Damjan Klobčar

A new approach to monitoring thermal fatigue cracks in die casting moulds

Abstract Die casting moulds are exposed to high cyclic temperatures, mechanical loads and severe chemical conditions. Thermal and mechanical loads cause high local stresses and consequently surface cracks. This paper introduces a new approach to establishing thermal fatigue cracks in die casting moulds by measuring the resulting defect-fins on aluminium alloy castings in the actual die casting process. The investigation showed that cracks occurred sooner and were bigger if the mould material had a lower hardness. The maximum depth of the defect-fin observed on the casting due to thermal fatigue crack on the mould with the lowest hardness (42 HRc) at 10000 cycles was 0.37 mm, the maximum width of the defect-fin was 0.76 mm and maximum length was 9.6 mm. The observed cracks were bigger closer to the entrance of melt flow, especially at the places with stress concentrators, due to higher melt temperature and higher melt flow.

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.

Superplastic Behaviour of AA5083 Aluminium Alloy with Scandium and Zirconium

The aim of the present investigation was to determine and to compare the superplastic behaviour of the AA5083 (Al-Mg-Mn) alloy with Sc and Zr additions. The investigated alloys were processed to form sheets by conventional hot and cold rolling. The superplastic properties were determined with strain rates in the range of 1x10-4 to 5x10-2s-1 and forming temperatures of 350 to 550°C. The results showed that the alloy with about 0.4% Sc exhibited a high superplastic ductility across a wide temperature range and strain rates up to 1x10-2s-1. The highest elongations to failure of about 2000% were attained at 550°C and at an initial strain rate of 5x10-3s-1. However, the alloy with about 0.15% Zr exhibited elongations up to 600%. The FSP processed Al-4.5Mg alloy with combined addition of about 0.2% Sc and 0.15% Zr exhibited good superplastic properties at higher strain rates (> 1x10-2s-1) with elongations up to 1500%.

The superplasticity of friction stir processed Al-5Mg alloy with additions of scandium and zirconium

Abstract The paper describes the effect of minor additions of scandium and zirconium on the superplastic behaviour of friction stir processed Al-5Mg based alloy. The measurements included the flow curves and tensile elongations of (in wt.%) Al-5Mg-0.1Zr, Al-5Mg-0.2Sc, and Al-5Mg-0.2Sc-0.15Zr alloys at initial strain rates ranging from 1 × 10−3 to 1 × 10−1 s−1, and at forming temperatures from 350 to 500°C. The inclusion of friction stir processing at tool rotation rates of 95 and 475 rpm considerably enhanced the superplastic behaviour of the Al-5Mg-0.2Sc-0.15Zr alloy, which was reflected in elongations without failure of over 1900%. Other friction stir processed alloys, treated at lower tool rotation rates, did not achieve superplasticity due to abnormal grain growth. The results yielded by the friction stir processed alloys are compared with the superplastic behaviour of the same alloys produced conventionally by cold rolling.

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.

New findings in welding of structural steels

Gas shielded arc welding is the most widely applied welding process in industry. H2 (1.0425) structural steel chosen can be welded very cost-effectively with VAC 60 welding wires in CO2 shielding gas. With only the replacement of a highly oxidising gas, i.e. CO2, with a less oxidising gas mixture, i.e. Ar+18% CO2, a nicer appearance of the weld face and a higher weld quality may be obtained. Still higher quality of welds may be accomplished by employing pulsed arc welding of structural steels. In the study and development of the existing welding process, special attention was paid to the metal transfer. In pulsed arc welding with VAC 60 wire in the protective gas mixture of Ar+18% CO2, the metal transfer is very smooth and uniform in a very wide range of welding parameters. Because of the low oxidising capability of the Ar+18% CO2 gas mixture and the very short time of droplet formation, however, in pulsed arc welding major chemical processes in the droplet will occur only in welding with a higher average welding current (281 A). Less alloyed surfacing welds with silicon and manganese will provide higher quality only because of the surfacing weld dilution resulting from the parent-metal fusion, i.e. penetration. In pulsed arc welding, a pulse shape and energy and base current may efficiently affect the degree of penetration.