An investigation of pore cracking in titanium welds

Two welded Ti-6A1- 4V pressure vessels leaked prematurely in service. The leaks were caused by cracks emanating from weld porosity. The cracks originated during fabrication, with subsequent growth in serv-ice leading to the formation of the leak paths. Pore cracking is thought to be caused by a mechanism that involves both sustained- load and cyclic contributions, with the former being the more prominent. It is shown that the tendency for cracking is influenced by pore position and that pore size is not a deciding factor in that regard. The factors that govern pore cracking are discussed, and the possible role of inter-stitial embrittlement is assessed.     

钛及钛合金的焊接

钛及钛合金因其优异的综合性能和独特功能被广泛应用于航空航天和海洋工程等重要领域,而不同类型的钛及钛合金的性能各不相同。介绍钛及钛合金的分类及其焊接特性,分别描述工业纯钛、α型钛合金、α+β型钛合金、近β和β型钛合金的分类方法、添加元素及基本特性,综述不同类型的钛及钛合金的国内外焊接研究现状,重点关注各种钛及钛合金的焊接方法、焊后组织和性能,归纳总结各种钛及钛合金的主要焊接方法为TIG焊、激光焊和电子束焊。     

Liquation and post-weld heat treatment cracking in Rene 80 laser repair welds

An extensive experimental study on a nickel-based superalloy, Rene 80 using autogenous laser welding has been undertaken to determine the effect of the process parameters and weld bead geometry on cracking in the as-welded and post-weld heat treated conditions. Little cracking was observed in the as-welded condition with low powers and beam diameters around 2.5 mm. Welding speed had little effect on the incidence of cracking in the as-welded condition. Investigation of the aspect ratio (penetration divided by width) indicated that little cracking occurred in the as-welded condition when the aspect ratio was approximately 0.5. The same effect was observed with the post-weld heat treated samples. An analysis of the microstructures indicated that the cracking was caused primarily by liquation in the as-welded condition and was exacerbated by post-weld heat treatment cracking during the subsequent heat treatment. Finally the study resolved some of the contradictory findings in the literature on the effect of process parameters on the incidence of cracking in the as-welded and post-weld heat treated conditions.     

Fatigue crack growth behaviors of a new burn-resistant highly-stabilized beta titanium alloy

This article presents the fatigue crack growth (FCG) behaviors of a new burn-resistant highly-stabilized beta Ti40 alloy. The FCG rates were analyzed. The fracture surfaces and the side surfaces of the test samples were explored. The results show that frequency affects the cracking behaviors of Ti40 alloy. Temperature also plays an important role in Ti40 alloy cracking. At room temperature (25°C), when the frequency increases, the cracking rate changes a little in the range of low stress intensity factor (ΔK), while it changes significantly when ΔK is high. At 500°C, the cracking rate of Ti40 alloy changes significantly during all the course of cracking. The frequency also affects the microstructure patterns of Ti40 alloy. A number of secondary cracks appear in the area more than 200 μm from the main crack at a high ΔK when the frequency is 1 Hz, but only a few secondary cracks exist when the frequency is 10 Hz. Facet image is the main image of the fracture surfaces when the frequency is 1 Hz. While, ductile striation occupies most of the area of fracture surfaces when the frequency is 10 Hz.     

Effect of intermetallics on susceptibility of Mg alloy welds to liquation cracking

ABSTRACT

The circular-patch welding test was used to study the liquation and liquation cracking of AZ-series Mg alloys. A heat treatment was carried out on the as-received AZ91 alloy to dissolve the γ(Mg₁₇Al₁₂) particles before welding. The circular-patch welding test was then conducted on the heat-treated, as-received AZ91 alloy using AZ61 and AZ91 filler wires. The results showed that the susceptibility of AZ91 alloy to liquation and liquation cracking was significantly reduced by the dissolution of massive γ(Mg₁₇Al₁₂) particles via a heat treatment before welding as the liquation mechanism was changed. Both constitutional liquation and incipient melting occurred in the partially melted zone of the as-received AZ91 welds, while only incipient melting occurred in the heat-treated AZ91 welds.     

Hydrogen behavior in titanium aluminide alloys

This is a synthetical report about hydrogen behavior in titanium aluminide alloys in our group. There are two kinds of hydrogen solubility in titanium aluminides, one is the overall solubility at high temperature in the matrix without hydride and the other is the terminal solubility at low temperature in the matrix in equilibrium with the hydride. The former decreases but the later increases with increasing temperature. Hydrogen as a temporary β stabilizer clearly decreases the size of the α2 phase, and increases greatly the amount of β phase, and then increases evidently the mechanical properties of Ti3Al＋Nb. The cathodic corrosion of TiAl during charging is due to hydride on the surface. The decrease of the strength, the strain to fracture and fracture toughness for hydrogenated samples is due to hydride. The enrichment of atomic hydrogen at the crack tip during charging under sustained load can enhance localized plastic deformation and cause hydrogen-induced delayed cracking.     

钛中氘致裂纹的原位研究

采用热台显微镜(HSM)和压力-体积-温度(PVT)相结合的方法原位研究了钛在恒定温度及升温过程中的氘致裂纹(DIC)现象。结果表明:在550℃恒温吸氘(D2)过程中,钛片表面只出现很少的裂纹;而在由室温升温至550℃吸氘过程中钛表面出现了由边缘向中心部分扩展的环状裂纹。钛在升温过程中的形貌变化特征与"边缘进攻"模型符合;X射线光电子能谱(XPS)测试显示,由钛氧化物、碳化物和氮化物组成的钛表面钝化层在环状裂纹形成过程中发挥了重要作用。     

铝合金焊接凝固裂纹高温动态开裂行为

详细研究了5083、6082、ZL101三种铝合金的凝固金相组织，观察和记录了三种材料高温拉伸开裂动态过程及开裂后的断口特征。结果表明，材料的冶金因素制约着凝固裂纹的动态开裂行为，即材料的冶金特性不同，它们的凝固裂纹开裂模型也不同。文中由此总结了凝固裂纹的三种开裂模型。第一，裂纹形成时存在“愈合作用”，ZL101属于此列。第二，裂纹形成时伴随着金属桥的变形与断裂，如5083材料。第三。裂纹形成时晶粒是沿液膜分离。如6082。     

The effects of gas nitriding on fatigue behavior in titanium and titanium alloys

Fatigue behavior has been studied on gas-nitrided smooth specimens of commercial pure titanium, an alpha/beta Ti-6Al-4V alloy, and a beta Ti-15Mo-5Zr-3Al alloy under rotating bending, and the obtained results were compared with the fatigue behavior of annealed or untreated specimens. It was found that the role of the nitrided layer on fatigue behavior depended on the strength of the materials. Fatigue strength was increased by nitriding in pure titanium, while it was decreased in the Ti-6Al-4V and Ti-15Mo-5Zr-3Al alloys. Based on detailed observations of fatigue crack initiation, growth, and fracture surfaces, the improvement and the reduction in fatigue strength by nitriding in pure titanium and both alloys were primarily attributed to enhanced crack initiation resistance and to premature crack initiation of the nitrided layer, respectively.     

A study on the porosity of CO2 laser welding of titanium alloy

0IntroductionTitanium alloys are increasingly applied in aeronauticindustry because of its higher strength to weight ratio thansteel and superior fatigue performance to aluminum alloy.At the same time there are many newtitanium-based alloysoccurring,such as Ti3Al-Nb titanium aluminide[1].Weldsof titanium alloy are prone to porosity,presenting a poten-tial problem for many application requiring sealing,corro-sion and fatigue resistance and good fracture toughness.Many studies have demonstrate…     

Mechanical properties and strengthening mechanisms in laser beam welds of pure titanium

Abstract

The mechanical properties and strengthening mechanisms in laser beam welds of pure titanium were investigated. Although grain coarsening is evidently observed in the heat affected zone (HAZ) and fusion zone (FZ) compared with the base metal (BM), the tensile and hardness tests indicate that the HAZ and FZ are stronger than the BM under the welding conditions employed in the present work. The strengthening mechanism in the HAZ is ascribed to the substructure strengthening and that in the FZ is attributed to the combination of the substructure strengthening and the solute solution strengthening.     

Enhancing the weldability of CP titanium friction stir welds with elemental foils

ABSTRACT

Friction stir welding (FSW) has proven to be a viable technique for joining a wide variety of alloys. However, thick section welding of alpha and near-alpha Ti alloys has proven particularly challenging. Previous research at the Naval Research Laboratory using Ni markers in CP Ti friction stir welds indicated that elemental additions of Ni to the joint line can provide substantial benefits for improved weldability of these alloys. The current study surveys the effects of Ni and other elemental additions to CP Ti friction stir welds to determine their influence on the resultant weld microstructure, weld surface finish, and welding machine forces. These results reveal that Ni provides the most benefits for the concentrations examined, but other elements may also provide benefits at lower concentrations. The addition of these elements may improve the weldability and weld quality for FSW of CP Ti, enabling thick section welding of this and similar alloys.     

电化学充氢条件下X70管线钢及其焊缝的氢致开裂行为

采用电化学充氢的方法研究了X70管线钢在不同浓度硫酸溶液中的氢致开裂(HIC)行为．结果表明,增大充氢电流密度、延长充氢时间以及降低充氢溶液的pH值能够促进氢进入X70钢基体．微观观察表明,X70钢中的非金属夹杂物如氮化物和氧化物等对其氢致开裂行为有不同的影响,氮化物夹杂并不是充氢裂纹的必然形核位置,而Mg,Al,Ca等的氧化物是更为有害的氢致裂纹源．通过氢渗透实验测得室温下氢在X70钢中的有效扩散系数为3．34×10-9cm2／s．对XT0管线钢基体及焊缝试样电化学预充氢后拉伸,焊缝试样的拉伸塑性较差,各项塑性指标在充氢前、后均低于X70钢基体材料．     

TC4钛合金光纤激光摆动焊抑制小孔型气孔的原因分析

采用光纤激光器对8 mm厚TC4钛合金板进行振镜摆动焊接,采用光谱仪和高速摄像机采集等离子体的光谱、图像及小孔的图像,分析摆动焊接抑制小孔型气孔的原因.结果表明,摆动光束对抑制钛合金小孔型焊接气孔具有显著作用;光束未摆动焊接时,焊缝中的气孔率达9.8%;光束摆动后焊缝中的气孔率均降低,其中焊接参数为5 kW、2 m/min,摆动参数为80 Hz、0.5 mm时,小孔形气孔被完全抑制.与光束未摆动相比,光束摆动焊接的小孔稳定性显著增加,其原因是光束摆动提高了光束与熔池液面的接触面积,金属蒸发增强,驱动小孔张开的径向力和轴向力得以增加.     

Prediction of solidification cracking in laser welds of type 310 stainless steels

The occurrence of solidification cracks in laser welds of type 310 stainless steels was predicted by numerical analyses of the solidification brittle range (ductility curve for cracking) and thermal strain in the weld metal. The solidification brittle range in laser welding was estimated from that in arc welding based on the numerical analyses of supercooling (for calculating dendrite tip temperature) and segregation (for calculating completely solidified temperature) during rapid solidification. The calculated solidification brittle range was reduced with an increase in the welding speed because of the enhanced supercooling and the inhibited solidification segregation. The thermal strain analysis by FEM suggested that solidification cracks would occur in SUS310S welds at laser travelling velocity of 60 mm/s applying the initial strain of 1.5%, while no solidification cracks in SUS310EHP welds at any laser travelling velocities applying the higher initial strain of 2.2%. The cantilever type cracking test in laser welding revealed that the predicted results of occurrence of solidification cracks were consistent with experimental ones.     

Effect of high-density current electropulsing on corrosion cracking of titanium aluminide intermetallic

The effect of electropulsing on the corrosion cracking of titanium aluminide produced by self-propagating high-temperature synthesis has been investigated. The electropulsing treatment led to improved corrosion resistance in sodium fluoride solution and also eliminated corrosion cracking at the α₂/γ interface during corrosion in a solution of nitric and hydrofluoric acids. This behavior was attributed to thermal and athermal effects resulting from electropulsing and leading to the interaction of conduction electrons with the defect structure. The effect of magnetic field accompanying electropusling on depinning of dislocations also has been discussed. Support for this is provided on the basis of X-ray diffraction analysis and microhardness testing.