Thermal stresses in aluminium alloy die casting dies Damjan Klobcˇar * , Janez Tušek University of Ljubljana, Faculty of Mechanical Engineering, Laboratory for Welding, Aškercˇeva 6, 1000 Ljubljana, Slovenia article info Article history: Received 5 November 2007 Accepted 7 March 2008 Available online 22 April 2008 PACS: 02.70.Dh 06.60.Vz 07.20.n 44.20.+b 44.25.+f 47.27.Te Keywords: High-pressure die casting Hot-work tool steel Maraging steel Immersion test Welding Finite element modeling Thermal fatigue cracking abstract The aim of this research is to analyze the inﬂuence of Aluminium Alloy die casting parameters, die mate- rial, and die geometry on in-service tool life. An innovative immersion testing apparatus is developed, at which Aluminium Alloy die casting is simulated. It enables controlled thermal fatigue cycling. Special specimens with different edge geometry and specimens with maraging steel welds deposited by Gas Tungsten Arc (GTA) welding are prepared. They are subjected to cyclic heating in bath of molten Alumin- ium Alloy 226 and cooling in bath of water-based lubricant. The specimens are continuously internally cooled with cold water. The microstructure, hardness proﬁle, and the surface cracks developed are peri- odically analyzed after completion of a particular number of cycles. Temperature transients at different locations of the specimens are measured and used in calibration of ﬁnite element model (FEM). The com- putation of transient stresses is performed by developed FEM. The inﬂuence of immersion test parame- ters, material, specimen edge geometry, and thickness of maraging steel surfacing welds on thermal stresses is studied. To improve thermal fatigue testing efﬁciency, a specimen of particular geometry and immersion test parameters are developed based on ﬁnite element analysis. The results showed sig- niﬁcant differences in produced thermal stresses for analyzed materials, test parameters, and edge geom- etries. Maraging steel is found to be superior material for die casting dies, due to generation of lower stresses. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction In high-pressure Aluminium Alloy die casting a production of more than 100 000 castings per die is a common series. These cast- ing are geometrically complex parts, produced in tight dimensional tolerances, and with high surface quality. The production cost of castings is highly depended on the tool life, which is affected by the tool design, material selection, its heat treatment, and casting process parameters. During aluminium die casting, molten alumin- ium at temperatures 670–710 °C is injected into the mold at veloc- ities of 30–100 m/s. The injection pressures are between 50 and 80 MPa [1]. The in-service tool life is shorter due to: (a) thermal fa- tigue, which causes heat checks on the surface of the die; (b) cor- rosion and soldering due to aluminium oxidation to the die surface; (c) erosion due to melt ﬂow; (d) catastrophic failures due to thermal shocks; and (e) heating of die material, which causes instability of mechanical properties [2–5]. Die casting die fail when the stress becomes larger than the strength of the steel [5]. These stresses are: (a) residual stresses, produced during tool manufacturing due to machining, EDM, grinding, non-optimal heat treatment, and die design failures; and (b) stresses produced during die casting, i.e. thermo-mechan- ical stresses. To enable longer in-service life, these stresses should be kept as low as possible. Residual stresses could be minimal, if die is designed optimally and if machining and heat treatments are done correctly. During the operation of the die, the one must prevent higher thermal shocks, local material overheating, and un- even pressures, which could lead to catastrophic die failures. Die casting dies are constantly heated and cooled down during the operation cycle. This generates thermal gradients and conse- quently thermal stresses, which cyclically changes its magnitude and direction, and produces thermal fatigue of die material. Surface cracks are thus produced, which propagates with increased num- ber of cycles, and produces unacceptable marks (burr) on the cast- ings surfaces. Such die must be replaced or the cracks must be repaired by welding. The surface cracking highly depends on ther- mal fatigue resistance of the die material, which depends on mate- rial and its heat treatment, and die casting parameters. Thermal fatigue resistance decreases with higher operating temperature and at die casting parameters, which produces severe thermal gra- dients. Thermal fatigue resistance also decreases in materials with 0927-0256/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2008.03.009 * Corresponding author. Tel.: +386 1 4771 205. E-mail address: damjan.klobcar@fs.uni-lj.si (D. Klobcˇar). Computational Materials Science 43 (2008) 1147–1154 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci