H13钢低温等离子体渗硼层的热熔损性能

研究喷丸预处理的H13钢低温等离子体渗硼试样的耐铝合金热熔损的性能.结果表明,经喷丸预处理的H13钢试样在590℃等离子渗硼2 h后,表面获得高硬度的硼化物层,其硬度值达10 GPa,厚度约为2 μm.通过动态热熔损实验,发现低温渗硼试样耐铝合金热熔损性能明显优于原始试样,并且当熔损时间较短时其耐热熔损效果尤其突出.     

S30432奥氏体钢管喷丸强化形变层组织和特性

针对S30432钢管样的内壁喷丸强化形变层,采用金相分析、扫描电子显微分析和硬度分析等方法,进行了强化形变层表面状态、形变组织、显微硬度及其氧化特性的分析研究,并依据喷丸层形变组织形态特征的差别将强化形变组织分为单滑移层、多滑移层和碎化晶层3层。研究结果表明:喷丸强化效果主要取决于碎化晶层和多滑移层的发育程度,尤其是取决于碎化晶层的发育程度;喷丸处理可以提高奥氏体钢管的抗氧化能力。     

高压水射流喷丸强化技术

基于目前在使用传统喷丸技术中存在的一系列问题的基础上,通过前人对水的不断研究和探索,提出了一种新的喷丸强化技术——水射流喷丸强化。它的使用将改变传统的热加工技术为冷加工技术。通过对某种合金的实验性研究分析,初步得出了该技术优越性所在。与传统喷丸技术的比较,进一步阐明了高压水射流喷丸强化技术的优越性、环保性和前景的广泛性。     

Depth profilometry of near-surface inhomogeneities via laser-photothermal probing of the thermal diffusivity of condensed phases

Several applications are presented of the Hamilton-Jacobi formulation of thermal-wave physics to the problem of laser photothermal depth profilometry of the thermophysical transport parameter (the thermal diffusivity) of inhomogeneous condensed phases (solids and liquids) with arbitrary, continuously varying thermal diffusivity profiles. A working general method for solving the inverse problem and obtaining arbitrary diffusivity depth profiles from the laser beam-intensity modulation frequency dependence of the photothermal signal (amplitude and phase) is described. Specific examples of profile reconstructions are presented, including magnetic field-induced thermophysical inhomogeneities in liquid crystals, laser processing inhomogeneities in steels and Zr-Nb alloys, and finally, evaluation of machining damage in metals. Whenever possible, profiles obtained photothermally are compared with those resulting from destructive methods, such as microhardness testing.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Fretting fatigue behaviour of shot-peened Ti-6Al-4V at room and elevated temperatures

Fretting fatigue behaviour of shot‐peened titanium alloy, Ti‐6Al‐4V was investigated at room and elevated temperatures. Constant amplitude fretting fatigue tests were conducted over a wide range of maximum stresses, σ_max= 333 to 666 MPa with a stress ratio of R= 0.1 . Two infrared heaters, placed at the front and back of specimen, were used to heat and maintain temperature of the gage section of specimen at 260 °C. Residual stress measurements by X‐ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Elevated temperature induced more residual stress relaxation, which, in turn, decreased fretting fatigue life significantly at 260 °C. Finite element analysis (FEA) showed that the longitudinal tensile stress, σ_xx varied with the depth inside the specimen from contact surface during fretting fatigue and the largest σ_xx could exist away from the contact surface in a certain situation. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from FEA results to characterize fretting fatigue crack initiation behaviour. It showed that stress relaxation during test affected fretting fatigue life and location of crack initiation significantly. MSSR parameter also predicted crack initiation location, which matched with experimental observations and the number of cycles for crack initiation, which showed the appropriate trend with the experimental observations at both temperatures.     

Dislocation engineering and its effect on the oxidation behaviour

Abstract

Shot-peening of the surface of steel prior to oxidation can have a beneficial effect. Shot-peening can improve the oxidation resistance by introducing a localised plastic deformation in the near surface region resulting in an increase of the dislocation density. These dislocations can act in Cr-containing steels as fast diffusion paths for Cr promoting the formation of protective Cr-oxides. However, the effect of shot-peening has some limitations such as working temperature and microstructure. It has different effects on austenitic steels and ferritic martensitic steels. The effect of shot-peening can become futile due to recovery and recrystallisation of the alloy when subjected to higher temperatures for longer periods. In the present work, the main emphasis is put on the type of dislocation arrangement promoting the positive effect on the oxidation behaviour. Dislocation engineering was applied on shot-peened samples by means of some pre-annealing procedures resulting in a recovery process. During the process, dislocations were assumed to rearrange and form certain combinations nearer to the alloy grain boundaries. These arrays of dislocations can result in different oxidation behaviour. In the present study, 18 wt% Cr and 12 wt% Cr steels were shot-peened and vacuum annealed at 750°C for 1 h, 2 h, 3 h, 5 h and 15 h. Subsequently these steels were oxidised at 750°C. The mass gain in all cases is different for both steels, and in the case of both 12 wt% Cr and 18 wt% Cr steels the best oxidation resistance was achieved for the shot-peened +1 h pre-annealed sample.     

基于多丸粒模型的喷丸表面强化过程数值模拟

针对喷丸表面强化过程,利用有限元法,建立了均匀分布的多丸粒喷丸强化数值模型,研究了喷丸速度、连续冲击及二次冲击弹丸速度对于目标靶体内残余应力场的影响;利用多丸粒偏置建模法,建立不同覆盖率的多丸粒仿真模型,研究喷丸覆盖率、连续冲击对于残余应力场的影响,对比了两种多丸粒喷丸模型。结果表明,提高喷丸速度可增加残余压应力层深度,但对残余应力最大幅值没有显著影响;连续冲击引起残余应力的饱和,残余应力分布没有明显变化;后续冲击弹丸的速度对于残余应力场有明显的影响;偏置高覆盖率喷丸强化模型可获得较均匀分布的残余应力场,压应力最大幅值与喷丸覆盖率及作用区域有关。     

基于SPH耦合有限元法的喷丸残余应力场数值模拟

针对有限元方法不能有效模拟喷丸加工过程中大量弹丸反复冲击的现状,使用光滑粒子流体动力学法(smoothed particle hydrodynamics,SPH)耦合有限元法(finite element method, FEM)模拟喷丸强化过程。工件采用FEM建模,弹丸采用SPH建模,通过接触算法实现SPH和FEM的耦合以模拟弹丸对工件的强化作用。提出弹丸流的材料模型,分析了相关参数对工件残余压应力分布和能量利用率的影响。结果表明,随着弹丸打击次数的增加,工件表面残余压应力分布逐渐趋于稳定;高覆盖率能有效改善工件表面残余压应力分布,低覆盖率则会降低喷丸效果;适当提高弹丸速度虽然可以使残余应力层深度和最大残余压应力值增加,但会降低能量利用率。通过与相关实验数据的比较,验证了仿真模型和结果的正确性。     

喷丸强化过程及冲击效应的数值模拟

运用大型有限元软件LS-DYNA建立了喷丸强化处理过程的三维有限元模型, 研究了弹丸冲击作用下, 铝合金材料Al 2024-T3动态响应过程中的应力波结构、应变率效应及应力波衰减效应等动态参量; 研究了弹丸搭接率对于残余应力场的影响, 建立了高覆盖率多丸粒强化模型; 研究了冲击顺序、材料应变率及初始残余应力对强化效应的影响; 研究了喷丸强化后的表面微观变形特征. 分析结果表明, 弹丸高速冲击引起的弹塑性双波会在材料内部形成高应变率效应; 弹丸搭接率ζ对于强化效应有明显影响, ζ=1/2是近似的临界值; 不同冲击顺序对于强化效果的影响较小, 材料应变率对于强化效果有显著影响; 初始残余应力对于喷丸强化最终形成的残余应力场的影响取决于弹丸冲击速度; 经过喷丸强化处理, 零件表面形成微米级凹坑, 并且随着冲击次数和喷丸覆盖率的提高, 凹坑深度逐渐增加.     

Microstructure and Corrosion Behavior of Friction Stir Welded Al Alloy Coated by In Situ Shot-Peening-Assisted Cold Spray

To improve the corrosion resistance of friction stir welded (FSW) joints, in situ shot-peening-assisted cold spray coating was applied to the surface of FSW joints. The microstructure and corrosion behavior of the coatings were investigated. The results showed that a dense coating was obtained by cold spraying technology. Moreover, the use of in situ shot-peening-assisted technology significantly reduced the porosity of the coating, which has a positive impact on the microstructure and the mechanical performance. Exfoliation corrosion tests showed that the coating affected positively on the corrosion resistance. On the coated surface, the craters caused by shot-peening particles were corroded in the form of exfoliation corrosion, while the exposed particles were corroded in the form of intercrystalline corrosion. This work provides a new approach to the corrosion protection of FSW joints.