Study and prevention of cracking during weld-repair of heat-resistant cast steels

International audienceHeat-resistant cast steels are highly sensitive to cracking as they are weld-repaired because of their very low ductility. To prevent weld-repair cracking of three different heat-resistant cast steels used for the manufacturing of superplastic forming (SPF) dies, the effect of various welding parameters, such as the choice of the filler material, the number of weld passes and the pre-heating temperature has been investigated. The choice of an appropriate filler metal and the pre-heating to 400 °C of the material prior to welding drastically lower the propensity to cracking, but remain unable to eliminate cracks entirely. To further reduce weld-repair cracking and hopefully prevent it completely, a buttering technique has been developed. Buttering of the base metal surface with nickel alloys before weld-repair has been shown to prevent cracking of the base metal, but results in some hot-cracking of the buttering layer itself. On the other hand, buttering with Ni-Fe alloys, less sensitive to hot-cracking, results in crack-free weld repairs

A microstructural and low-cyle fatigue investigation of weld-repaired heat-resistant cast steels

The multi-pass weld-repair of heat-resistant cast steels is carried out using an automated shielded metal arc welding (SMAW) process, with various filler materials and pre-heating at 400°C. Specimens weld-repaired with a filler material more resistant than the heat-resistant cast steel (over-matching) generally crack within the base metal following the tenth filling pass, whereas specimens buttered with a soft alloy prior to welding remain free of cracks. The high temperature strain-controlled fatigue lifetime of material weld-repaired without buttering is lower than that of bulk initial material. This is due to an increase of the stress amplitude as a result of the so-called over-matching. In the case of material welded following a prior buttering, the fatigue lifetime is reduced because of the stress tri-axiality generated in the thin soft layer which prevents its plastic flow. As a consequence, it is concluded that even though buttering prevents cracking efficiently during welding, it is not acceptable as far as fatigue performance, especially lifetime, is concerned

New investigations on aluminium-steel joining with Nd-YAG laser

Inexistan

Réalisation d'assemblages acier-aluminium par mouillage réactif induit par laser : approche expérimentale et numérique

Inexistan

Steel to aluminium joining by laser and TIG reactive wetting

The laser joining of a low carbon steel to a 6000 series aluminium alloy was realised in key-hole welding mode in a steel-on-aluminium overlap configuration and was investigated in a three-fold approach: (1) process optimisation, (2) material characterisation and (3) mechanical testing. No-defect welds, composed of a solid solution of aluminium in iron and richer aluminium “white solute bands” of FeAl phases were obtained when limiting steel penetration in aluminium to below 500 m. Embrittlement of the joining zone was observed, mainly located on the weld–aluminium interfaces composed of Fe2Al5 and/or FeAl3 phases with thicknesses between 5 m and 20 m. Limiting penetration to below 500 m allowed to restrict steel to aluminium dilution in order to confine the hardness of the welds. With such penetration depths, up to 250 N/mm in linear strength could be achieved, with failures located in the weld–aluminium interfaces. Increasing penetration depth led to a change in the assembly weak points (in the weld and on the steel–weld interfaces) and induced a severe decrease in strength

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

Steel to aluminium brazing by laser and TIP processes

The low carbon steel to 6000 aluminium alloys brazing was investigated by laser and TIG processes. The configuration used in Nd:YAG laser brazing was an overlap one with the aluminium placed upon steel. Two brazing modes were studied, in the first one the joints were obtained between steel and aluminium alloys by reactive wetting whereas in the second mode an aluminium-silicium filler alloy was employed. For TIG brazing the arc was diriged toward steel in a lap configuration where the aluminium was placed upon steel. In the first mode, the brazed joints were realized without filler metal and in the second mode, the influence of silicium, zinc and nickel were studied in powder shape. The joints were observed by optical microscope and scanning electronic microscope. The phase composition was characterized by energy dispersive X-rays and the microhardness was obtained by Vickers hardness test. Two mechanical test were used to characterize fracture strengths of the assemblies. The first one was a transverse tensile test which stressed the interfaces in shearing mode, the second test used was a “tearing off test” which stretched the interfaces in tensile mode. The first results obtained show that that the use of an aluminium-silicium filler reduced the reaction layer thickness for interfaces formed by laser brazing. In the case of reaction layers obtained by TIG brazing, the use of silicium showed a change in morphology with a reduced thickness of the reaction layers. The use of nickel seemed to lower the layer thickness. The fracture strengths of the joints obtained between steel and aluminium without filler metal measured with the transverse tensile test were about 180 N.mm-1 for the joints brazed by laser process and about 140N.mm-1 for the joints obtained by TIG brazing

Soudage hétérogène acier/aluminium par les procédés laser et TIG :étude métallurgique et mécanique de l'interface

Non renseign

Galvanised steel to aluminium joining by laser and GTAW processes,

A new means of assembling galvanised steel to aluminium involving a reaction between solid steel and liquid aluminium was developed, using laser and gas tungsten arc welding (GTAW) processes. A direct aluminium melting strategy was investigated with the laser process, whereas an aluminium-induced melting by steel heating and heat conduction through the steel was carried out with the GTAW process. The interfaces generated during the interaction were mainly composed of a 2-40 μm thick intermetallic reaction layers. The linear strength of the assemblies can be as high as 250 N/mm and 190 N/mm for the assemblies produced respectively by laser and GTAW processes. The corresponding failures were located in the fusion zone of aluminium (laser assemblies), or in the reaction layer (GTAW assemblies)