Denis Favez
In jewelry manufacturing, joining dissimilar materials is usually achieved by brazing or soldering, in which the materials comprising the joint are heated to a suitable temperature in the presence of a filler metal having a liquidus below the solidus of the base metals. This simple and cost-effective method has however several disadvantages, including undesired phases transformations in the bulk base metals during heat treatment. An alternative joining methodology is laser beam welding. Due to its high energy density, a laser beam is able to melt locally the interface, without affecting the bulk microstructure of the parts to be joined. With this method, however, it is necessary to have a thorough understanding of microstructure formation during solidification. In this thesis, the process of laser welding an 18 carat yellow gold to a superaustenitic stainless steel has been studied and developed. In order to understand the microstructure formation during this welding process, a fundamental metallurgical analysis of the gold-steel system was first carried out, partially based on the binary Au-Fe elemental system. Following this, laser welds have been characterized and a concept for gold-steel welding proposed and successfully tested. First, Differential Thermal Analysis (DTA) and directional solidification in a Bridgman-type furnace followed by rapid quenching were used to acquire an understanding of the microstructure evolution upon cooling in Au-Fe alloys. During the (γ-Fe) primary phase solidification, micro- and macrosegregation phenomena influence the final composition profile. The peritectic phase forms rapidly and instabilities develop during solid-phase peritectic transformation. Below the peritectic temperature, solid phases decompose in a miscibility gap, leading to continuous or discontinuous precipitation, depending on cooling conditions. Finally, (α-Fe) ferrite precipitation gives insights on orientation relationships between primary and peritectic phases. Second, yellow gold-stainless steel alloys have been studied at low cooling rates by DTA. By combination of these observations with the Au-Fe metallurgical analysis, the microstructure formation in gold-steel laser welding could be explained. It appears that the microstructure formed during laser welding is strongly related to the local melt pool composition. Three different cases were identified: (1) Due to its small solidification interval and little segregation, goldrich liquid solidifies quickly as columnar grains, avoiding formation of cracks or porosities. (2) Due to the high cooling rate, liquid of intermediate composition is brought rapidly to low temperature, reaching the metastable liquid miscibility gap, resulting in demixing into two liquids. (3) A liquid containing mainly iron will solidify with a dendritic microstructure rejecting large amount of gold in the interdendritic liquid. In this last case, this large solidification interval combined with the considerable stresses around the weld pool leads certainly to cracking. Based on the above analysis, a concept of gold-steel laser welding can be proposed. By focusing the laser beam on the gold side of the interface, a growing liquid pool can be formed in this metal. When the melt pool reaches the steel surface, a layer of peritectic phase will form, without melting much steel. Since the liquid pool contains mainly gold, hot cracking is avoided. After solidification, the peritectic layer ensures a smooth transition between the mechanical properties of gold and steel. This concept has been successfully applied on a number of real pieces, forming sound welds with suitable properties. This thesis has also shown that a fundamental metallurgical approach is a powerful tool for complex welding development. It allows not only to develop a viable industrial solution, but also to acquire a knowledge that can be used to expand the process to other materials or different geometries.
EPFL2009
