J. Tušek

Improved modelling of a parallel plate active magnetic regenerator

Much of the active magnetic regenerator (AMR) modelling presented in the literature considers only the solid and fluid domains of the regenerator and ignores other physical effects that have been shown to be important, such as demagnetizing fields in the regenerator, parasitic heat losses and fluid flow maldistribution in the regenerator. This paper studies the effects of these loss mechanisms and compares theoretical results with experimental results obtained on an experimental AMR device. Three parallel plate regenerators were tested, each having different demagnetizing field characteristics and fluid flow maldistributions. It was shown that when these loss mechanisms are ignored, the model significantly over predicts experimental results. Including the loss mechanisms can significantly change the model predictions, depending on the operating conditions and construction of the regenerator. The model is compared with experimental results for a range of fluid flow rates and cooling loads.

Laser Cladding of Cold-Work Tool Steel by Pulse Shaping

Repair welding of cold-work tool steels in cold is very risky and almost impossible by conventional processes. The application of pulse shaping in laser cladding with wire to avoid the solidification problems in relevant steel is demonstrated. The results show that sound remelting and/or cladding can be achieved by the right selection of laser parameters and pulse shape, i.e. long pulse duration, moderate pulse peak powers and ramped-down pulse shape. Despite the defects and softening in the cladding due to the formation of retained austenite, the cladding shows better wear resistance at lower loads compared to the heat-treated base material.

A mathematical model for the melting rate in welding with a multiple-wire electrode

The paper deals with the development of a mathematical model for the calculation of melting rates obtained in gas-shielded arc welding with a multiple-wire electrode or in submerged arc welding with a multiple-wire electrode. The first part provides a very general and short description of welding with a multiple-wire electrode, the main advantages and characteristics, as well as variants of welding with a multiple-wire electrode applicable in practice to various cases. The major part of the paper treats the development of the mathematical model for the calculation of the melting rate on the basis of the physical principles of the welding arc and of the wire extension heating due to current conduction and mutual influence of the welding arcs. Finally, a comparison is made between the melting rate results obtained by practical measurements and those obtained theoretically by the mathematical model. The mathematical model for the calculation of melting rates in welding with a multiple-wire electrode is accurate enough to be used in practice and for further studies.

Suitability of maraging steel weld cladding for repair of die casting tooling Part II : influence of ageing during aluminium alloy die casting on maraging steel weld microstructure, mechanical properties and crack growth

Abstract This study was done to evaluate precipitation annealing of 18 % Ni maraging steel repair welds during aluminium alloy die casting and to predict the prolonged in-service tool life. The emphasis of this study was the influence of post-weld precipitation annealing heat treatment and aluminium die casting thermal cycling on metallurgical and mechanical properties. A series of specimens of 1.2344 tool steel was prepared to which 1.6356 maraging steel was gas tungsten arc weld clad. Analysis of weld microstructure and hardness was made in order to understand the metallurgical processes during heat treatment at elevated temperatures and at prolonged time. The Response Surface model for prediction of hardness after heat treatment was developed. The edges of immersion test specimens are gas tungsten arc weld clad with 1.6356 maraging steel and machined to the final edge geometry. Two specimens were tested in as-welded condition and two in optimally aged condition. Testing was performed on specially developed immersion test apparatus, which enables the simulation of thermal fatigue during aluminium alloy die casting. After completion of a particular number of thermal fatigue cycles the weld microstructure and hardness were evaluated. A hypothetical model for prediction of precipitation annealing time during aluminium alloy die casting was proposed. The results showed that tool heating lasts 30 % of molten metal injection time, preparing conditions for precipitation annealing at 10 % of injection time, and precipitation annealing at 60 % of injection time. These results, together with a finite element model for prediction of tool temperature and response surface model for prediction of hardness after precipitation annealing, enable accurate prediction of maraging steel tool hardness after any particular number of die casting cycles and consequently in-service tool life. The thermal fatigue resistance of maraging steel welds was compared to AISI H13 tool steel. The results showed superior thermal fatique resistance for AISI H13.

Impact toughness of welds deposited on H13 hot work tool steel

Abstract This paper presents the experimental results of the impact toughness and hardness of welds deposited on H13 hot work tool steel. To obtain the optimum combination of high hardness and high toughness, multipass gas tungsten arc welding was used to melt four different commercially available filler materials suitable for repair welding of HWTS. Special geometry of the welding sample is presented in this work. The focus is also on the technique of the preparation of the V-shaped notch in order to precisely evaluate the impact toughness. Two different preheating temperatures and two different post weld heat treatments were applied to the samples in order to obtain the effect of heat treatment on the impact toughness and hardness. Data show that weld deposited with filler material C on the base of the hot work tool steel and alloyed with vanadium exhibits high impact toughness in combination with high hardness in the as-welded condition and also after tempering.

Tungsten heavy alloy as a filler metal for repair welding of dies for high pressure die casting

Abstract This paper presents the preliminary test of tungsten heavy alloy as a filler metal for repair welding of high pressure die casting dies. H13 grade hot work tool steel was used as a base material. Multipass gas tungsten arc welding was used to fill a single-V butt joint with tungsten heavy alloy in the form of a welding rod. The microstructure of the weld was examined using scanning electron microscopy and optical microscopy. It was found that tungsten grains are not dissolved completely during welding and areas of fine eutectic were formed during solidification of the weld. The hardness of the weld was measured in three courses across the weld and increased in the heat affected zone while the weld hardness was below 380 HV0.3. Charpy V-notched samples were prepared from the weld. The high content of tungsten grains distributed in the nickel–iron weld matrix resulted in low impact toughness of 5.1 J. Face bend testing of the weld was conducted in order to study weld ductility.

The influence of laser pulse shape in repair welding of tool steels

The majority of tool steels are commonly considered as non-weldable because of their high carbon and high alloy elements content. Repair welding of such steels with conventional methods is very difficult due to cracking during remelting or cladding and is generally performed with preheating. Besides its common benefits, repair welding with laser technology also offers a possibility of tool repair without preheating. This paper presents the influence of different pulse shapes on welding of high carbon, high chromium tool steel with the pulsed Nd:YAG laser. Repair welding tests were carried out on AISI D2 tool steel which is commonly used for deep drawing, blanking, forming and thread rolling dies; shear and granulator blades and high wear resistant and intricate moulds for plastic injection. The steel specimens were quenched and tempered to hardness of 56 HRc. Afterwards, microstructural analysis, micro-hardness analysis and investigation of defects with scanning electron microscopy were carried out. The test results suggest that it is possible to obtain sound welds without preheating, with the right selection of welding parameters and appropriate pulse shape.The majority of tool steels are commonly considered as non-weldable because of their high carbon and high alloy elements content. Repair welding of such steels with conventional methods is very difficult due to cracking during remelting or cladding and is generally performed with preheating. Besides its common benefits, repair welding with laser technology also offers a possibility of tool repair without preheating. This paper presents the influence of different pulse shapes on welding of high carbon, high chromium tool steel with the pulsed Nd:YAG laser. Repair welding tests were carried out on AISI D2 tool steel which is commonly used for deep drawing, blanking, forming and thread rolling dies; shear and granulator blades and high wear resistant and intricate moulds for plastic injection. The steel specimens were quenched and tempered to hardness of 56 HRc. Afterwards, microstructural analysis, micro-hardness analysis and investigation of defects with scanning electron microscopy were carried out. The t...

Laser Grooving and Welding of Cracks Occuring at Dies for Die Casting

The paper treats the application of laser to repair of cracks occurring at dies for die casting of non-ferrous metals (particularly aluminium, magnesium and their alloys). The first part describes a suitable laser unit enabling crack grooving and then welding. An Nd:YAG laser source is shown with its equipment for laser-beam transfer, control and directing of laser-beam focus. Dies for die casting are made of quality steels and are of very complex shape. As far as their repair is concerned this means that they are to be welded at their edges, corners, narrow gaps and vertical walls, i.e. in various positions and in various directions. In the second part the grooving technology is described, and in the third part laser welding of grooved cracks using a filler material, i.e., a thin welding wire. At the end some conclusions are drawn. It is stated that from the viewpoints of technology and economics, it is sensible to laser groove and then weld the thermal cracks with a suitable material. The filler material should have such a chemical composition that after welding a weld having adequate mechanical properties, without any additional heat treatment, is obtained.