Distortion control in welding by mechanical tensioning

Abstract

This paper investigates the potential of mechanical tensioning (MT) to reduce the magnitude of residual stresses in welds and to eliminate buckling distortion. Both friction stir (FSW) and arc welds have been produced from the aluminium alloy AA2024, with different levels of tensile stress applied along the weld line either during or after welding. The resulting welds have been characterised in terms of out of plane distortion, residual stresses and microstructure. Buckling distortion was eliminated by stretching plates to between 35 and 70% of the yield stress of the material during welding. For each set of welding parameters investigated, an optimum tensioning stress has been identified, which eliminates the tensile residual stress peak across the weld zone, along with distortion. This optimum tensioning stress increases in line with the heat input of the welding process. When MT stresses are increased beyond this optimum value, then distortion arises once more and a band of compressive stress is formed across the weld zone.     

Process–microstructure–property correlations in Al–Li AA2199 friction stir welds

Abstract

This paper examines the effect of three friction stir welding process parameters on the residual stresses, hardness and distortion for butt welded aluminium–lithium AA2199 alloy, a novel, low density high strength alloy with potential in the aerospace sector. A systematic set of nine trial welds is examined at different tool rotation and traverse speeds as well as tool downforces. The tensile residual stresses (～50% of parent material yield strength) and the hardness drop in the weld line varied little with any of the friction stir welding process parameters. However, their breadth increased with rotation speed and downforce and decreased with increasing translation speed, which is consistent with increasing heat input. Weld distortion took the form of a saddle with the longitudinal bending distortion correlating strongly with the width of the tensile zone consistent with it being driven by the magnitude of the tensile buckling forces in the weld region.     

Microstructure,hardness and residual stress distribution in maraging steel gas tungsten arc weldments

Abstract

The distribution of residual stresses due to welding has been studied in maraging steel welds. Gas tungsten arc welding process was used and the effect of filler metal composition on the nature of residual stress distribution has been investigated using X-ray diffraction technique with Cr K_α radiation. Three types of filler materials were used, they include: maraging filler, austenitic stainless steel and medium alloy medium carbon steel filler metal. In the case of maraging steel weld, medium alloy medium carbon filler, the residual stress at the centre of the weld zone was more compressive while, less compressive stresses have been identified in the heat affected zone of the parent metal adjacent to the weld metal. But, in the case of austenitic stainless steel filler the residual stresses at the centre of the weld and heat affected zone were tensile. Post-weld aging treatment reduced the magnitude of stresses. The observed residual stress distribution across the weldments has been correlated with microstructure and hardness distribution across the weld.     

Residual stress measurement of Al/steel dissimilar friction stir weld

ABSTRACT

The dissimilar welds between aluminium (Al) alloy, A6061-T6, and stainless steel, type 304, were fabricated by a friction stir welding (FSW) technique. The FSW tool was offset to Al side and the probe was inserted only into Al plate. The softening occurred in Al side due to the heat input during FSW, while the hardness increased by the post-ageing treatment. Tensile strength of dissimilar weld also increased about 8.5% by the post-ageing. The residual stresses were measured based on the cos α method. The residual stresses parallel to the weld line, σ_xr, were predominantly tensile, while those perpendicular to the weld line, σ_yr, were compressive. Post-ageing treatment had little effect on the residual stresses.     

Rolling to control residual stress and distortion in friction stir welds

Abstract

Considerable residual stress and distortion can be produced by friction stir welding, impeding industrial implementation. Finite element analysis has been used to develop three innovative rolling methods that reduce residual stress and distortion in friction stir welds. Of the three methods, post-weld direct rolling where a single roller is applied to roll the top surface of the weld after the weld metal has cooled to room temperature proved the most effective. The residual stress predictions from the model compared favourably with residual stress measurements reported in an accompanying paper. Finally, the effectiveness of using post-weld direct rolling is illustrated with an industrial example of a large integrally stiffened panel, where the distortion was virtually eliminated.     

搅拌摩擦焊工艺参数对铸态A356铝合金抗拉强度的影响(英文)

A356是一种高强度铝硅铸造态合金,广泛用于食品、化工、船舶、电器和汽车行业。熔焊这种铸造合金时存在许多问题,如孔隙、微裂隙、热裂等。然而,用搅拌摩擦焊(FSW)来焊接这种铸造态合金可以避免上述缺陷发生。研究了搅拌摩擦焊工艺参数对铸造态A356铝合金抗拉强度的影响;对旋转速度、焊接速度和轴向力等工艺参数进行优化;从宏观和微观组织分析角度对焊接区的质量进行分析;对焊接接头的抗拉强度进行了测定,并对抗拉强度与焊缝区硬度和显微组织的相关性进行了研究。在旋转速度1000r/min、焊接速度75mm/min和轴向力5kN的条件下得到的焊接接头具有最高的抗拉强度。     

Establishing empirical relationships to predict grain size and tensile strength of friction stir welded AA 6061-T6 aluminium alloy joints

AA 6061-T6 aluminium alloy(Al-Mg-Si alloy) has gathered wide acceptance in the fabrication of light weight structures requiring a high specific strength and good corrosion resistance.Compared with the fusion welding processes that are routinely used for joining structural aluminium alloys,friction stir welding(FSW) process is an emerging solid state joining process in which the material welded does not melt and recast.Joint strength is influenced by the grain size and tensile strength of the weld nugget region.Hence,an attempt was made to develop empirical relationships to predict grain size and tensile strength of friction stir welded AA 6061-T6 aluminium alloy joints.The empirical relationships are developed by response surface methodology(RSM) incorporating FSW tool and process parameters.A linear regression relationship was also established between grain size and tensile strength of the weld nugget of FSW joints.     

铝合金搅拌摩擦焊过程热力演变的三维数值模拟

对搅拌摩擦焊过程中搅拌头速度变化进行分析,建立了考虑搅拌摩擦焊过程中焊缝产热的热源模型.对2024铝合金搅拌摩擦焊温度场和应力场进行了三维有限元模拟,表明焊缝两侧温度和应力分布的不对称现象不明显,主要由于焊接速度远小于搅拌头转速所致,但随着焊接速度加快,这种不对称现象逐渐加强.焊接过程中焊缝中心温度低于搅拌头边缘温度,焊接前方和两侧均为压应力,后方为拉应力;焊接结束后与搅拌头接触区的横向和纵向残余应力为较大拉应力,远离焊缝残余应力较小;沿厚度方向上,横向和纵向残余应力均逐渐降低.有限元计算结果与短波长X射线应力测试结果进行对比,结果表明,二者趋势基本吻合.     

Knowledge discovery for friction stir welding via data driven approaches

Abstract

For a comprehensive understanding towards friction stir welding (FSW) which would lead to a unified approach that embodies materials other than aluminium, such as titanium and steel, it is crucial to identify the intricate correlations between the controllable process conditions, the observable internal process variables, and the characterisations of the post-weld materials. In Part 1 of this paper, multiple correlation analyses techniques have been developed to detect new and previously unknown correlations between the internal process variables and weld quality of aluminium alloy AA 5083. Furthermore, a new exploitable weld quality indicator has, for the first time, been successfully extracted, which can provide an accurate and reliable indication of the as welded defects. All results relating to this work have been validated using real data obtained from a series of welding trials that utilised a new revolutionary sensory platform called ARTEMIS developed by TWI Ltd, the original inventors of the FSW process.     

2195-F态铝锂合金TIG焊和FSW焊后残余应力分析

采用盲孔法和压痕法,分别对2195-F态铝锂合金手工TIG焊和FSW焊后残余应力进行测量. 结果表明,盲孔法的测量值普遍高于压痕法. 两种焊接方法,近焊缝区的纵向应力均高于横向应力;横向应力整体表现为压应力或小于50 MPa的拉应力;纵向应力在热影响区附近表现为大于焊缝的拉应力. 焊缝区附近,手工TIG焊纵向残余应力大于FSW,且纵向残余应力表现为较大的拉应力,最大值接近于接头的屈服强度;焊缝区外,手工TIG焊和FSW残余应力值相差不大,其横向残余应力基本表现为很小的拉应力或者压应力.     

New technique of in situ rolling friction stir welding

Abstract

A new technique of in situ rolling friction stir welding (IRFSW) has been developed to reduce the residual stress and distortion. It can eliminate the weld flashes and improve corrosion resistance of the FSW seam. A new kind of FSW tool consisting of rolling balls was designed to achieve IRFSW. The residual stresses across the weld were measured by ultrasonic stress measurement experimental installation. The 3·5%NaCl solution was used to corrode the surface of welded joint, and corrosion current and potential were determined by an electrochemical analyser. The results shown that the residual tensile stress was reduced, the weld flashes were eliminated, and the corrosion resistance was improved. By applying IRFSW, the reduction of distortion was ～34·4% compared with that of conventional FSW joint for aluminium plant with 3 mm in thickness and 300 mm in length.     

Mitigation of welding induced buckling distortion using transient thermal tensioning

Abstract

Welding induces residual stresses in structures and may cause buckling distortion if the stresses exceed the critical buckling stress of the structure. Reducing the welding heat input or increasing the structural stiffness reduces or eliminates buckling distortion. However, where, because of the design constraints, structure geometry and weld size are fixed, the transient thermal tensioning process is effective in reducing buckling distortion. An experimental verification and demonstration of transient thermal tensioning for minimising welding induced buckling distortion is presented. Conventional welding was carried out to demonstrate buckling distortion and establish a baseline case. Buckling distortion was eliminated using transient thermal tensioning during welding under the same welding conditions. After buckling distortion was eliminated, angular distortion became evident, which was eliminated using mechanical restraints alongside transient thermal tensioning. Residual stress measurements were obtained using the blind hole drilling method and a comparison of residual stresses for the baseline panel and for the panel with transient thermal tensioning is presented.     

Art of friction stir welding to produce weld joint without rotation of shoulder with narrow heat-affected zone and high corrosion resistance

ABSTRACT

This present study uses an indigenously designed non-rotating shoulder friction stir welding (NRSFSW) tool to weld AA2014 aluminium alloy. This tool is harnessed with a penetrated material removal mechanism. Generally, the heat-affected zone (HAZ) is the most severely affected region in the conventional FSW (CFSW) joint of heat-treatable aluminium alloys. The prime novelty of this work is to reduce the width of most corrosion susceptible region of the FSW joint using a non-rotating shoulder tool. The corrosion behaviour of the NRSFSW joint has been investigated and compared to the CFSW joint. The NRSFSW tool resulted in focused heat input which eventually reduced the width of the HAZ and improved corrosion resistance as compared to the CFSW tool.     

Effect of low transformation temperature weld filler metal on welding residual stress

Abstract

The effect of weld filler metal austenite to acicular ferrite transformation temperature on the residual stresses that arise during the gas metal arc welding of a low carbon steel has been examined using a finite element model. It was found that the stress levels in the weld can be tailored by the appropriate selection of the filler metal and compressive, near zero or tensile residual stresses produced. Reasonable agreement was obtained between the model and the stresses measured using neutron diffraction both in welds using conventional and low transformation temperature filler metal.     

Linear friction welding of aluminium to magnesium

Abstract

Joining of an aluminium alloy to a magnesium alloy (AA 6082-T6 to AZ31) has been carried out by linear friction welding. The joining of this material combination is of particular significance for automotive components. Results show that welds with reasonable strength (comparable to the yield strength of the parent materials in O temper) can be produced. Weld microstructures were characterised by backscattered scanning electron microscopy, hardness testing and laboratory based X-ray diffraction. A particular emphasis was placed on determining the effects of welding parameters on the relative amounts of detrimental intermetallic phase at the weld line.     

Microstructure and mechanical properties of friction stir welding joint during post weld heat treatment with different zigzag lines

Zigzag line is a common defect in friction stir welding(FSW) joint.The formation mechanism of the zigzag line in Al-Cu alloy FSW joint and its influence on the microstructure and mechanical properties during post weld heat treatment(PWHT) were studied by scanning electron microscopy(SEM),microhardness and tensile tests.It is found that the occurrence of zigzag line for PWHT joint is determined by PWHT process which in nature depends on residual stress and thermal stress of FSW joint.The optimization of PWHT process to reduce the residual and thermal stress can trigger for the deterioration of mechanical properties of PWHT joints with zigzag line.No obvious decrease of tensile properties is observed for T6-450 and T6-495 joints although zigzag line appears in the weld.PWHT determines the sizes of zigzag line cracks and consequently determines the fracture location and characteristics of FSW joint.     

Microstructure,mechanical properties and residual stresses as a function of welding speed in aluminium AA5083 friction stir welds

Friction stir welding (FSW), like other friction welding techniques, has the advantage that many of the welding parameters, e.g. tool design, rotation speed and translation speed, can be controlled in a precise manner, thus controlling the energy input into the system. However, the effect of different welding speeds on the weld properties remains an area of uncertainty. In this paper, we report the results of microstructural, mechanical property and residual stress investigations of four aluminium AA5083 friction stir welds produced under varying conditions. It was found that the weld properties were dominated by the thermal input rather than the mechanical deformation by the tool.     

Influence of time on residual stresses in friction stir welds in agehardenable 7xxx aluminium alloys

Friction stir welding (FSW) can be used to weld 7xxx series wrought aluminium alloys. As these materials are age-hardenable, their properties can change with time. This work used the neutron diffraction technique to measure the residual stresses around a FSW in a 7xxx alloy and to determine how these residual stresses changed with time. It was found that the residual stresses associated with the weld nugget decreased, while those associated with the heat-affected zone increased with time. This is in contrast to strength and hardness values that increased in all regions of the weld with time.     

A pinless embedded tool used in FSSW and FSW of aluminum alloy

A pinless tool design was proposed to achieve high butt tensile strength in aluminum alloy sheets (∼3 mm thick) in friction stir welding (FSW), where a 7 mm long cylindrical insert rod made of aluminum alloy was tightly embedded into a hollow cylindrical tool. The effects of rod diameter (d) on the welding properties and mechanism of weld formation were investigated. For friction stir spot welding (FSSW), the maximum rate of temperature rise was about 12.8 °C/s, and the failure load was about 2 kN for this pinless tool (d = 10 mm), with these being about 7.4 °C/s and 1.35 kN for the plain one. For FSW, the failure load was about 1.46 kN for this tool, and about 0.72 kN for the plain one. The abutting edges of sheets with a thickness of 3 mm could be welded using this pinless tool.     

Optimisation of friction stir welding process parameters to weld cast aluminium alloy A413 – an experimental approach

Abstract

A413 is a high strength eutectic aluminium silicon cast alloy used in the food, chemical, marine, electrical and automotive industries. Fusion welding of these cast alloys can lead to problems such as porosity, microfissuring and hot cracking, etc. However, friction stir welding can be used to weld these cast alloys effectively, without defects. In this investigation, an attempt was made to optimise the friction stir welding process parameters for joining the cast aluminium alloy A413. Joints were made using four levels each of tool rotation speed, welding speed and axial force. The quality of the weld zone was analysed using macrostructure and microstructure analysis. Tensile strength of the joints were evaluated and correlated with the weld zone hardness and microstructure. The joint fabricated using a tool rotation speed of 900 rev min^?1, a welding speed of 75 mm min^?1 and an axial force of 3 kN showed the best tensile strength.