Belgian Welding Institute

The demand for joining dissimilar metals has exponentially increased due to the global concerns about climate change, especially for electric vehicles in the automotive industry. Ultrasonic welding (USW) surges as a very promising technique to join dissimilar metals, providing strength and electric conductivity, in addition to avoid metallurgical defects, such as the formation of intermetallic compounds, brittle phases and porosities. However, USW is a very sensitive process, which depends on many parameters. This work evaluates the impact of the process parameters on the quality of ultrasonic spot welds between copper and aluminium plates. The weld quality is assessed based on the tensile strength of the joints and metallographic examination of the weld cross-sections. Furthermore, the welding energy is examined for the different welding conditions. This is done to evaluate the influence of each parameter on the heat input resulting from friction at the weld interface and on the weld quality. From the obtained results, it was possible to optimise parameters to achieve satisfactory weld quality in 1.0 mm thick Al–Cu plate joints in terms of mechanical and metallurgical properties.

Aluminium alloy sheets have come into widespread use in the design of stringerstiffened aerospace structures. This paper evaluates experimentally and numerically the response of stringer-stiffened Alclad 7075-T6 aluminium alloy panels subjected to impact loading. The aim of the study was to compare the impact resistance of riveted structures with the structures with the impact resistance of structures welded using the refill friction stir spot welding (RFSSW) process, which is a relatively new technique now used to create joints in aircraft structures. The experimental tests were carried out using a drop-weight type impact testing machine. Finite element modelling of stringer-stiffened panels under impact loading was carried out by using the nonlinear finite element-based MSC.Marc + Mentat program. The results of the numerical computations were validated against the experimental data. Studies were also made of the resistance of the joints to failure, the springback response and the stiffness of joints in the conditions of the dynamic impact test.

Many aircraft structures are thin walled and stringer-stiffened, and therefore, prone to a loss of stability. This paper deals with accurate and validated stability analysis of the model of aircraft skin under compressive loading. Both experimental and numerical analyzes are conducted. Two different methods of joining panel elements are considered. In the first case, the panel is fabricated using rivets. In the second variant, the refill friction stir spot welding technique is used. Both types of panels are loaded in axial compression in a uniaxial tensile testing machine. The geometrically and physically nonlinear finite element analyzes of the panels were carried out in ABAQUS/Standard. The Digital Image Correlation measurement system ARAMIS has been utilized to monitor the buckling behavior and failure mode in the skin-stringer interface of the stiffened panels. The results of experiments and the digital image correlation system are presented and compared to the numerical simulations.

In the aerospace industry, the refill friction stir spot welding (RFSSW) method is increasingly used to join thin sheets, especially those made of 2xxx and 7xxx series aluminium alloys. This paper presents the results of spot welding lap joints of 7075-T6 aluminium alloy sheets. The load capacity of joints was determined by tensile/shear tests using a universal testing machine. The effect of tool plunge depth, tool rotational speed and duration of welding on joint load capacity and failure mode was investigated. The polyoptimisation of the values of welding process parameters was carried out to ensure the highest load capacity of the joint at a minimum variance of the results obtained. The selection of the optimal parameters of the RFSSW process was carried out using an adequate mathematical model obtained on the basis of Weierstrass’ theorem. It was found that tool rotational speed had the greatest impact on the load capacity of the joints. It was possible to increase the load capacity of the joint by increasing the duration of welding but only to a limited extent. The selection of optimal welding parameters requires a compromise solution, i.e. the selection of a tool rotational speed that ensured adequate plasticisation of the material with a welding duration that ensured that an appropriate joint microstructure was obtained and assured the required load capacity of the joint. The methodology of mathematical modelling of polyoptimisation presented can be useful in optimising similar joining processes.

This study investigated joining of Al to Cu sheets by electromagnetic pulse welding, which is a solid-state welding process that uses electromagnetic forces to join materials. The interfacial morphology and mechanical properties of the Al/Cu joints were analysed and related to the welding process parameters and weld properties. The centre section of the Al/Cu joints evolved from a non-welded to a welded zone. The welded zone started with a wavy interface, consisting of thick interfacial layers with defects and evolved to a relatively flat interface without an interfacial layer. The interfacial layer thickness is determined by both the discharge energy and the stand-off distance. A higher tensile force, up to 4.9 kN, was achieved at a higher energy and a lower stand-off distance of 2 mm. The tensile force is directly related to the weld width, since a higher tensile force is achieved for a higher weld width. In addition, the presence of interfacial layers can contribute to a small extent to a higher tensile force.

The goal of this paper is to give a clear summary of the literature review performed during the master thesison friction welding on dissimilar materials. Of main interest for this work is the welding of steel to a ceramicmaterial such as alumina (Al2O3Almost all research articles that have been found in open literature state that welding steel to a ceramic isnearly impossible without using an interlayer consisting of a compatible metal. The most frequently reportedtype of weld is steel to alumina with an aluminium interlayer. For this reason the literature study included inthis thesis, also focuses on friction welding of aluminium to steel and aluminium to ceramic. A detailedexplanation will be given for the most challenging difficulties like the creation of a brittle intermetalliccompound layer in a steel-aluminium weld interface and the creation of fatal thermally induced stresses inan aluminium-ceramic weld interface. Both of these problems must be dealt with in order to produce astrong bond. A redesigned rotary friction welding machine is currently being assembled and will be used forthe experimental part of this research.). Because of the difficulties involved in producing a sound weld for thismaterial combination, not a lot of literature is available on this topic. This paper starts with a discussion onthe basics of friction welding and typical problems encountered in welding of steel to ceramic.

Hot dip galvanizing is a very popular and well known process in corrosion protection of steel.However, very occasionally, cracks appear on structures when they leave the zinc bath. The responsiblecrack phenomenon appears to be liquid metal embrittlement. This phenomenon is already known for a longtime, but it is still not yet fully understood. The lack of fundamental theoretical knowledge and the absenceof accurate models to predict liquid metal embrittlement oblige engineers to set up extensive test programsto determine an area of process parameters in which safe design is guaranteed. A qualitativeunderstanding of the various influencing parameters during galvanizing is necessary to explain theoccurrence of liquid metal embrittlement. This knowledge is also helpful to design an experimental test setup and procedure for evaluating the influence of one parameter where the effect of other parameters shouldremain constant. This master thesis deals with the occurrence of liquid metal embrittlement whengalvanizing welded high strength steels. This paper gives an overview of the most important processparameters and gives a short description of possible future experimental work.

Ferritic stainless steel X2CrNi12 (EN 10088) is generally appreciated for its relatively low cost and good resistance to wet abrasion and mild environments, but unfortunately its weldability is restricted. Typical applications up to now include railway wagons for coal and iron ore, mining and mineral process and transport equipment, bus frames and chassis, silos, etc. This steel grade nowadays can be fabricated cost effectively with low carbon and impurity levels appreciably improving both the weldability and mechanical properties. In this case, long-term maintenance costs of assemblies produced using this ‘clean’ X2CrNi12 stainless steel will be low, with a suitable coating providing sufficient protection for several decades. For other applications, the use of weldable X2CrNi12 is also more economical than higher alloyed stainless steels. Moreover, joining the steel by laser welding without filler metal should be considered. The present paper provides an overview of results of a research project initiated by the Belgian Welding Institute investigating the possibilities of this modified low carbon ferritic stainless steel. The main objective of this work was to demonstrate that it offers great application ranges for constructing purposes which should expand its application field substantially. In the near future broadening this steel family with even higher mechanical properties allowing further reduction of plate or wall thickness and of production costs will be explored.

Grade 91 pipes and tubes have become standard materials for the design of new and refurbishment of existing power plants. Several major failures have been encountered after fabrication of these new creep strength enhanced steels and are mainly attributable to improper practices. Another critical situation, however, is arising when it comes to repair welding. Gas backing and the required post-weld heat treatment (PWHT) are a time consuming factor in the total weld repair schedule. While PWHT is a prerequisite for obtaining the correct microstructure, power generators will tend to avoid this thermal treatment in order to reduce downtime of the power plant. In a collaborative research program, the Belgian Welding Institute and Laborelec undertook the challenge to investigate the feasibility of a cold weld repair methodology in order to weld repair T91 tubes without the use of gas backing and PWHT. The feasibility study so far demonstrated that a repair method based on the selection of an undermatched consumable and a controlled deposition procedure can result in an acceptable microstructure and hence inherent properties. The results of the research programme are very promising for temporary repair welding of T91 tubes within the containment of the boiler.