Non-destructive Neutron Diffraction Research Articles

Evolution of Material Properties and Residual Stress with Increasing Number of Passes in Aluminium Structure Printed via Additive Friction Stir Deposition.

Additive friction stir deposition (AFSD) is an emerging solid-state additive manufacturing process with a high deposition rate. Being a non-fusion additive manufacturing (AM) process, it significantly eliminates problems related to melting such as cracking or high residual stresses. Therefore, it is possible to process reactive materials or high-strength alloys with high susceptibility to cracking. Although the residual stresses are lower in this process than with the other AM processes, depending on the deposition path, geometry, and boundary conditions, residual stresses may lead to undesired deformations and deteriorate the dimensional accuracy. Thermal cycling during layer deposition, which also depends on the geometry of the manufactured component, is expected to affect mechanical properties. To this day, the influence of the deposit geometry on the residual stresses and mechanical properties is not well understood, which presents a barrier for industry uptake of this process for large-scale part manufacturing. In this study, a stepped structure with 4, 7, and 10 passes manufactured via AFSD is used to investigate changes in microstructure, residual stress, and mechanical property as a function of the number of passes. The microstructure and defects are assessed using scanning electron microscopy and electron backscatter diffraction. Hardness maps for each step are created. The residual stress distributions at the centreline of each step are acquired via non-destructive neutron diffraction. The valuable insights presented here are essential for the successful utilisation of AFSD in industrial applications.

Influence of multi-layer laser cladding depositions and rail curvature on residual stress in light rail components

Rapid deterioration of critical rail components due to wear and fatigue is a significant challenge faced by the railway industry. Laser cladding has been applied to straight and curved light rail substrates which are particularly prone to these methods of degradation using a Stellite 21 hardfacing alloy. The influence of multi-layer cladding depositions, grinding-based finishing procedures and substrate rail grade on residual stress was analyzed using non-destructive neutron diffraction techniques. As cladding is a thermal process, microstructural changes from the heat inputs can result in a high internal stress state which reduces the loading capacity whilst a high hardness increases the likelihood of brittle failure. Stellite laser cladding depositions were found to result in low tensile residual stresses within the cladding layer, which become compressive in the heat affected zone (HAZ). Repeated thermal inputs from multi-layer cladding depositions did not negatively impact the hardness or microstructure after double layer laser cladding and increased the residual stress to 100 MPa at the cladding surface after grinding. Laser cladding on a curved rail substrate also produced increased internal stress after cladding due to higher strains resulting from the rail geometry but remained below the yield limit of the cladding and substrate material. These outcomes were compared to current literature indicating this critical combination of low internal strain and a cladding and HAZ hardness complementing the substrate material is difficult to achieve. Therefore, ensuring laser cladding is compatible with a variety of light rail components is essential in developing viable maintenance techniques to recondition critical railway infrastructure to avoid disruptive replacement procedures.

Application of a New Alloy and Post Processing Procedures for Laser Cladding Repairs on Hypereutectoid Rail Components.

The development of a laser cladding repair strategy is critical for the continued growth of heavy-haul railway networks. Premium hypereutectoid rails have undergone laser cladding using a new martensitic stainless-steel alloy, 415SS, developed for high carbon rails after standard cladding metals were found to be incompatible. Non-destructive neutron diffraction techniques were used to measure the residual stress in different layers generated across a dissimilar metal joint during laser cladding. The internal stress distribution across the cladding, heat-affected zone (HAZ), and substrate was measured in the untempered rail, after 350 °C and 540 °C heat treatment procedures and two surface grinding operations. The martensitic 415SS depositions produce compressive stress in the cladding, regardless of tempering procedures, which may inhibit fatigue crack propagation whilst grinding operations locally relive surface stress. Balancing tensile stresses were recorded below the fusion boundary in the HAZ due to thermal gradients altering the microstructure. The combination of 540 °C tempering and 0.5 mm surface layer removal produced a desirable combination of compression in the cladding deposition with significantly reduced tensile stresses in the HAZ. A comparison with the current literature shows that this alloy achieves a unique combination of desirable hardness, low tensile stress, and compression in the cladding layer. Data obtained during strain scanning has been used to determine the location of microstructural changes at the fusion boundary and HAZ through correlation of the stress, strain, full width at half maximum (FWHM), and intensity profiles. Therefore, neutron diffraction can be used for both the accurate measurement of internal residual stress and to obtain microstructural information of a metallurgical join non-destructively.

Neutron diffraction residual stress analysis and mechanical properties of additively manufactured high strength steel hollow sections

Selective laser melting (SLM) is regarded as one of the most attractive metal additive manufacturing (AM) techniques, which has offered a promising solution to fabricating unique members and can be used for a wide range of applications. However, the SLM is often associated with fast cooling rate and a resultant large thermal gradient; which may lead to a large amount of plastic strains and coexistent residual stresses, and sometimes even result in cracking and failure during the fabrication. These plastic strains and residual stresses play crucial roles in determining the mechanical performance of the produced structural members. Plastic strains cannot be directly measured but can be characterized indirectly by hardness values. Thus, it is essential to study the distributions and magnitudes of both residual stresses and hardness in SLM-fabricated members. In the present study, comprehensive experimental studies on micro-hardness, material properties and residual stresses in SLM-fabricated H13 high strength steel hollow sections were conducted. Non-destructive neutron diffraction was utilized to measure residual stresses and their distributions across the wall thickness and over the cross-sections. The effects of both scanning patterns and cross-section geometries on the residual stresses and hardness were also studied. Linear relationships between the hardness values and the material strengths were established for the additively manufactured H13 steel. The Vickers hardness value was found to be highly correlated to the ultimate strength, while it was correlated to the 0.2% tensile proof stress with a small deviation.

Neutron diffraction residual stress determinations in titanium aluminide component fabricated using the twin wire-arc additive manufacturing

In recent years, twin wire-arc additive manufacturing (T-WAAM) technique has been considered as a promising method of fabricating and shaping titanium aluminide components with high efficiency and low cost. However, excessive thermal input of the non-consumable tungsten electrode arc deposition induces significant residual stresses in the buildup component, thus accurate measurement of residual stresses is necessary for T-WAAM buildup part quality assessment. In the present research, non-destructive neutron diffraction residual stresses measurements on as-fabricated and heat-treated T-WAAM produced Ti48Al titanium aluminide components are performed. To exclude the influence of initial large T-WAAM residual stresses on d 0 hkl sample alignment, different d 0 hkl sample dimensions are designed: one is normal thin slice and the other is the meshed sample. According to the obtained results, the conducted post-production heat treatment has partially released initial residual stresses. Also, the meshed design of d 0 hkl sample shows better measurement accuracy than the simple sliced d 0 hkl sample. In addition, T-WAAM fabricated titanium aluminide wall component performs featured tensile-compressive alternating residual stress distribution induced by the layer-by-layer arc deposition.

Residual stress measurements of lean duplex stainless steel welded sections

Lean duplex stainless steel (LDSS) has been increasingly utilised in engineering applications due to its excellent durability, corrosion resistance, as well as superior structural and economic benefits. Moreover, compared to cold-form sections, welded members have significant structural advantages, and thus, have been widely used in many engineering practices. However, as one of the key factors affecting the performance of structural components, residual stresses of LDSS welded sections have not been sufficiently investigated. Therefore, square and H-shaped LDSS welded sections were considered in the present experimental programme. Accurate measurements of the tensile and compressive residual stresses were conducted through the non-destructive neutron diffraction method. As LDSS is a dual-phase material, neutron diffraction measurement was repeated twice for each specimen to obtain the individual phase residual stresses (ferrite-phase, α and austenite-phase, γ). Hardness analysis, as well as microstructural characterisation using optical microscopy and electron backscatter diffraction (EBSD) were thereafter performed to ascertain the volume fraction of each phase, based on which the residual stresses along the direction of interest were successfully converted. According to the obtained experimental results, the authors recommended analytical models for the LDSS fabricated square sections and proposed new models for the H-sections, through which the residual stress distributions for welded LDSS sections can be accurately predicted. In addition, the recommended/proposed analytical models for LDSS sections were further compared with their counterparts previously developed for high-strength steel (HSS) and ultra-high-strength steel (UHSS) welded sections.

On the determination of residual stresses in additively manufactured lattice structures.

The determination of residual stresses becomes more complicated with increasing complexity of the structures investigated. Additive manufacturing techniques generally allow the production of 'lattice structures' without any additional manufacturing step. These lattice structures consist of thin struts and are thus susceptible to internal stress-induced distortion and even cracks. In most cases, internal stresses remain locked in the structures as residual stress. The determination of the residual stress in lattice structures through nondestructive neutron diffraction is described in this work. It is shown how two difficulties can be overcome: (a) the correct alignment of the lattice structures within the neutron beam and (b) the correct determination of the residual stress field in a representative part of the structure. The magnitude and the direction of residual stress are discussed. The residual stress in the strut was found to be uniaxial and to follow the orientation of the strut, while the residual stress in the knots was more hydro-static. Additionally, it is shown that strain measurements in at least seven independent directions are necessary for the estimation of the principal stress directions. The measurement directions should be chosen according to the sample geometry and an informed choice on the possible strain field. If the most prominent direction is not measured, the error in the calculated stress magnitude increases considerably.

Residual stress analysis of friction stir welded AA6063 pipe butt joint by neutron diffraction technique of ENGIN-X

Residual stress refers to stress that remains in a structure when all applied stresses have been removed. It is important because the combination with applied stress can cause failure at the level below at which the failure could occur. A study has been carried out using the non-destructive neutron diffraction (ND) technique of ENGIN-X to quantify the redistribution of residual stresses in a pipe. A three-dimensional residual stress-measurements test is fully performed on a 89 mm (3.5 in) diameter pipe in AA6063 grade aluminium (5 mm wall thickness) containing a girth friction stir weld. The results have shown an opposite residual stress effect due to tool rotational speed increment especially on the retreating side of the weld. This weld residual stress analysis is an important factor for Fitness-For-Purpose Assessments. This residual stress measurement data provides a basis for structural integrity assessment and inspection planning.

Residual stresses of box and I-shaped columns fabricated from S960 ultra-high-strength steel

Ultra-high-strength steel (UHSS) with a nominal yield stress of 960 MPa is of significant interest for the design of heavily loaded compressive members, such as high-rise buildings, long-span bridges and large-scale infrastructure. However, the residual stresses induced by the fabrication process can be detrimental to the structural members. In particular, compressive residual stresses can result in premature buckling and the ultimate strength of fabricated columns will be reduced. To date, research pertaining to the distribution of the residual stresses for UHSS column is limited. An experimental program about the measurements of residual stress distributions for UHSS box and I-shaped columns is therefore presented herein. Measurements of residual stress were undertaken on two box and two I-shaped columns having various width-to-thickness ratios by using the non-destructive neutron diffraction method. The distributions of residual stress in three orthogonal directions for each specimen was measured. The influences of column width-to-thickness ratio on residual stress distributions were investigated and discussed. In addition, simplified analytical models for the box and I-shaped column were established to facilitate the buckling analysis for such UHSS columns. Comparisons of the residual stress distributions between the experimental results and analytical model demonstrated a good agreement and this model can be safely used for the fabricated high-strength-steel (HSS) and UHSS column.

Residual stresses in welded high-strength steel I-Beams

This study investigates a unified residual stress model applicable for welded high-strength steel (HSS) I-beams. In the experimental program, the homogeneous specimens including two prismatic I-beam samples and a web-tapered I-beam fabricated from Australian BISPLATE-80 and BISPLATE-100 steel plates having nominal yield stresses of 690 MPa and 890 MPa respectively were inspected to determine their residual stress distribution using a non-destructive neutron diffraction technique. Details of this neutron diffraction method for measuring residual stresses are presented. It is shown that the technique can achieve high spatial resolution of the residual stresses as well as capturing the high stress gradient in the heat-affected zone, as a consequence of the deep penetration of the neutron particles into the material. The pattern of residual stresses in the specimens reveals that the tensile stresses peak at the flange-web junctions at an average of 70% of the parent material yield stresses, and that the compressive residual stresses have an approximately uniform distribution that dominates large regions of the flange and web. The test results reconfirm the compressive residual stresses being related to the geometry of the cross-section and independent of the steel grade. The interaction of the residual stresses in the flanges and web was found to be negligible for both prismatic and web-tapered beams. A residual stress model applicable for welded thin-walled I-section members for steel grades between 460 MPa and 1000 MPa is proposed by fitting the test results and collective data available in the literature. This representation was incorporated into a detailed finite element (FE) model and it is shown that the FE predictions are in good agreement with the results of experiments conducted on a wide range of HSS I-section beams tested to failure caused by buckling and/or yielding.

Residual stress in laser cladded rail

Abstract To improve the fatigue life of components subject to loads with high surface strain gradients, it is possible to coat them with an alloy of higher durability. The present study focuses on the effect of cladding high value track components, made of a standard rail steel UIC 900A/grade 260, with a layer of a premium martensitic stainless steel to reduce wear and fatigue. The laser cladding process inevitably generates residual stresses in the clad and parent metal, which could be detrimental to the integrity of the component. Therefore, measurements to determine the residual stress state of cladded rail were performed using semi-destructive centre-hole and deep hole drilling and non-destructive neutron diffraction techniques. Subsequently, the effects of cycling loading and wear, representative of typical service loads, on the redistribution of the residual stress field were investigated. It was observed that laser cladding causes a triaxial compressive residual stress field in the clad and near the interface and a tensile stress field in the parent material. The stress field is shown to change when the first cycle of load is applied but reaches a steady state after only 10 cycles: After the 10th cycle there is no evidence that the clad continues accumulating strain which could indicate that there is low risk of ratcheting. Wear effect on residual stress redistribution was found to be local on the surface of the specimen only.

Neutron Diffraction Measurement of Residual Stress in High Melting Point Metals Processed by 3D Printing

Recently metal 3D printing technology has been considered as one of the most innovative manufacturing technology due to its various advantages. In particular, there have been urgent needs for technology using high melting point metal (HMPM) for the extreme environmental applications such as defense weapon, aerospace rocket, power plant. In this study, we conducted feasibility test for fabrication of Mo and W alloys using direct energy deposition (DED), which is one of the major additive manufacturing (AM) technologies. It is of great concern that the as-processed HMPMs are expected to have significant residual stress due to the use of high laser power in the 3D printing process. So we measured the residual stress and microstructure of Mo and W alloys processed by DED using non-destructive neutron diffraction method. The result demonstrated that the residual stress in the as-processed HMPMs is not significant raging form -100 to 200 MPa regardless of the high applied heat input due to the low thermal expansion and stress relief caused by intergranular micro-cracking. Key words: 3D printing, Direct energy deposition, High melting point metal, Residual stress, Neutron diffraction

A multi-technique investigation of the incuse coinage of Magna Graecia

This paper focuses on the application of different neutron techniques to characterize the manufacturing process of ‘incuse’ coins minted by Greek colonies in Southern Italy during the 6th and 5th centuries BC. In order to provide an insight into incuse coinage minting, numismatic and historical studies were combined with metallurgical research based on non-destructive neutron diffraction, neutron texture analysis and neutron tomography. The most significant scientific data collected during our campaign of investigation will be showcased.

PIND: High spatial resolution by pinhole neutron diffraction

A pinhole neutron diffraction (PIND) technique was developed to enable improving the spatial resolution down to 250 μm. Instead of the conventional engineering diffraction method which integrates all the diffraction signals on the detector plane, the PIND setup utilizes the diffraction pattern of each pixel on 2D detectors. The proposed PIND arrangement enables improving the spatial resolution of time-of-flight instruments and allows solving problems involving steep gradients of strain or texture. The phase content and preferential orientation of grains inside samples can be spatially resolved in 2D/3D. Further, PIND retains the capability of in-situ non-destructive neutron diffraction mapping of lattice strain and grain orientation under external stimuli such as temperature and force.

Residual Stress State of X65 Pipeline Girth Welds Before and After Local and Furnace Post Weld Heat Treatment

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. All pipe spools were fabricated using identical pipeline production procedures for manufacturing multipass narrow gap welds. Nondestructive neutron diffraction (ND) strain scanning was carried out on girth welded pipe spools and strain-free comb samples for the determination of the lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses were measured on two pipe spools in as-welded condition and two pipe spools after local and furnace PWHT. Measurements were conducted through the thickness in the weld material and adjacent parent metal starting from the weld toes. Besides, three line-scans along pipe length were made 3 mm below outer surface, at pipe wall midthickness, and 3 mm above the inner surface. PWHT was carried out for stress relief; one pipe was conventionally heat treated entirely in an enclosed furnace, and the other was locally heated by a flexible ceramic heating pad. Residual stresses measured after PWHT were at exactly the same locations as those in as-welded condition. Residual stress states of the pipe spools in as-welded condition and after PWHT were compared, and the results were presented in full stress maps. Additionally, through-thickness residual stress profiles and the results of one line scan (3 mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry's R6 procedure. The residual stress profiles in as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

Dynamic Strain Evolution around a Crack Tip under Steady- and Overloaded-Fatigue Conditions

We investigated the evolution of the strain fields around a fatigued crack tip between the steady- and overloaded-fatigue conditions using a nondestructive neutron diffraction technique. The two fatigued compact-tension specimens, with a different fatigue history but an identical applied stress intensity factor range, were used for the direct comparison of the crack tip stress/strain distributions during in situ loading. While strains behind the crack tip in the steady-fatigued specimen are irrelevant to increasing applied load, the strains behind the crack tip in the overloaded-fatigued specimen evolve significantly under loading, leading to a lower driving force of fatigue crack growth. The results reveal the overload retardation mechanism and the correlation between crack tip stress distribution and fatigue crack growth rate.

Residual stresses in high strength steel welded box sections

Induced residual tensile and compressive stresses generated by the fabrication process can cause premature buckling and reduce the ultimate axial strength of welded steel box sections subjected to compressive loads. Previous research pertaining to induced residual stresses has been restricted to box sections fabricated from mild steel and 460MPa high strength steel (HSS) using single pass welds or lightly welded sections. To date, very limited research has been undertaken to investigate the residual stresses in box sections fabricated from 690MPa HSS plates. This paper investigates the induced residual stress distributions in square welded sections fabricated from 690MPa HSS plates with single and multiple weld passes. The non-destructive neutron diffraction technique has been used to investigate the residual stresses in six test specimens: four specimens fabricated from 5mm HSS plates using single weld passes, and the other two specimens fabricated from 16mm HSS plates using six weld passes. Experimental results suggest that the 16mm thick heavily welded sections exhibited higher tensile residual stresses than the 5mm thick lightly welded sections, while the compressive residual stresses of the heavily welded sections were similar to the compressive residual stresses of the lightly welded sections. Considering the experimental results, a relationship between compressive residual stresses and width to thickness ratio of the specimens is formulated. In addition, a residual stress distribution model is proposed for lightly and heavily welded box sections.

Residual stress in a laser welded EUROFER blanket module assembly using non-destructive neutron diffraction techniques

Whilst the structural integrity and lifetime considerations in welded joints for blanket modules can be predicted using finite element software, it is essential to prove the validity of these simulations. This paper provides detailed analysis for the first time, of the residual stress state in a laser-welded sample with integral cooling channels. State-of-the-art non-destructive neutron diffraction was employed to determine the triaxial stress state and to understand microstructural changes around the heat affected zone. Synchrotron X-ray diffraction was used to probe the variation of strain-free lattice reference parameter around the weld zone allowing correction of the neutron measurements. This paper details an important experimental route to validation of predicted stresses in complex safety-critical reactor components for future applications.

Measurement of Interface Thermal Resistance With Neutron Diffraction

A noncontact, nondestructive neutron diffraction technique for measuring thermal resistance of buried material interfaces in bulk samples, inaccessible to thermocouple measurements, is described. The technique uses spatially resolved neutron diffraction measurements to measure temperature, and analytical or numerical methods to calculate the corresponding thermal resistance. It was tested at the VULCAN instrument of the Spallation Neutron Source, Oak Ridge National Laboratories on a stack of three 6061 alloy aluminum plates (heat-source, middle-plate, and heat-sink), held in dry thermal contact, at low pressure, in ambient air. The results agreed with thermocouple-based measurements. This technique is applicable to all crystalline materials and most interface configurations, and it can be used for the characterization of thermal resistance across interfaces in actual engineering parts under nonambient conditions and/or in moving/rotating systems.

Neutron Diffraction Investigation of Residual Stresses Induced in Niobium-Steel Bilayer Pipe Manufactured by Explosive Welding

Recently, reliable and hermetic joining of stainless steel to niobium pipes has been achieved with the explosive bonding technique. Joining of these two materials are essential to ensure production of a bimetallic transition element of pipe-type for its further use as a part of charged beam acceleration systems of the new generation. A non-destructive neutron diffraction investigation of the tri-axial strains along a radial cross-sectional line through the joint section has been performed. Residual stress results indicate inherently different natures in the residual stress values within the respective pipe sections. In the external stainless steel pipe the residual stresses are tensile, showing a sudden increase to 600 MPa as the interface is approached, whilst being compressive in the internal niobium pipe, not exceeding 650 MPa. A characteristic abrupt stress discontinuity exits at the interface region.