M. Amirthalingam

Stress relaxation due to ultrasonic impact treatment on multi-pass welds

Abstract Ultrasonic impact treatment (UIT) is a relatively novel technique applied to the toe of welded joints to improve the fatigue life by changing the weld geometry and the residual stress state. In this study, the stress relaxation due to ultrasonic impact treatment is investigated on a six pass welded high strength quenched and tempered steel section. Stress measurements in two orthogonal directions were conducted by energy dispersive synchrotron X-ray diffraction. Results show that the application of only ultrasound to a welded component re-distributes the residual stresses more uniformly, while mechanical impacts in combination with ultrasound is an effective way to release the residual stresses. After welding, diffraction peak broadening due to the lattice distortion, characterised by the full width at half maximum (FWHM), is observed in the region of the weld toes. Ultrasonic impact treatment reduces the FWHM at these locations.

Multipurpose furnace for in situ studies of polycrystalline materials using synchrotron radiation

We report a multipurpose furnace designed for studies using synchrotron radiation on polycrystalline materials, namely, metals, ceramics, and (semi)crystalline polymers. The furnace has been designed to carry out three-dimensional (3D) x-ray diffraction measurements but can also be used for other types of synchrotron radiation research. The furnace has a very low thermal gradient across the specimen (<0.2 degrees C/mm). Accurate determination of the temperature can be carried out by welding a thermocouple to the specimen. The furnace can be rotated over an angle of 90 degrees in order to determine the crystallographic orientation of each individual grain. It is possible to follow growth kinetics of all grains in the illuminated volume of the specimen. The specimen environment can be controlled varying from vacuum (up to 10(-5) mbar) to gas or air filled. The maximum temperature of operation is 1500 degrees C, with the possibility of achieving high heating (up to 20 degrees C/s) and cooling rates (up to 30 degrees C/s without quenching gas). 3D maps of the microstructure of the specimen can be generated at elevated temperatures by bringing the high-resolution detector close to the specimen. We show an example of a simulation of the heat affected zone during the thermal cycle of a weld in a transformation-induced plasticity steel carried out using the furnace. The unique characteristics of the furnace open possibility of new fields in materials research using synchrotron radiation.

Elemental segregation during resistance spot welding of boron containing advanced high strength steels

The partitioning behaviour of carbon, phosphorous and boron during the solidification of a resistance spot weld pool was studied using experimental simulations and a phase field model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to a range of resistant spot welding thermal cycles. Mechanical properties were evaluated by hardness and cross tension tests and correlated with the weld microstructure. Phase field modelling results and experimental predictions show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt% for a steel with a bulk concentration of 0.08 wt%. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength and plug diameter, the failure mode switches to a complete plug mode, resulting from the phosphorous depletion at the grain/inter-phase boundaries.

Pass-by-pass stress evolution in multipass welds

Abstract In multipass welding, each successive thermal cycle will introduce local melting, solid state phase transformations, grain growth, grain refinement, recrystallisation and recovery, all of which lead to a complicated stress state. Most stress measurements performed on multipass welded components represent the final residual stress state. Information concerning stress evolution on a pass-by-pass basis is difficult to find. In this investigation, six pass welds were made on high strength quenched and tempered steel sections, and depth resolved strain measurements in two orthogonal directions were carried out after each weld pass using energy dispersive synchrotron X-ray diffraction. The residual stresses were calculated using biaxial Hooke’s law. A thermal–metallurgical–mechanical welding model was constructed and validated with temperature and pass-by-pass stress measurements, which improves the reliability of the model. Cross-sectional stress distributions are presented after each pass, revealing the weld stress development in multipass welds.

In situ strain investigation during laser welding using digital image correlation and finite-element-based numerical simulation

ABSTRACT In situ strain evolution during laser welding has been measured by means of digital image correlation to assess the susceptibility of an advanced high strength automotive steel to solidification cracking. A novel method realised using auxiliary illumination and optical narrow bandpass filter allowed strain measurements as close as 1.5 mm from the fusion boundary with good spatial and temporal resolution. A finite-element thermomechanical model of the welding process supports the experimentally measured transverse strain. The validated finite-element numerical model can be used to assess the local strain and associated stress conditions which influences weldability and in particular, solidification cracking.

In Situ Phase Transformation Studies on a Transformation Induced Plasticity Steel Under Simulated Weld Thermal Cycles Using Synchrotron Diffraction

In situ phase transformation behaviour of aluminium-containing transformation induced plasticity steels, while subjecting them to heat affected zone weld thermal cycles have been studied. Experiments were carried out at ID11 of the European Synchrotron Radiation Facility, Grenoble, France. A specially designed oven was used to simulate the weld thermal cycles. Time–temperature resolved 2D synchrotron diffraction patterns were recorded and used to calculate volume fractions and lattice parameters of the phases. Results show that during heating, the retained austenite starts to decompose to ferrite and iron carbides once the temperature reaches 290°C. The lattice parameter of austenite increases linearly up to 290°C, followed by an increase in slope due to the formation of iron carbides. The combined effect of carbon concentration and thermal expansion causes scatter in the lattice parameter of austenite once the temperature reaches the inter-critical (α + γ) region. It is also observed that a significant amount of austenite (6–7%) was found to be retained at room temperature despite a high cooling rate (>20°C s−1). Even after cooling the samples to room temperature, austenite was found to continue decomposing upon further holding and the volume fraction of retained austenite decreased continuously with time at room temperature.

Residual Stress Measurements in Multi-Pass Welded High Strength Steel Using Energy Dispersive Synchrotron X-Ray Diffraction

Six pass welds were made on a 16 mm thick high strength quenched and tempered structural steel plate (S690QL1, Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt.%)). Depth resolved measurements in two orthogonal directions were carried out using energy dispersive synchrotron X-ray diffraction at the ID15 beamline of the European Synchrotron Radiation Facility. The strains were calculated from the shift in the local d-spacing for four bcc planes ({200}, {211}, {220}, {310}). The planar stresses were calculated from the biaxial Hooke’s law, using the diffraction elastic constants of the individual planes. A two dimensional cross-sectional residual stress map with a depth resolution of 2 mm was obtained. Transverse compressive stresses were found at the weld toes and root. Transverse tensile stresses were present in the middle of the plate. Longitudinal tensile stresses concentrated along the fusion line. This work describes the procedures to obtain the depth resolved residual stress map and the generated results provide necessary information to validate thermal mechanical finite element model of multi-pass welding.

Transformation-Induced Diffraction Peak Broadening During Bainitic and Martensitic Transformations Under Small External Loads in a Quenched and Tempered High Strength Steel

Insitu phase transformation behavior of a high strength S690QL1 steel during continuous cooling under different mechanical loading conditions has been used to investigate the effect of small external loads on the transformation-induced plasticity during bainitic and martensitic transformations. The results show that during phase transformations, the untransformed austenite undergoes plastic deformation, thereby retarding further transformation to bainite/martensite. This occurs independent of external load.

Microstructural Evolution during Gas Tungsten Arc, Laser and Resistance Spot Welding of Al-Containing Transformation Induced Plasticity (TRIP) Steel

In this work, the microstructural evolution of aluminum containing commercial grade TRIP steels during gas tungsten arc (GTA), Laser beam (LB) and resistance spot (RS) welding have been studied. Microstructural analysis was carried out using optical and scanning electron microscopy. Results show that fusion zones of welded TRIP steels contain complex inclusions with similar size distribution. The energy dispersive spectroscopy analysis of inclusions indicated that these inclusions are primarily oxides of aluminum with epitaxial enrichment of manganese and phosphorous. The fusion lines of GTA and LB welded aluminum containing TRIP steel contain a zone of polygonal ferrite with a size of about 200 m and 50 m respectively. It is found that aluminum partitioned from the liquid weld to the solidified delta ferrite in the fusion line causing enrichment and resulting in ferrite stabilisation. This ferrite zone was not found in the case of resistance spot welded samples due to faster cooling rates.

Anisotropy in Thermal Expansion of Bainitic Ferrite

The evolution of local d-spacings between lattice planes of bainitic ferrite in a high strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)), has been determined to calculate the thermal expansion behavior. For this purpose, in-situ continuous cooling tests have been carried out in a high-energy synchrotron X-ray diffractometer. The results indicate thermal anisotropy in the bainitic ferrite planes.