Aging of maraging steel welds during aluminium alloy die casting

The aim of this study is to evaluate precipitation annealing of 18% Ni maraging steel repair welds during aluminium die casting and to predict the prolonged in-service tool life. The emphasis of this study is 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 is prepared to which 1.6356 maraging steel is GTA weld cladded. Analysis of weld microstructure and hardness is made in order to understand the metallurgical processes during heat treatment at elevated temperatures and at prolonged time. The Response Surface (RS) model for prediction of hardness after heat treatment is developed. To the edges of immersion test specimens 1.6356 maraging steel is GTA welded. The edges are machined to the final edge geometry. Two specimens are tested in as-welded condition and two in optimally aged condition. Testing is preformed on developed immersion test apparatus, which enables the simulation of thermal fatigue during aluminium die casting. After completion of a particular number of thermal fatigue cycles the weld microstructure and hardness is evaluated. Based on these results a hypothetical model for prediction of precipitation annealing time during aluminium die casting is proposed. The results showed that tool heating takes place at 25% of molten metal injection time, preparing conditions for precipitation annealing at 15% of injection time, and precipitation annealing at 60% of injection time. These results, together with a finite element (FE) model for prediction of tool temperature and RS model for prediction of hardness after precipitation annealing, enable accurate prediction of maraging steel tool hardness after particular number of die casting cycles and consequently in-service tool life. (C) 2008 Elsevier B.V. All rights reserved.