钛合金拉伸高速铣削试验研究

通过对钛合金TC4在拉伸状态下的铣削试验,重点研究了高速铣削对钛合金TC4(Ti6A14V)表面残余应力和表面粗糙度的影响,得到了在不同切削参数下钛合金TC4表面残余应力和表面粗糙度的实验数据.实验结果表明,拉伸装夹基本不影响表面粗糙度,但可以大大提高加工表面残余压应力并增大残余压应力层的厚度,为开展钛合金拉伸高速铣削加工提供了依据.     

不同喷丸强度下镍铝青铜的表面喷丸强化效果

采用0.15,0.20,0.25mm的喷丸强度对铸态镍铝青铜进行了表面喷丸强化处理,对比分析了喷丸前后镍铝青铜表层的残余应力、显微硬度、微观形貌及表面粗糙度,并计算了表层的亚晶尺寸和显微畸变。结果表明:随着喷丸强度的提高,镍铝青铜表面的残余压应力和加工硬化程度都逐渐增大;但喷丸强度过大时,表面粗糙度明显增大并伴随有显微裂纹产生,从而导致表层应力释放,使得残余压应力增大幅度下降,减弱了强化效果。     

喷丸对重载齿轮用18CrNiMo7-6钢抗胶合性能的影响

分别采用传统喷丸、微粒子喷丸、传统喷丸+微粒子喷丸工艺对渗碳淬火齿轮钢18CrNiMo7-6进行表面处理,研究了喷丸处理对表面粗糙度、硬度、残余应力及胶合载荷的影响。结果表明:3种喷丸工艺均可以提高试验钢的表面硬度,传统喷丸+微粒子喷丸对表面硬度的提高幅度最大,其次为微粒子喷丸;传统喷丸增大了试验钢的表面粗糙度,微粒子喷丸和传统喷丸+微粒子喷丸降低了表面粗糙度,微粒子喷丸处理后的表面粗糙度最低;3种喷丸工艺均会在试验钢表面引入残余压应力,微粒子喷丸和传统喷丸+微粒子喷丸引入的最大残余压应力位于表面,而传统喷丸引入的位于次表面;传统喷丸+微粒子喷丸处理后试验钢的胶合载荷最大,其次为微粒子喷丸处理后的,这与微粒子喷丸提高了表面硬度和表面残余压应力,同时降低了表面粗糙度有关。     

预应力切削钛合金的有限元分析

为揭示预应力切削对钛合金Ti6Al4V加工表面残余应力的调整机理,探讨切削时锯齿形切屑的形成过程,基于预应力切削原理建立了钛合金的预应力切削有限元模型,模拟了0、280 MPa和560 MPa这3种预应力下的锯齿形切屑形成过程以及已加工表面的残余应力分布。结果表明:采用预应力切削方法可以调整钛合金已加工表面的残余应力状态;预应力对锯齿形切屑的形成过程和切屑特征无明显影响;在材料弹性极限内施加越大的预应力,表面层残余压应力效果越显著,次表层最大残余压应力值越高,残余压应力层分布也越深。     

42CrMoA钢的单次喷丸和复合喷丸强化

对42CrMoA高强钢进行了单次喷丸、精整复合喷丸、抛光复合喷丸强化处理,研究了不同喷丸工艺下钢的表面粗糙度、疲劳性能、表层残余应力、组织细化程度及显微硬度。结果表明:喷丸方式对42CrMoA钢最大残余压应力和应力层深度影响不大;单次喷丸、精整复合喷丸、抛光复合喷丸后钢的表面残余压应力、表面组织细化程度、显微硬度及疲劳极限依次增大,表面粗糙度则依次减小;复合喷丸尤其是抛光复合喷丸可显著降低42CrMoA钢的喷丸表面粗糙度。     

切削用量对钛合金已加工表面残余应力的影响

为了得到切削用量对钛合金加工表面残余应力的影响,本文根据金属切削有限元分析相关理论,建立了三维斜角切削有限元模型,对钛合金Ti6Al4V的铣削加工进行了模拟分析,得到了切削速度和切削深度对工件加工表面残余应力的影响。由模拟结果得出:残余应力随切削速度的增大而逐渐增加,切削速度对加工表面残余应力的影响较大,同时随着切削速度的改变,表面残余应力层的厚度有上升趋势;残余应力在已加工表面外表面表现为拉应力,沿深度方向由拉应力逐渐过渡到压应力;切削深度对残余应力及残余应力层的影响都较小。     

Ti6Al4V钛合金冲蚀机理研究

为研究Ti6Al4V钛合金在气固两相流使用环境中的损伤机理,进行了室内力学性能试验以及不同攻角下的冲蚀试验,并借助扫描电子显微镜对试验后试样表面的显微形貌进行分析。试验结果表明:较低角度30°时,"显微切削"和"挤压锻打"作用对Ti6Al4V钛合金试样表面会造成较为严重的损伤。     

两种砂轮磨削Ti5Al4V钛合金的表面质量比较

分别用SiC、CBN两种砂轮对Ti5Al4V钛合金进行了干磨削试验,研究了在0.01,0.03,0.05mm三种磨削深度下钛合金的表面硬度、表面粗糙度和表面形貌。结果表明:随着磨削深度的增加,经SiC砂轮磨削后,Ti5Al4V钛合金表面形貌逐渐恶化,出现了磨削烧伤和磨削裂纹,次表层的硬度下降,出现了软化层;在相同的磨削深度下,CBN砂轮磨削的Ti5Al4V钛合金表面质量优于SiC砂轮磨削的,主要原因是CBN砂轮磨削时产生的磨削力小和磨削热较少。     

工艺参数对TC4钛合金喷丸强化影响的仿真分析

利用有限元法针对TC4钛合金的喷丸强化过程,建立了数值模型。首先,运用Abaqus建立的TC4钛合金喷丸强化的有限元模型,分别计算喷丸速度分别为40、60、80、100 m/s的模型,结果表明：当喷丸速度逐渐增大的过程中,表面残余应力,最大残余压应力和残余压应力层厚度都逐渐增大;其次,建立的TC4钛合金喷丸强化的有限元模型,分别计算了垂直入射、入射角为10°和入射角为20°的模型,结果表明：随着入射角的增大,靶材的表面粗糙度并没有显著变化,但对残余压应力的影响不可忽视,主要体现在最大残余应力由607.3 MPa逐渐减小为504.4 MPa,降幅20.4%,残余压应力层的厚度由158.5μm降低到145.2μm,降幅8.4%。入射角度控制在0～10°之间最合适时可以使得喷丸强化效果最优。     

喷丸对TC4钛合金残余压应力场及疲劳寿命的影响

研究了不同弹丸及喷丸参数对喷丸后TC4钛合金表面形貌、表面塑性变形程度、残余压应力场和疲劳寿命的影响。结果表明:与铸钢弹丸相比,陶瓷弹丸喷丸强化后TC4钛合金表面的起伏程度变化不大,但能有效地覆盖加工刀痕;随喷丸压力增大和喷丸时间延长,试样表面的累积塑性变形先快速增大后趋于饱和;当喷丸压力达到0.25 MPa、铸钢弹丸喷丸时间大于40 s或陶瓷弹丸喷丸时间大于80 s时,最大残余压应力可达到610 MPa,残余压应力场深度超过250μm;两种弹丸喷丸强化后,TC4钛合金的疲劳寿命分别可提高10倍和20倍以上。     

Analysis of Fretting Fatigue of Cavitation Shotless Peened Ti–6Al–4V

Fretting fatigue behavior of cavitation shotless peened titanium alloy, Ti–6Al–4V coupons was investigated using finite element method and a critical plane-based multi-axial fatigue parameter. Cavitation shotless peening (CSP)-induced compressive residual stress, which was larger at the contact surface than its counterpart from the shot peening (SP). However, compressive residual stress decreased more sharply with distance from the contact surface in CSP than in SP. Analysis using a critical plane-based multi-axial fatigue parameter demonstrated that the crack initiation would occur inside the cavitation shotless peened specimen which matched with the experimental observations. On the other hand, crack initiation would occur on the contact surface in the shot peened specimen which again was in agreement with experiments. The analysis also showed that the crack propagation part of the total fretting fatigue life was longer in the shot peened specimen than in the cavitation shotless peened specimen while the crack initiation part was almost equal from both peening methods. Therefore, CSP could not improve the fretting fatigue life/strength as much as the SP did but it improved relative to the un-peened specimen.     

IMPROVING FATIGUE STRENGTH OF METALS USING ABRASIVE WATERJET PEENING

Abrasive waterjet (AWJ) peening has been proposed as a viable method of surface treatment for metal orthopedic devices. In this study the influence of AWJ peening on the compressive residual stress, surface texture and fatigue strength of a stainless steel (AISI 304) and titanium (Ti6Al4V) alloy were studied. A design of experiments (DOE) and an analysis of variance (ANOVA) were used to identify the primary parameters contributing to the surface texture and magnitude of surface residual stress. The influence of AWJ peening on the fatigue strength of the metals was evaluated under fully reversed cyclic loading. It was found that AWJ peening results in compressive residual stress and is primarily influenced by the abrasive size and treatment pressure. The residual stress of the AISI 304 ranged from 165 to over 460 MPa. Using the optimum treatment parameters for maximizing the residual stress, the endurance strength of Ti6Al4V was increased by 25% to 845 MPa. According to results of this study AWJ peening is a viable method of surface treatment for applications that require an increase in surface roughness and maintenance or increase in fatigue strength, qualities that most often are not available from a single process.     

IMPROVING FATIGUE STRENGTH OF METALS USING ABRASIVE WATERJET PEENING

Abrasive waterjet (AWJ) peening has been proposed as a viable method of surface treatment for metal orthopedic devices. In this study the influence of AWJ peening on the compressive residual stress, surface texture and fatigue strength of a stainless steel (AISI 304) and titanium (Ti6Al4V) alloy were studied. A design of experiments (DOE) and an analysis of variance (ANOVA) were used to identify the primary parameters contributing to the surface texture and magnitude of surface residual stress. The influence of AWJ peening on the fatigue strength of the metals was evaluated under fully reversed cyclic loading. It was found that AWJ peening results in compressive residual stress and is primarily influenced by the abrasive size and treatment pressure. The residual stress of the AISI 304 ranged from 165 to over 460 MPa. Using the optimum treatment parameters for maximizing the residual stress, the endurance strength of Ti6Al4V was increased by 25% to 845 MPa. According to results of this study AWJ peening is a viable method of surface treatment for applications that require an increase in surface roughness and maintenance or increase in fatigue strength, qualities that most often are not available from a single process.     

A non-local fatigue approach to quantify Ti-10V-2Fe-3Al fretting cracking process: Application to grinding and shot peening

This paper presents an experimental study on the tribological behaviour and cracking response of a Ti-10V-2Fe-3Al titanium alloy under fretting loading with a cylinder on plane configuration. Three types of surfaces were investigated: a polished one considered as the reference, a ground one and a shot peened surface. Surfaces were compared with respect to residual stress, hardness and roughness. The first step of this study was to determine sliding conditions and coefficient of friction of the three contact types. Next, fretting tests under stabilized partial slip regime were carried out to investigate crack nucleation and propagation. Results show that whatever surface roughness or residual stress in the material, tribological behaviour is the same. These latter confirm that sliding condition and coefficient of friction in partial slip regime is due to material effect and not to roughness or surface hardness. Then, residual stress induced by grinding or shot peening have no influence on the crack nucleation threshold under fretting solicitation because crack nucleation is only induced by a sufficient tangential loading. The crack nucleation threshold is formalized by applying the Crossland criterion taking into account the stress gradient and the ensuing “size effect”. As expected, cracks propagation is influenced by residual stress under the surface. Compared to the reference case, for a same loading parameters set, residual stress induced by grinding is not sufficient to decrease the crack length reached whereas effects of shot peening decrease highly these latter. So, there is a threshold of residual stress from which residual stresses are useful against cracking.     

Slurry Erosion Wear Resistance and Impact-Induced Phase Transformation of Titanium Alloys

Erosion wear resistance and impact-induced phase transformation of titanium alloys TA2 (pure Ti), TC4 (Ti–6Al–4V) and TC11 (Ti–6.5Al–3.5Mo–1.5Zr–0.3Si) were investigated using a slurry jet tester. The slurry erosion wear resistance of TA2 is comparable to that of 304 stainless steel, especially at the impingement angle 90°. Although TC4 and TC11 have higher hardness, TA2 possesses the best erosion wear resistance except TC11 at 15°. With the increasing erosion time, the eroded surface hardness of TC11 at the impingement angle 90° increases and then decreases, while the volume loss rate drops in the first 15 min, then increases until 30 min, and then slightly decreases again. With XRD characterization and SEM observation, erosion-induced phase transformation from metastable β-phase to α-phase is proved on the surface of titanium alloy TC11. And the thickness of visible phase transformation layer is about 10 μm. Phase transformation influences the erosive wear mechanism of titanium alloys. At the impingement angle of 30°, the material removal of TC4 and TC11 can be described as micro-plowing and lip extruding, while plowing mark is not a typical surface morphology of TA2, indicating a better work-harden ability. So, stabilizing β-phase can be an effective way to improve the erosion wear resistance of titanium alloys.     

Solid particle erosion behaviour of Ti6Al4V alloy

Abstract

The effects of particle impingement angle, impingement velocity and erodent particle size on the erosion rate and surface morphology of the Ti6Al4V alloy have been investigated comprehensively in order to evaluate solid particle erosion behaviour of Ti6Al4V alloy. Samples were eroded in a specially designed sandblasting system under various parameters using alumina (Al₂O₃) erodent particles. Surface morphology investigations were examined by scanning electron microscope using various analysis and modes (energy dispersive X-ray analysis, elemental mapping and compositional contrast). Ti6Al4V alloy showed ductile behaviour with a maximum erosion rate at 30° impingement angle. Erosion rate of Ti6Al4V alloy increased with increases in velocity and decreased with increases in erodent particle size. Scanning electron microscopy investigations of eroded surfaces of Ti6Al4V alloy samples reveal the dominant erosion mechanism such as microploughing, microcutting and plastic deformation. Embedded erodent particles on the surfaces of Ti6Al4V alloy nearly at all particle impingement angles and velocities were clearly detected.     

喷丸三维残余应力场的有限元模拟

运用大型有限元计算软件ABAQUS建立了模拟喷丸残余应力场的三维有限元模型,预测了在相同喷丸强度下玻璃丸和钢丸两种类型弹丸喷射所产生的残余应力场。模拟过程中,分析了线性减缩积分单元的沙漏参数、材料的应变硬化率、喷丸覆盖率以及初始残余拉应力等因素对304不锈钢靶材残余应力分布的影响。从计算结果可以看出,钢丸喷丸产生的残余压应力层较深,但在高覆盖率时,玻璃喷丸产生的残余压应力的平均值比钢丸喷丸处理后产生的大。在有初始残余拉应力(250 Mpa)存在的情况下,两种类型的喷丸处理均能使304不锈钢靶材表面形成残余压应力层,这说明喷丸工艺可以提高奥氏体不锈钢焊接构件的抗应力腐蚀开裂能力。本研究成果为进一步探讨喷丸强化不锈钢焊接头抗应力腐蚀性能的机理奠定了基础。     

The influence of laser assistance on the machinability of the titanium alloy Ti555-3

The Ti533-3 alloy is a new titanium alloy which is starting to see increased use in the aeronautical domain to improve the durability of components and to optimize the weight/resistance ratio. This alloy is characterized by greater resistance compared to the more commonly used titanium alloys such as Ti6Al4V. However, a disadvantage of the Ti533-3 alloy is that it is very difficult to machine. In this work, the use of laser-assisted machining has been tested to improve chip formation by a thermal softening phenomenon and to improve the machining productivity of the alloy. A parametric investigation of laser assistance on the machinability of the Ti555-3 titanium alloy shows that: (1) the cutting forces can be greatly decreased if the surface temperature is high; (2) the thermal gradient induced by laser heating modifies the surface integrity in terms of strain hardening and residual stresses in the workpiece; and (3) the chip formation mechanisms are also changed, by increasing the sawteeth frequency when using laser assistance.     

喷丸强化与应力因素对2E12铝合金晶间腐蚀行为的影响

为有效控制2E12铝合金的晶间腐蚀失效行为,采用浸泡腐蚀试验、金相分析技术、扫描电镜分析技术和X射线衍射分析技术等研究了喷丸强化和应力因素对2E12-T3铝合金晶间腐蚀行为的影响规律,并探讨了作用机理。研究结果表明:2E12-T3铝合金具有很高的晶间腐蚀敏感性,拉应力会明显增强其敏感程度,而压应力则会降低其敏感性。采用合理的喷丸强化处理(喷丸强度取0.15 mmN,覆盖率取100%)能够明显降低2E12-T3铝合金的晶间腐蚀敏感性,因为喷丸在铝合金表面引入合理分布的残余压应力,同时使表层晶粒细化,铝合金的电化学活性降低,晶间的选择性腐蚀得到有效控制。然而,喷丸强度过高或喷丸覆盖率过大时,铝合金表面会存在较严重的脱层等损伤性破坏,反而会使2E12-T3铝合金的晶间腐蚀敏感程度提高。