Frédéric Deschaux-Beaume

Control of mass and heat transfer for steel/aluminium joining using Cold Metal Transfer process

Abstract The Cold Metal Transfer process is investigated to join zinc coated steel with aluminium alloy by braze-welding. A 4043 filler metal is deposited on the surface of the coated steel, and the effect of the current waveform on the metal transfer and the heat transferred to the base metal is investigated. The reduction in the ‘boost phase’ duration of the current waveform decreases the volume of liquid drops at the wire tip and allows to increase the short-circuit frequency, inducing a similar or higher deposit rate. Regular deposits are obtained when the linear energy remains below ∼500 J mm−1. The heat transferred to the base metal, and thus the thickness of the intermetallic layer formed at the Al/steel interface, is lower for an equivalent deposit rate when the short-circuit frequency is high.

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.

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.

In-situ observation and modelling of solidification and fluid flow on GTAW process

An experimental setup is presented in order to obtain experimental data during solidification of a static weld pool after arc extinction with a GTAW process. Several devices have been set up to extract three kinds of measurements: (i) solidification front velocity (ii) fluid flow velocity at the vicinity of the front (iii) temperature field in the solid part. A high-speed camera is used to film the interface during welding at microscopic level and an infra-red in order to take the temperature field around the weld pool in the solid part. After processing and calibration of the videos, the experimental results are compared to theoritical results founded on an adapted model from the KGT [1] and from the one of Gandin et al. [2]. All the tests are done thin plate of Cu-30wt.%Ni.

In situ observations and measurements during solidification of CuNi weld pools

A two-scale in situ observation set-up has been used to investigate the physical phenomena around the solid/liquid interface in welding. A spot gas tungsten arc welding process is used to observe weld pool solidification phenomenon after arc extinction. The solidification front and the fluid flow in the weld pool have been observed at a microscale by a high speed camera. The whole weld pool and the surrounding base metal have been filmed at a macroscale by an infrared camera to estimate temperature fields around the weld pool and bead shape. A qualitative and quantitative dataset was extracted from these observations that involve useful results for further improvements of the understanding of weld solidification mechanisms and to enhance solidification models.

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...