping behaviour was explained in the framework of the dislocation string model of Granato and Lücke.     

Effect of Rare Earth Doping on Impedance,Modulus and Conductivity Properties of Multiferroic Composites:0.5(BiLa_xFe_(1-x)O_3)–0.5(PbTiO_3)

The Lanthanum-doped bismuth ferrite–lead titanate compositions of 0.5(Bi LaxFe1-xO3)–0.5(Pb Ti O3)(x = 0.05,0.10,0.15,0.20)(BLxF1-x-PT) were prepared by mixed oxide method.Structural characterization was performed by X-ray diffraction and shows a tetragonal structure at room temperature.The lattice parameter c/a ratio decreases with increasing of La(x = 0.05–0.20) concentration of the composites.The effect of charge carrier/ion hopping mechanism,conductivity,relaxation process and impedance parameters was studied using an impedance analyzer in a wide frequency range(102–106Hz) at different temperatures.The nature of Nyquist plot confirms the presence of bulk effects only,and non-Debye type of relaxation processes occurs in the composites.The electrical modulus exhibits an important role of the hopping mechanism in the electrical transport process of the materials.The ac conductivity and dc conductivity of the materials were studied,and the activation energy found to be 0.81,0.77,0.76 and 0.74 e V for all compositions of x = 0.05–0.20 at different temperatures(200–300 °C).     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

Growth Kinetics of Single Inclusion Particle in Molten Melts

On the basis of the single-particle framework, a new theory on inclusion growth in metallurgical melts is developed to study the kinetics of inclusion growth on account of reaction and collision. The studies show that the early growth of inclusion depends on reaction growth and Brawnian motion collision, and where the former is decisive, the late growth depends on turbulence collision and Stokes＇ collision, and where the former is dominant; collision growth is very quick during the smelting process, lessened in the refining process, but nearly negligible in the continuous casting process.     

Theoretical Analysis for the Motion and Temperature of Melt Droplets in Spray Degassing Process

The motion of melt droplets in spray degassing process was analyzed theoretically. The height of the treatment tank in spray degassing process could be determined by the results of theoretical calculation of motion of melt droplets. To know whether the melt droplets would solidify during spraying process, the balance temperature of melt droplets was also theoretically analyzed. Then proof experiments for theoretical results about temperature of melt droplets were carried. In comparison, the experimental results were nearly similar to the calculation results.     

Using Finite Element and Contour Method to Evaluate Residual Stress in Thick Ti-6Al-4V Alloy Welded by Electron Beam Welding

This work was to reveal the residual stress profile in electron beam welded Ti-6Al-4V alloy plates(50 mm thick) by using finite element and contour measurement methods.A three-dimensional finite element model of 50-mmthick titanium component was proposed,in which a column–cone combined heat source model was used to simulate the temperature field and a thermo-elastic–plastic model to analyze residual stress in a weld joint based on ABAQUS software.Considering the uncertainty of welding simulation,the computation was calibrated by experimental data of contour measurement method.Both test and simulated results show that residual stresses on the surface and inside the weld zone are significantly different and present a narrow and large gradient feature in the weld joint.The peak tensile stress exceeds the yield strength of base materials inside weld,which are distinctly different from residual stress of the thin Ti-6Al-4V alloy plates presented in references before.     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.