Fracture toughness and fatigue crack growth in Ti‐6Al‐4V friction stir welds

Friction stir welding of titanium holds the promise for producing joints with microstructures and mechanical properties that are more comparable to wrought material than traditional fusion welding processes. Extensive data exist on the microstructure and static mechanical properties of titanium friction stir welds, but very little are available on the durability (fatigue) and even less on the damage tolerance (fracture toughness and fatigue crack growth). This paper presents the results of an investigation into the damage tolerance of friction stir welds made in 6 mm thick Ti‐6Al‐4V after a post‐weld heat treatment. It was found that the apparent fracture toughness was lower than the wrought base material, 7–25% depending on the crack orientation relative to the weld, but the crack growth performance (ΔK vs. da/dN) of the weld in the absence of weld‐induced residual stresses was identical to the base material.     

Fatigue behaviour of dissimilar friction stir spot welds between A6061‐T6 and low carbon steel sheets welded by a scroll grooved tool without probe

A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile‐shear fatigue tests were performed using lap‐shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile‐shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.     

The potential adaptation of stationary shoulder friction stir welding technology to steel

Stationary shoulder friction stir welding is a newly developed technique currently used for joining plates of relatively soft metals at different angular planes. The process is not currently applicable to steel, hence the present study was developed to investigate the theoretical and technical viability of stationary shoulder technology in DH36 steel. Aluminium welds were produced using both conventional rotating shoulder and stationary shoulder friction stir welding techniques, and steel welds were produced using only conventional friction stir welding techniques. The effects of stationary shoulder technology on both the microstructural evolution and resultant mechanical properties of aluminium have been evaluated so that the likely effects on steel could be predicted. In the aluminium welds, the stationary shoulder technique results in a distinct transition between stirred and unstirred material, contrasting to the gradual change typically seen in conventional friction stir welds produced with a rotating shoulder. An investigation of weld properties produced in DH36 steel has demonstrated that if the stationary shoulder weld technique was used, the microstructure likely to be formed, would be dominated by a bainitic ferrite phase and so would exhibit hardness and tensile properties in excess of the parent material. It is predicted that if the same abrupt transition between unstirred and stirred material observed in aluminium occurred in steel, this would lead to crack initiation, followed by rapid propagation through the relatively brittle weld microstructure. Hence, these findings demonstrate that without further design and process improvements, stationary shoulder friction stir welding is unlikely to be applicable to steel.     

Friction stir welding of thick section Al‐Zn‐Mg‐Cu aluminum alloy

Friction stir butt welding of 25 mm thick AA7075–T651 plates has been investigated. Careful process parameter selection resulted in single pass, full‐penetration defect free welds. The weld nugget exhibits a significant grain refinement while facing the dissolution of strengthening precipitates. Microhardness survey gives a W‐shaped profile with lower hardness values recorded in the thermo‐mechanically‐affected zone. Tensile fractures occur, again, in the thermo‐mechanically‐affected zone, where minimum hardness occurred. The friction stir welds demonstrate an excellent root bend performance while falling behind base material in face bend test. The welds also displayed outstanding impact toughness compared to that of parent material. It is concluded that defect free single pass friction stir welds can be successfully made on 25 mm thick AA7075–T651 plates.     

Effect of post-weld heat treatment on fracture toughness and fatigue crack growth behaviour of electron beam welds of a titanium (α+β) alloy

The effects of a post-weld heat treatment on the fracture toughness and fatigue crack growth behaviour of electron beam welds of an α + β titanium alloy, Ti–6.5Al–1.9Zr–0.25Si have been studied. Welds in the stress-relieved condition exhibited poor fracture toughness due to poor energy absorbing capacity of the thin α and α' phases. Post-weld heat treatment which resulted in the decomposition of α' to α + β and the coarsening of intragranular and intergranular α resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and intergranular α phase. Fatigue crack growth resistance of welds was superior to the base metal in the α + β heat-treated condition. The superior crack growth resistance of the welds is due to the acicular α microstructure which results in a tortuous crack path and possible crack closure arising from crack path tortuosity.     

R‐curve behaviour of friction stir welds in aluminium‐lithium alloy 2195

ABSTRACT The fracture resistance of friction stir welds in 2195‐T8 is described in this paper. R‐curves were produced for several crack planes, parallel to the welding direction and situated at varying distances from the weld centreline. The friction stir weld was also characterized by hardness and tensile testing. Fracture resistance of the friction stir weld is compared to the base metal and to a variable polarity plasma arc weld. Results show that the material in and around the friction stir weld is tougher than the base metal and the variable polarity plasma arc weld. The friction stir weld fracture performance is discussed in the light of the observed hardness and fractographic data presented.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.     

Mikrostrukturelle und mechanische Eigenschaften von laserstrahlgeschweißtem Magnesiumfeinblech AZ31‐HP mit und ohne Zusatzwerkstoffe

Microstructural and mechanical properties of laser welded sheets of magnesium AZ31‐HP with and without filler wires This paper describes Nd:YAG laser beam welding experiments carried out on rolled 2.5 mm thick magnesium sheet AZ31‐HP. For the butt welds in flat position, filler wires AZ31X and AZ61A‐F were used, diameter 1.2 mm. The microstructure and mechanical properties of the different laser beam welded joints were examined and compared with one another. The obtained results show that the laser beam welding of AZ31‐HP sheet is possible without hot crack formation, both without and with filler wires. The determined tensile strength, ductility, fracture toughness and microhardness of laser beam welded joints without filler wire were not effected by AZ31X nor AZ61A‐F. By use of these filler wires loss of zinc was minimized and the shape of weldments was optimized. The values of fracture strength, yield strength and microhardness of the joints and base material are quite similar. It is found that the ductility of the joints is lower than the base materials due to the heterogeneous microstructure of the fusion zones and geometrical notches of the weld seams. Both, weld and base material of AZ31‐HP, showed stable crack propagation. Furthermore, for base material slightly lower fracture toughness values CTOD than for the joints were determined.     

Comparison of fatigue properties between friction stir welds and TIG welds

对比分析了搅拌摩擦和氩弧焊两种工艺方法对铝合金焊接接头疲劳性能的影响,建立了焊接接头的S-N曲线,结果表明：在相同的载荷条件下,搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头。搅拌摩擦焊接头疲劳寿命N=106次的疲劳强度值约为59~65MPa之间。对焊接接头显微组织的分析表明：搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区,阻碍了滑移带的形成和裂纹的扩展,从而提高了接头的疲劳性能。TIG焊接接头疲劳端口分析显示,焊接缺陷是主要的疲劳裂纹源。     

Elucidation of fatigue characteristics and fracture mechanism of friction stir spot‐welded tension–shear joint steels

The fatigue life and fracture mechanism of friction stir spot welded tension–shear joints using 590‐MPa class steel as a base material under constant‐amplitude conditions were investigated with focus on welding dimension variations caused by tool wear. The fatigue limit of the friction stir spot welding (FSSW) joint used for this study is significantly low compared with the static tensile strength of the joint itself. It was clarified that the FSSW joint in this study exhibited two different failure morphologies regardless of the applied load level: base metal fracture and weld area fracture. Although the welding state changes due to the tool wear phenomenon that produce two types of fracture modes in relation to the welding rip diameter, they have no effect on the fatigue strength, regardless of the applied load.     

Analysis of the effect of structural defects on the fatigue strength of RFSSW joints using C‐scan scanning acoustic microscopy and SEM

Solid‐state refill friction stir spot welding (RFSSW) technology offers significant benefits in the fabrication of aluminium structures in the transport and aerospace industries. In this paper, the joining of 1.6‐mm‐thick Alclad 7075‐T6 aluminium alloy sheets is investigated. High‐cycle fatigue strength tests of single‐lap welded joints were carried out on an Instron E10000 testing machine with a limited number of cycles equal to 2 × 10⁶. The welding of overlap fatigue specimens was conducted using an RPS100 spot welder by Harms & Wende GmbH & Co KG. C‐mode scanning acoustic microscopy (C‐SAM) and scanning electron microscopy (SEM) were utilised to evaluate the joint quality and characterise the microstructure. The paper discusses the effect of the maximum load force and defects (voids, hook, kissing bond, bonding ligament, etc) associated with the material flow in the weld on the failure mechanism. Insufficient plasticisation of sheet material and mixing of the material in the weld area are crucial defects that influence the number of destructive cycles. The weld defects in the joint structure are a source of a decrease in the fatigue life compared with the fatigue life of defect‐free welds. It was also found that RFSSW joint defects can be effectively detected by the nondestructive C‐SAM method.     

Fatigue crack growth behaviour of gas tungsten arc,electron beam and laser beam welded Ti–6Al–4V alloy

The present investigation is aimed to evaluate fatigue crack growth parameters of gas tungsten arc, electron beam and laser beam welded Ti–6Al–4V titanium alloy for assessing the remaining service lives of existing structure by fracture mechanics approach. Center cracked tensile specimens were tested using a 100 kN servo hydraulic controlled fatigue testing machine under constant amplitude uniaxial tensile load. Crack growth curves were plotted and crack growth parameters (exponent and intercept) were evaluated. Fatigue crack growth behavior of welds was correlated with mechanical properties and microstructural characteristics of welds. Of the three joints, the joint fabricated by laser beam welding exhibited higher fatigue crack growth resistance due to the presence of fine lamellar microstructure in the weld metal.     

Effect of welding parameters on microstructure and mechanical properties of AA7075‐T6 friction stir welded joints

The effects of advancing speed and rotational speed on the microstructure and the mechanical properties of friction stir welded 7075‐T6 aluminium alloy sheets were studied. The fatigue strength of sound joints was measured and compared to tensile testing results. Macrographs and microhardness maps were carried out to reveal the microstructure transformations. Fractographic observations were made to identify the failure mechanisms. The effects of welding parameters on the fatigue strength are discussed in terms of welding pitch k (mm/rev) and heat input (J/mm). At a high welding pitch, crack initiation at the root of the circular grooves left by the tool on the weld surface is the most detrimental failure mechanism. As the size and the depth of the grooves are related to the welding pitch, the fatigue strength increases when the welding pitch is reduced. However, when the heat input is excessive, the failure is caused by sub‐surface defects produced after abnormal stirring and/or by softening of the heat‐affected zone. Lateral lips on the weld surface edges also have an effect on the fatigue strength for intermediate welding pitch values.     

搅拌摩擦与氩弧焊铝合金接头疲劳性能的比较

建立了铝合金焊接接头的S-N曲线,对比分析了搅拌摩擦和氩弧焊两种工艺对其焊接接头疲劳性能的影响,结果表明:在载荷相同的条件下,铝合金搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头,搅拌摩擦焊接头的疲劳寿命N=106次的疲劳强度为59-65 MPa,搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区,阻碍了滑移带的形成和裂纹的扩展,从而提高了接头的疲劳性能,铝合金焊接接头的缺陷是主要的疲劳裂纹源.     

Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti‐6Al‐4 V

Experimental investigation was conducted to evaluate the fracture toughness and fatigue crack growth characteristics in selective laser‐melted titanium 6Al‐4 V materials as a follow‐on to a previous study on high cycle fatigue. For both the fracture toughness and crack growth evaluation, the compact tension specimen geometry was used. It was found that the fracture toughness was lower than what would be expected from wrought or cast product forms in the same alloy. This was attributed to the rapidly cooled, martensitic microstructure, developed in the parts. At low stress ratios, the crack growth rates were faster than in wrought titanium but became comparable at higher ratios. The fracture toughness appears to be higher when the crack is oriented perpendicular to the build layers. The difference in the average threshold and critical stress intensity values for the crack growth results for the three orientations was within the scatter of the data, so there was essentially no difference. The same was true for the empirically derived Paris Law constants. Residual stresses were likely to have overshadowed any variation in crack growth because of microstructural directionalities associated with build orientation.     

Fracture toughness and fatigue crack growth rate properties in wire + arc additive manufactured Ti‐6Al‐4V

This paper presents an experimental investigation of the fracture and fatigue crack growth properties of Ti‐6Al‐4V produced by the Wire + Arc Additive Manufacture (WAAM®) process. First, fracture toughness was measured for two different orientations with respect to the build direction; the effect of wire oxygen content and build strategy were also evaluated in the light of microstructure examination. Second, fatigue crack growth rates were measured for fully additive manufactured samples, as well as for samples containing an interface between WAAM® and wrought materials. The latter category covers five different scenarios of crack location and orientation with respect to the interface. Fatigue crack growth rates are compared with that of the wrought or WAAM® alone conditions. Crack growth trajectory of these tests is discussed in relation to the microstructure characteristics.     

Fatigue performance of friction stir welded marine grade steel

An extensive study on the fatigue performance of friction stir welded DH36 steel was carried out. The main focus of this experimental testing programme was fatigue testing accompanied by tensile tests, geometry measurements, hardness and residual stress measurements, and fracture surface examination. The S–N curve for friction stir butt welded joints was generated and compared with the International Institute of Welding recommendations for conventional fusion butt welds. Friction stir welds of marine grade steel exceeded the relevant rules for fusion welding. This newly developed S–N curve is being proposed for use in the relevant fatigue assessment guidelines for friction stir welding of low alloy steel. Fracture surfaces were examined to investigate the fatigue failure mechanism, which was found to be affected by the processing features generated by the friction stir welding tool.     

Microstructure and mechanical properties of friction stir welded 18Cr–2Mo ferritic stainless steel thick plate

In this study, microstructure and mechanical properties of a friction stir welded 18Cr–2Mo ferritic stainless steel thick plate were investigated. The 5.4 mm thick plates with excellent properties were welded at a constant rotational speed and a changeable welding speed using a composite tool featuring a chosen volume fraction of cubic boron nitride (cBN) in a W–Re matrix. The high-quality welds were successfully produced with optimised welding parameters, and studied by means of optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and standard hardness and impact toughness testing. The results show that microstructure and mechanical properties of the joints are affected greatly, which is mainly related to the remarkably fine-grained microstructure of equiaxed ferrite that is observed in the friction stir welded joint. Meanwhile, the ratios of low-angle grain boundary in the stir zone regions significantly increase, and the texture turns strong. Compared with the base material, mechanical properties of the joint are maintained in a comparatively high level.     

Macromechanics study of stable fatigue crack growth in Al–Cu–Li–Mg–Ag alloy

This study was made on a fresh variety of Al–Li base alloy to investigate the role of ageing precipitates and microstructure dimensions in the fatigue crack growth resistance. The fatigue crack growth rate was measured in three different states of the material (i.e. base metal in T8 condition, friction stir weld and laser beam weld in full‐aged condition). Metallurgical analysis showed that the base metal in T8 temper is precipitation hardened by an equivalent amount of δ′ (AL₃Li), T₁ (AI₂CuLi) and θ′ (AI₂Cu) precipitates. The friction stir weld retained the morphology of strengthening precipitate; however, coarsening of Cu containing precipitates has occurred. On the other hand, laser beam weld showed a different type of CuAl phase morphology, which is characteristic of cast metal. The results of fatigue tests confirmed that fatigue crack growth resistance largely depends on microstructural features, specifically the strengthening phases. The fatigue crack resistance was in the order of base metal > laser beam weldment > friction stir weldment. The CuAl phase played a vital role in the crack closure of the laser beam weldment, thus enhancing the fatigue life as compared with the friction stir weldment, which was evident from the plot between log of da/dN (crack growth in each cycle) and log of ΔK (stress intensity range).     

Strength and fatigue strength of a similar Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si linear friction welded joint

Strengths for monotonic and cyclic loadings of similar overmatching Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si (Ti6242) linear friction welds (LFW) were studied and compared with the parent material (PM) behaviour. Non‐destructive synchrotron observations revealed the presence of pores in the weld interface. The weld centre zone (WCZ) showed a higher strength leading to lower macroscopic ductility of the cross‐weld samples. Local strain and normalized strain rate have been assessed by stereo digital image correlation (DIC) and revealed an early plastic activity at yielding in the vicinity of the WCZ attributed to residual stresses. For the target life, the fatigue strength was slightly reduced but compromised by a strong scatter. Indeed, an internal fish‐eye fatigue crack initiation was found on an unexpected dendritic defect that was very different from the PM microstructure and the known martensitic α′ in the WCZ. The dendritic defect was linked to surface contamination prior to welding and led to melting.