Modelling hot cracking in 6061 aluminium alloy weld metal with microstructure based criterium

International audienceHot cracking in welding is a complex phenomena due to coupling between process, metallurgy and mechanical loading. In this paper, a coupled approach that integrates microstructural prediction based on the process simulation and criteria at the microstructural scale is proposed. The criteria is based on some development of Rappaz et al [1] and takes into account the influences of grain morphology, mechanical and thermal fields induced by welding, on hot cracking. To integrate all factors influencing hot cracking, a methodology is developed that couples process simulation, simple microstructural prediction and criteria along columnar grains. The criteria based on the microstructure behavior is able to predict crack onset location in columnar grains zones

Weldability of New Ferritic Stainless Steel For Exhaust Manifold Application

International audienceIn the current context of fossil energy scarcity, car manufacturers have to optimize vehicles energy efficiency. This global and continuous improvement includes a change of the exhaust manifold design. Usually in cast iron, exhaust manifolds tend to be mechanically welded in order to fit new constraints such as lightness, durability, efficiency and small size. To achieve such requirements, ferritic stainless steels with high chromium content (19%) and molybdenum (2%) are developed. For the welding, the use of existing filler wire does not satisfy fully the application requirements. This leads to oxidation problems and / or thermal fatigue strength that drastically reduces assembly lifetime. New flux cored wires are developed in the context of this study in order to provide molten zone characteristics close to those of the base metal. Different chemical compositions are tested in order to highlight the influence of stabilizing element on microstructure. Welding tests revealed the major influence of titanium on the grain refinement in the molten zone. A minimum Ti content of 0.45 weight % in the filler wire is required to be efficient as grain refiner

Aging of a cast 35Cr-45Ni heat resistant alloy

International audienceThe microstructural evolution during aging and its effect on the mechanical properties of a centrifugally cast 35Cr-45Ni heat resistant alloy was studied by means of optical and electron microscopy, and by mechanical testing in samples aged in air at 750 ◦C for a period of time of up to 1000 h. The as-cast microstructure consisted of an austenitic matrix and a network of two types of primary carbides that were identified as NbC and M7C3 by their light and dark tones when viewed in backscattered electron mode in a scanning electron microscope. Aging promoted the occurrence of different phenomena such as the transformation of primary M7C3 to M23C6 carbides, precipitation of secondary M23C6 carbides and the transformation of NbC to Nb3Ni2Si. It was found that aging promoted an increase in Vickers microhardness of more than 50%, the increment in tensile strength of around 20% and the reduction in ductility of close to 70%