Gilles Fras

Steel to aluminium braze welding by laser process with Al–12Si filler wire

Abstract The joining of DC04 steel to 6016-T4 Al alloy is achieved by laser braze welding using a 4047 (Al–12Si) filler wire and a brazing flux. The dissimilar joining is obtained both by welding the parent 6016 alloy to the 4047 filler wire, producing a continuous bonding without apparent macroscopic flaws, and by reactive wetting of the molten Al alloy on the solid steel, resulting in the formation of a thin layer of Fe–Al–Si intermetallic compounds at the steel/bead interface. The linear strength of the assemblies can be as high as 190 N mm−1, with a failure generally located in the reaction layer of the steel/bead interface. Last, the strength of the assemblies is shown to increase linearly with the reaction layer width.

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

Abstract Hot cracking in welding is a complex phenomenon due to coupling between process, metallurgy and mechanical loading. A methodology based on process simulation, simple microstructural prediction and a pressure model along columnar grains is developed in order to integrate all factors that influence hot cracking. The model is based on some developments of Rappaz, Drezet and Gremaud and takes into account the influences of grain morphology, mechanical and welding thermal loading, on hot cracking. The model based on the microstructure behaviour is able to predict crack onset location in columnar grains on 6061 aluminium alloy.

TWO DIMENSIONAL SIMULATION OF DEHYDRATION OF A HIGHLY DEFORMABLEGEL : MOISTURE CONTENT, STRESS AND STRAW FIELDS

Abstract A model taking into account the simultaneous influence of mechanical and hygrometric actions (Mrani and al., 1995a, 1995b) was applied to study of the dehydration of a cylinder of highly deformable biphase gel (agar gel) (Mrani and al., 1995b). Numerical solution of the water transport equation and the mechanical equilibrium equation provided access to the water content and stresses and strains in the cylinder. The numerical results were validated for water content and overall deformation of gel cylinders. The model clearly shows the inversion of the curve of the faces of the cylinder observed experimentally. Three deformation phases were observed. In the first phase, when the concaveness of the faces of the cylinder was turned inwards, tractive stress appeared at the surface of the sample. More uniform distribution of the water content became established in a second phase and the cylinder recovered its initial shape corresponding to the relaxation of the stress state. In the third phase, the con...

Numerical Study of Drying Stresses in Agar Gel

ABSTRACT ABSTRACT A numerical model of isothermal drying of deformable substances is proposed. The model is applied in a uni-directional case to analysis of water transport in a sphere of agar ge. The simulation program makes it possible to calculate the water content fields and def rmations and stresses in the sphere during drying. Validation of the water content profile results is proposed by the experimental study of the dehydration of gel spheres in a sugar solution. The sensitivity of internal stresses to drying conditions is then analysed.

Image processing and geometrical analysis for profile detection during pulsed gas metal arc welding

This article describes an image analysis algorithm used to detect profiles during arc welding processes. The new algorithm is an aggregation of image processing (segmentation, filtering), computational geometry (alpha shape) and graph theory (cycle detection). It allows to extract precise geometrical profile entities, whether open or closed contours, that could be used for the monitoring of the welding process. The algorithm is shown to be really efficient and could be used for real-time monitoring of gas metal arc welding process.

Hot tearing test for TIG welding of aluminum alloys: application of a stress parallel to the fusion line

Defects control such as solidification cracking in aluminum alloys welding is an important industrial issue and must be carefully examined. This phenomenon is a complex problem involving process, material and mechanical loading due to clamping. Several tests have been previously developed in order to characterize the material propensity to hot cracking. The purpose of the present work is to study, using a new hot cracking test and numerical simulation, the relationship between hot cracking sensitivity and mechanical or metallurgical factors in order to better identify the parameters leading to hot tearing during welding. The originality of the test presented here is that an initial stress is applied on the test specimen parallel to the welding direction. During the test, a fusion line is made using Gas Tungsten Arc Welding (GTAW) process on a thin sheet of aluminum alloy (6061). The crack initiation occurs once steady state thermal conditions are reached. The present test enables to distinguish between the structural effects on a global scale and the microstructural effects on a local scale. Microstructure control is made possible by adjusting welding power, welding speed and sample geometries. The grain structure, which is characterized by the shape, size and the growth direction, and which depends on welding current and speed, plays a crucial role in the crack initiation. Microstructural features are observed using high speed camera recording and post mortem micrographs. Mechanical factors are varied by adjusting the welding parameters. The relationship between welding parameters, grain morphology, and sensitivity to hot cracking are discussed. Experimental measurements and numerical results will help to better determine global and local conditions at the onset of hot tearing and to compare those conditions using existing hot tearing model.

Strains and Stresses Induced by Water Transport in Hygroscopic Elastic Media

We propose to analyse the phenomenological equation of water transport in a biconstituent medium with stresses. The work is limited to the case of elastic, isotropic behaviour and an isothermal transfer process.

Weldability of New Ferritic Stainless Steel for Exhaust Manifold Application

In the current context of fossil energy scarcity, car manufacturers have to optimize vehicles energy efficiency. This 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.

Which laser process for steel to aluminium joining

Non-galvanized and 10 µm zinc-coated 1.2 mm thick DC04 steel was joined to 6016-T4 aluminium alloy by using three different laser processses : a key-hole welding mode, with a precise control of the aluminium – steel dilution, a reactive wetting mode where solid steel – liquid aluminium reaction occurred driving to a uniform Fe2Al5 intermetallic layer between the two overlapped sheets and a braze-welding mode involving direct fusion of aluminium and an Al-12Si filler wire on solid steel.For liquid aluminium to liquid steel interactions obtained by key-hole mode, rather sound and resistant assemblies were realized either on non-galvanized or galvanized steel provided steel was placed upon aluminium with penetration in aluminium limited to 0.5 mm. The influence of galvanized layer was only detectable on the fusion zone of aluminium where occluded zinc bubbles were observed. Mechanical resistances of 150 N/mm were obtained for one joint assemblies and could be increased up to 250 N/mm making two joints per assembly.For liquid aluminium to solid steel interactions carried out by defocused laser, 180 N/mm transverse tensile strengths were obtained on non-galvanized steels by using a brazing flux. Due to a better wetting on non-galvanized steels, good assemblies could be obtained without using flux leading to lower mechanical resistances of up to 140 N/mm. However, using flux conduced to 220 N/mm maximal mechanical resistance.For this kind of interaction solid/liquid), using an Al-12Si filler wire allows to obtained also 180 N/mm mechanical strengths on non-galvanized steels using a brazing flux. Same characteristics are obtained in the reaction layers composition with a decrease in maximal layer thickness under 1 µm compared to the 2-40 µm thickness obtained without filler wire.Finally, comparisons are made between the three processes investigated focusing on the mechanical properties and the robustness of each process.Non-galvanized and 10 µm zinc-coated 1.2 mm thick DC04 steel was joined to 6016-T4 aluminium alloy by using three different laser processses : a key-hole welding mode, with a precise control of the aluminium – steel dilution, a reactive wetting mode where solid steel – liquid aluminium reaction occurred driving to a uniform Fe2Al5 intermetallic layer between the two overlapped sheets and a braze-welding mode involving direct fusion of aluminium and an Al-12Si filler wire on solid steel.For liquid aluminium to liquid steel interactions obtained by key-hole mode, rather sound and resistant assemblies were realized either on non-galvanized or galvanized steel provided steel was placed upon aluminium with penetration in aluminium limited to 0.5 mm. The influence of galvanized layer was only detectable on the fusion zone of aluminium where occluded zinc bubbles were observed. Mechanical resistances of 150 N/mm were obtained for one joint assemblies and could be increased up to 250 N/mm making two joints per as...