Richard Moat

Design of weld fillers for mitigation of residual stresses in ferritic and austenitic steel welds

Abstract Residual stresses that arise as a result of welding can cause distortion, and also have significant implications for structural integrity. Martensitic filler metals with low transformation temperatures can efficiently reduce the residual stresses generated during welding, because the strains associated with the transformation compensate for thermal contraction strains during cooling. However, it is vital that a low weld transformation temperature is not obtained at the expense of other important material properties. This article outlines the alloy design process used to develop appropriate low transformation temperature filler materials for the mitigation of residual stresses in both low alloy ferritic and austenitic stainless steel welds. Residual stresses in single pass, 6 mm bead in groove welds, on 12 mm thick plates, have been measured and compared against those obtained with commercially available conventional austenitic and ferritic filler materials. The filler metals developed here exceeded requirements in terms of weld mechanical properties, while significantly reducing the maximum residual stress in the weld and heat affected zone.

Effect of interpass temperature on residual stresses in multipass welds produced using low transformation temperature filler alloy

Abstract Weld filler alloys that exploit transformation plasticity through low austenite to martensite transformation temperatures offer an effective method of reducing residual stresses in strong steel welds. However, in multipass welds, the heat input from later weld passes may be insufficient to retransform prior welding passes, leading to the accumulation of thermally induced strains and elevated residual stresses. In this work, the residual stress distributions produced around arc welds fabricated with a martensitic weld filler alloy that transforms at a low temperature have been studied as a function of the number of passes deposited and the interpass temperature. It is found that when the interpass temperature is above the transformation temperature of the weld metal, the entire multipass weld transforms as a single entity, thus permitting the optimum exploitation of the transformation plasticity. In contrast, the deposition of new metal with a relatively low interpass temperature leads to increased residual stresses in the underlying layers, reducing or eliminating the beneficial stress states previously created.

Linear friction welding of aluminium to copper

Abstract The joining of dissimilar materials is attaining increasing importance as there is a drive to utilise hybrid structures and reduce the weight or cost of products. The present work therefore studied the linear friction welding of commercially pure grades of aluminium to copper (AA 1050 to C101) for potential power transmission applications. Results showed that welds with very good mechanical and electrical properties can be produced. The weld microstructure was analysed using optical microscopy, backscattered scanning electron microscopy, hardness testing and high energy synchrotron X-ray diffraction. These techniques were used in order to identify any cross-weld grain size variations and possible formation of intermetallic phases close to the weld line.

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.

Surface residual stresses in multipass welds produced using low transformation temperature filler alloys

Abstract Tensile residual stresses at the surface of welded components are known to compromise fatigue resistance through the accelerated initiation of microcracks, especially at the weld toe. Inducement of compression in these regions is a common technique employed to enhance fatigue performance. Transformation plasticity has been established as a viable method to generate such compressive residual stresses in steel welds and exploits the phase transformation in welding filler alloys that transform at low temperature to compensate for accumulated thermal contraction strains. Neutron and X-ray diffraction have been used to determine the stress profiles that exist across the surface of plates welded with low transformation temperature welding alloys, with a particular focus on the stress at the weld toe. For the first time, near surface neutron diffraction data have shown the extent of local stress variation at the critical, fusion boundary location. Compression was evident for the three measurement orientations at the fusion boundaries. Compressive longitudinal residual stresses and tensile transverse stresses were measured in the weld metal.

ENGIN-X – instrument for materials science and engineering research

Engin-X is a leading neutron diffractometer for materials science and engineering, with high resolution and versatile capabilities at the ISIS spallation source, UK. Over the past 10 years Engin-X has continually redefined the frontier of stress characterisation capability through investment in state-of-the-art equipment, attracting academic and industrial users from 24 countries. Measurements are typically carried out in collaborative experiments between universities, industry and ISIS to address a wide range of engineering problems: manufacturing challenges surrounding magnesium alloys for the automotive industry, creep deformation of nickel-base superalloys for aero engines, structural integrity of welds for nuclear power plants, residual stresses in a range of samples from complex aerospace components to ancient steel making manufacturing techniques.

Modelling the Interpass Temperature Effect on Residual Stress in Low Transformation Temperature Stainless Steel Welds

Weld residual stresses often have serious implications for the integrity of engineering structures (distortion, stress-corrosion cracking, hydrogen-induced cracking). Previously, it has been demonstrated by the authors that the use of a stainless steel welding consumable with a low martensite start temperature in single-pass welding can lead to lower (potentially harmful) tensile residual stresses or even compressive stress within the fusion zone and heat affected zones compared to non-transforming austenitic fillers. However, such effects may not carry over to multi-pass welding if the filler transforms fully on cooling from the first pass. In this paper finite element modelling is used to examine the use of interpass hold temperatures on the residual stresses introduced using such weld fillers in multi-pass welding of 304L stainless steel plate. Four levels of interpass temperature have been studied. The model has also been verified against experimental data obtained using the contour method for two welded plates having two different inter-pass temperatures. It is demonstrated that interpass hold temperatures above, or around, the transformation temperature can have very significant effects, allowing residual stress management of the resulting welded joint.Copyright

Stress distributions in multilayer laser deposited Waspaloy parts measured using neutron diffraction

During layered manufacturing of functional metallic parts by laser direct metal deposition (LDMD) residual stresses are generated by the transient thermal fields. Various methods to reduce or control the development of these stresses have been previously attempted. This paper reports a novel method using the pulse parameters of a diode laser deposition system as a tool for residual stress control. Stresses in LDMD manufactured Waspaloy blocks are calculated from strain measurements in three directions carried out at the neutron diffraction beam line SALSA at the European ILL facility, Grenoble, France. The results show tensile stresses close to the top of the structures of similar magnitude for all parameters and large compressive stresses normal to the deposition surface close to the base.

Effect of Sn on Corrosion Mechanisms in Advanced Zr-Cladding for Pressurised Water Reactors

The desire to improve the corrosion resistance of Zr cladding material to allow high burnup has resulted in a general trend among fuel manufacturers to develop alloys with reduced levels of Sn. While the detrimental effect of Sn on high temperature aqueous corrosion performance is widely accepted, the reason for it remains unclear. High-Energy synchrotron X-ray diffraction was used to characterise the oxides formed by autoclave exposure on Zr-Sn-Nb alloys with tin concentrations ranging from 0.01 to 0.92 wt.%. The alloys studied included the commercial alloy ZIRLO® and two variants of ZIRLO with significantly lower tin levels, referred to here as A-0.6Sn and A-0.0Sn. The nature of the oxide grown on tube samples from each alloy during autoclave testing at 360°C was investigated by cross-sectional Scanning and Transmission Electron Microscopy (SEM & TEM). Non-destructive synchrotron X-ray diffraction analysis on the oxides revealed that the monoclinic and tetragonal oxide phases display highly compressive in-plane residual stresses with the magnitudes dependent on both phase and alloy. Additional in-situ Synchrotron X-ray diffraction experiments during oxidation at 550°C provided further confirmation of the trends seen for autoclave tested samples and demonstrated the presence of elevated levels of tetragonal phase in the initial stages of oxidation. In-situ and ex-situ measurements demonstrate unambiguously that the amount of tetragonal phase present and, more importantly, the degree of transformation from tetragonal to monoclinic oxide both decrease with decreasing tin levels, suggesting that tin stabilises the tetragonal phase. It is proposed that in Zr-Nb-Sn alloys with low Sn, the tetragonal phase is mainly stabilised by very small grain size and therefore remains stable throughout the corrosion process. By contrast, in alloys with higher tin levels larger, stress stabilised, tetragonal grains can form initially, but then become unstable as the corrosion front progresses inwards and stresses in the existing oxide relax.

An anisotropic enhanced thermal conductivity approach for modelling laser melt pools

It is well established that the Marangoni flow dominated circulation within a laser melt pool significantly modifies the pool profile and temperature distribution. Detailed computational fluid dynamics models are required to accurately predict this but these are complicated and computationally expensive. Many researchers have in the past used an enhanced thermal conductivity approach, but the validity of this approach for accurately predicting the melt pool geometry and temperature distribution is largely unproven.This paper presents an analysis of the widely-used isotropic enhanced thermal conductivity approach and compares it with a more advanced anisotropic approach for modelling the laser melting of Inconel 718. Experimental and modelled results for the geometry of a melt pool created by a moving laser beam are compared. It is found that the conventional enhanced thermal conductivity approach does not change the melt pool size and shape; it only reduces the maximum surface temperature. The anisotropic enhanced thermal conductivity approach on the other hand is able to modify the melt pool size and geometry and yields a better agreement with the experimental results.It is well established that the Marangoni flow dominated circulation within a laser melt pool significantly modifies the pool profile and temperature distribution. Detailed computational fluid dynamics models are required to accurately predict this but these are complicated and computationally expensive. Many researchers have in the past used an enhanced thermal conductivity approach, but the validity of this approach for accurately predicting the melt pool geometry and temperature distribution is largely unproven.This paper presents an analysis of the widely-used isotropic enhanced thermal conductivity approach and compares it with a more advanced anisotropic approach for modelling the laser melting of Inconel 718. Experimental and modelled results for the geometry of a melt pool created by a moving laser beam are compared. It is found that the conventional enhanced thermal conductivity approach does not change the melt pool size and shape; it only reduces the maximum surface temperature. The anisotropic...