Mikrostruktur und mechanische Eigenschaften der reibgeschweißten γ‐TiAl‐Feingusslegierung Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si)

Microstructure and mechanical properties of friction welded γ‐TiAl based alloy Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) in investment cast condition. This paper describes properties of joints produced by friction welding of the intermetallic γ‐TiAl based alloy Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) in investment cast and hot‐isostatically pressed condition. The effect of friction welding parameters on microstructure and local properties are examined and discussed. It is found that the properties of the joint are essentially affected by properties of as‐cast Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) base material, both at room temperature and 700 °C.     

Numerical investigation of droplet impact on the welding pool in gas metal arc welding

A transient three‐dimensional model that describes physical phenomena inside a welding pool during gas–metal arc welding process is presented. The model considers such phenomena as heat‐mass transfer, electromagnetics, hydrodynamic processes and deformation of the weld pool free surface. The fluid flow in the weld pool is induced due to the presence of the mechanical impact of the droplets, thermo‐capillary surface tension, thermal buoyancy and electromagnetic forces. The weld pool surface deformation is calculated by considering arc pressure and droplet impact force. A comparative analysis of the impact of the electric current of the welding arc and different force factors causing the motion of liquid metal in the weld pool on the shape of the welded seam was carried out and discussed.     

Numerical fatigue life analysis of a high frequency mechanical impact treated industrial component based on damage mechanics models

After post weld treatment with high frequency mechanical impact (HFMI) treatment of welds, a significant increase of fatigue life (up to a factor of 10) can be achieved. During the last years numerous experimental tests of welded joints with simple geometry under constant amplitude loading have been performed to quantify the positive effect of high frequency mechanical impact treatment. Due to the lack of methods for the prediction of the high frequency mechanical impact benefits, a widespread use of this process is not the case yet. Furthermore, it is still not clear if the results of these fatigue tests can be transferred to complex geometries and complex loading conditions such as in industrial applications. Therefore, an approach to assess the fatigue life of complex welded structures under variable amplitude loading was developed. For this purpose, high frequency mechanical impact treatment and fatigue load of simple welded specimen made of S690QL steel were simulated with finite element analysis (FEA) firstly. Then, the needed damage parameters for the fatigue life correlation were evaluated from the finite element post‐processing. The calculated life time to crack initiation was in good agreement with the experimental fatigue test results. In the next step, this procedure was implemented on a welded arm of an evacuator of type EW180B of the company Volvo Construction Equipment made of S700MC. The variable amplitude load measured under real service condition was transferred to single constant amplitude load cycles using a rainflow‐counting algorithm. By simulation and damage mechanics evaluation of each load cycle the total damage sum could be calculated and compared with the experimental results from Volvo Construction Equipment.     

Conventional and in situ tensile test of friction stir welded steel – Optimization of processing parameters

In the present investigation, steel plates were joined at different tool traversing speed by friction stir welding keeping other parameters same. Microstructural characterization was carried out with optical and scanning electron microscopes. At weld nugget pearlite and bainite were present within ferrite matrix. Thermo‐mechanically and heat affected zones microstructure consisted of pearlite and ferrite. Second phase area fraction and matrix grain size at different regions were varied depending on welding parameters. Weld nugget exhibited substantial improvement in microhardness with respect to base metal. In this respect heat affected zone revealed minimum microhardness and was below base metal value. Tensile tests were carried out on standard and miniature specimens in scanning electron microscope. Highest joint efficiency to the tune of ～82 % and ～120 % of that of base metal obtained for standard and miniature specimens, respectively machined from weld fabricated at lowest welding speed. With increment in welding speed assembly strength was reduced for both types of specimens. Standard specimens failed from heat affected zone and miniature specimens failed through centre of weld nugget. Apart from matrix grain size and second phase area fraction, precipitation of microalloyed carbide / carbonitride was responsible for altering the joint strength.     

Friction stir welding of thick section Al‐Zn‐Mg‐Cu aluminum alloy

Friction stir butt welding of 25 mm thick AA7075–T651 plates has been investigated. Careful process parameter selection resulted in single pass, full‐penetration defect free welds. The weld nugget exhibits a significant grain refinement while facing the dissolution of strengthening precipitates. Microhardness survey gives a W‐shaped profile with lower hardness values recorded in the thermo‐mechanically‐affected zone. Tensile fractures occur, again, in the thermo‐mechanically‐affected zone, where minimum hardness occurred. The friction stir welds demonstrate an excellent root bend performance while falling behind base material in face bend test. The welds also displayed outstanding impact toughness compared to that of parent material. It is concluded that defect free single pass friction stir welds can be successfully made on 25 mm thick AA7075–T651 plates.     

Laserstrahl‐Kurzpulsschweißen von Metallfolien

Laser Beam Short Pulse Welding of Metal Foils Joining of metallic foils by laser beam spot welding is applied in different fields of electronic production and micro joining technology. The main challenges hereby are the realization of weld spots with high quality, i. e. weld spots with high stability and smooth shape, as well as the stabilization of the process. By the application of short laser pulses usually used for drilling or cutting the time at which the melt pool exists can be reduced and therefore the probability of arising instabilities, too. Short laser pulses are pulses with a length lower than one millisecond; pulses with a pulse length lower than one nanosecond are called ultra short pulses. The attempt of welding metallic foils by short laser pulses provides also the advantage of a lower thermal influence of the material. And a more simple system technology is possible then because different manufacturing methods can be realized with one laser. The main intention of the shown results of some investigations is the qualification of the welding process. Hereby the reachable weld spot quality as well as the enlargement of the process window is of special importance.     

Beeinflussung der Hartstoffverteilung beim Plasma‐Pulver‐Auftragschweißen

Influencing the distribution of reinforcing particles in plasma transfer arc welding A study was made to examine the possibilities of modification the distribution of reinforcing particles in plasma transfer arc welding by using pulsed gas flows. It is shown, how modulated gas flows can be created. The effects of different modulated plasma gas flows were analysed by measuring the pressure distribution in the arc. By using a two powder feeding system, a cobalt based hardfacing alloy with different amounts of tungsten carbides was deposited on plates of carbon steel. It was investigated, how variable pulsed gas flows affect the distribution of the reinforcing carbides. The metallographic analysis shows a uniform distribution of the tungsten carbide in contrast with specimens, welded with constant plasma gas flow.     

Hochfrequenzschweißen von Feinblech aus mikrolegiertem Stahl mit integrierter thermomechanischer Nahtbehandlung

High frequency welding of low alloyed sheet steel with integrated thermo‐mechanical treatment Described are the seam characteristics of high frequency welded low‐alloyed sheet steel blanks. The focus of the investigations is directed to static and fatigue strength. In addition, a possibility to perform a thermo‐mechanical treatment of the welding seam will be presented. Also the effect of the thermo‐mechanical treatment on the cold formability of the welding seam is analysed.     

Blends of carbon dioxide and HFCs as working fluids for the low-temperature circuit in cascade refrigerating systems

This paper describes an analysis on the performances of a cascade refrigeration cycle operated with blends of carbon dioxide (CO₂, or R744) and hydrofluorocarbons (HFC) as the low-temperature working fluid. The aim of this work was to study the possibility of using carbon dioxide mixtures in those applications where temperatures below CO₂ triple point (216.58 K) are needed. The analysis was carried out by developing a software based on the Carnahan–Starling–De Santis (CSD) equation of state (EoS) using binary interaction parameters derived from our experimental data. The properties of the investigated blends (R744/R125, R744/R41, R744/R32, R744/R23) were used to simulate the behavior of a cascade cycle using ammonia (R717) as the high-temperature-circuit working fluid and operating at evaporating temperatures down to −70 °C. The use of a suction–liquid heat exchanger on the low-temperature side of the circuit was also investigated. Results show that the R744 blends are an attractive option for the low-temperature circuit of cascade systems operating at temperatures approaching 200 K.     

Physico‐chemical and mathematical modeling of phase interaction taking place during fusion welding processes

The quality of metallic materials depends on their composition and structure and these are determined by various physico‐chemical and technological factors. To effectively prepare materials with required composition, structure and properties, it is necessary to carry out research in two parallel directions: 1. Comprehensive analysis of thermodynamics, kinetics and mechanisms of the processes taking place at the solid‐liquid‐gaseous phase interface during welding processes. 2. Development of mathematical models of specific welding technologies. We have developed a unique method of mathematical modeling of phase interaction at high temperatures. This method allows us to build models taking into account: thermodynamic characteristics of the processes, influence of the initial composition and temperature on the equilibrium state of the reactions, kinetics of heterogeneous processes, influence of the temperature, composition, hydrodynamic and thermal factors on the velocity of the chemical and diffusion processes. The model can be implemented in optimization of various technological processes in welding, surfacing, casting as well as in manufacturing of steels and non‐ferrous alloys, materials refining, alloying with special additives, removing of non‐metallic inclusions.