Materials Science
A novel production route for oxide dispersion strengthened steels ⁑
Oxide dispersion strengthened (ODS) ferritic/ martensitic steels are promising high-performance structural materials for advanced fusion and fission reactors. ODS Eurofer shows a pronounced increase in tensile strength, yield strength and creep strength from room temperature to temperatures up to 973 K, mainly attributed to the presence of the highly dispersed and extremely stable yttrium oxide particles, which act as obstacles for the movement of dislocations and trapping sites for point defects.
A new, low cost powder production route was explored involving a combination of mechanical alloying and spark plasma sintering (SPS). Different combinations of mechanical treatment and sintering parameters were examined in order to optimise the fabrication process. Results show that ball milling for 30 h and sintering at a temperature of 1373 K and a pressure of 60 MPa resulted in a high density and microhardness. As-produced ODS Eurofer shows a bimodal microstructure with homogeneously dispersed nanoscale yttrium oxide; subsequent heat treatment results in a good balance between material strength and ductility.
⁑ Fu, J.; Brouwer, J.C.; Richardson, I.M. and Hermans, M.J.M. ‘Effect of Mechanical Alloying and Spark Plasma Sintering on the Microstructure and Mechanical Properties of ODS Eurofer’, Materials and Design, 177, 107849, 2019.
Variation of the powder size as a function of milling time.
Energy Dispersive X-Ray Spectroscopy maps of samples produced by milling for different times and sintered at 1373 K. The dashed rectangles indicate uneven distributions of elements.
Crystallite size and lattice strain as a function of milling time for the mechanical alloyed powders.
Optical micrographs of samples milled for 30 h and sintered at (a) 1323 K and (b) 1373 K.
Morphology of as-produced (a), (b) and heat-treated (c), (d) samples with the location of precipitates marked.
Vickers hardness profile of the cross section of the as-produced and heat-treated samples.
Correction of welding induced distortion *
Correction of welding induced distortion during structure or component fabrication increases both the production time and production costs. Distortion arises due to a combination of factors related to inhomogeneous stress distributions, temperature dependent thermal expansion and localised plastic deformation. Welding induced distortion can take a number of forms, driven both thermal shrinkage of the weld deposit during cooling to room temperature as well as accompanying phase transformations.
Buckling distortion arises on welded components that are generally thin relative to the component size (vehicle bodies, ship hulls etc.). Unlike other forms of distortion, buckling is an instability triggered when the compressive stress exceeds the critical buckling stress for the structure or component. One promising technique to mitigate buckling distortion is the application of thermal tensioning, whereby additional heating is applied parallel to the welding direction. The additional heating leads to the introduction of stress peaks away from the weld line and a re-distribution of the stresses after final cooling to room temperature. With appropriate choice of heating strength and location, the final stress redistribution can avoid stresses exceeding the critical buckling stress and hence eliminate buckling distortion.
* Pazooki, A.M.A.; Hermans, M.J.M. and Richardson, I.M. ‘Finite Element Simulation and Experimental Investigation of Thermal Tensioning During Welding of DP600 Steel’, Science and Technology of Welding and Joining, 22(1), 7-21, 2017.
See also
Pazooki, A.M.A.; Hermans, M.J.M. and Richardson, I.M. ‘Control of Welding Distortion During Gas Metal Arc Welding of AH36 Plates by Stress Engineering’ International Journal of Advanced Manufacturing Technology, 88(5), 1439-1457, 2017.
Illustration of different distortion modes.
Schematic and experimental indication of additional heating burner positions relative to the welding torch.
Distortion of a conventional weld (left) and a weld made with thermal tensioning (right).
Photos showing out of plane distortion of a conventionally welded plate (left) and a plate welded with side heating (right).
Measured and simulated stress distributions for a conventional weld (above) and a weld made with thermal tensioning(below).
Avoiding transverse solidification cracking in welded high strength aluminium alloys ♰
Fusion welding of high strength aluminium alloys presents many challenges. Laser and hybrid laser/arc welding can be applied successfully; however, for some alloys, large numbers of transverse solidification cracks are observed in the weld fusion zone. These differ in direction from the more common centre line cracks typically associated with solidification cracking. Nevertheless, scanning electron microscopy confirmed that the cracks occurred when the weld fusion zone was in a semi-solid state. Using the thermal histories in the workpiece under representative welding conditions in constitutive models of the thermal-mechanical response of the materials revealed that transverse cracking is related to the elongated temperature distribution in the welding direction, which in turn induces a tensile strain in the weld fusion zone during the cooling phase. One of the possible solutions to the cracking problem is to employ an additional heat source to alter the temperature and associated stress distribution, reduce the cracking tendency.
Typical transverse cracks found in welded AA7075 and 2024 alloys. The microstructure showing cracks follow the dendrite boundaries. SEM images indicate crack surfaces with a mixture of (a) a smooth liquid state (b) the solid-state fractures and (c) liquid state fractures.
Plastic strain and elastic strain development with corresponding temperature history on the weld central line for welding conditions leading to transverse cracking (above) and conditions for which no cracking occurs (below).
Marco images and microstructure of the cracks formed on the surface of the samples produced by laser beam welding at a speed of 120 mm/s (above). No cracks were found on the surface of the samples produced by laser beam welding with an additional heat source at the same welding speed (below).
♰ Hu, B. and Richardson, I.M. ‘Mechanism and Possible Solution for Transverse Solidification Cracking in Laser Welding of High Strength Aluminium Alloys’. Materials Science and Engineering A-Structural Materials Properties, 429(1-2), 287-294, 2006.
