Reducing process lead time and design time are important aspect to reduce total cost of die casting process. Minimizing the trial-and-error stage of the production can further assist in reducing the cost. The shape of the cavity and die geometry is directly related to soldering tendency of a die casting. The more complicated geometry a die has, the more likely soldering can occur. The dies of complicated geometry usually procure sharp angles, core pins, nooks, and the part that acts as hot spots. These hot spots can induce soldering, due to their higher temperatures than other areas [
1]. Moreover, prolonging die service life and preventing catastrophic die failures is essential because the die cost contributes considerably to the overall process cost. Die-casting dies are subjected to high mechanical and thermal loads, which can cause appalling or delayed die failure due to mount up damages. Most failures developed gradually and can be predicted. However, ongoing failures after few cycles can root for a larger economical losses due to wrecked tools, expensive down times, and disordered delivery schedules [
2].
Thermal fatigue is one of the most common complications encountered in a die casting process. It is a result of the cyclic, rapid, non-uniform heating and cooling of dies. Thermal fatigue gets amplified by mechanical loadings and other damage mechanisms, such as erosion and corrosion. Thermal fatigue is one of the major foundations of poor-quality castings and die failures. The initial phase of the thermal fatigue initiates with the formation of micro-crack networks denoted as heat checking. Heat checking results in the deterioration of die cavity surfaces. Further damage is due to heat checking leads to clasping, defective castings, and subsequently die damage and failure [
3,
4,
5,
6,
7,
8]. When an unconstrained object is slowly and uniformly heated or cooled, it will expand or contract proportionally to its coefficient of thermal expansion, and a temperature change will occur in relation to the mentioned temperature. However, if the body is constrained during the heating and cooling process, it will develop stresses and strains because it cannot reach its unconstrained dimensions properly. Similarly, if a body does not load and release through the same stress–strain path, repeated or cyclic heating and cooling might outcome in the build-up of inelastic deformation. If the time-varying stress within the body is tensile, accumulation may lead to a thermal fatigue. Even an unconstrained body may develop large thermal stresses if exposed to non-uniform or rapid heating and cooling.
Objects with sudden geometric and compositional variations retain internal constraints which are caused by temperature distribution. In a case of rapid surface temperature changes in a solid body, the temperature of a nearby surface layer increases and decreases promptly, whereas the rest of the body cannot react fast enough to the alterations in temperature. Different expansions and contractions within the body provide internal constraints, and various layers expand and contract at different degrees. Each region gets constrained by its neighbouring region, which results in a stress-strain field within the body [
7,
8,
9]. Thermally induced stresses are not the only factors involved in the thermal fatigue phenomenon. Die material properties at elevated temperatures, mechanical loads from filling and locking, residual stresses, and cavity surface conditions also affect a material’s response to heat checking and a thermal fatigue. Tempering, decarburization, and phase or structure changes due to an exposure to elevated temperatures accelerate thermal fatigue.
To predict and avoid die failures and their consequences, thermal management of dies, the interactions between thermal and stress/strain fields, material properties, and the influence of all these factors on die service life must be understood. By investigating the effects of these factors on die life during the design stage of dies and the casting process, prolonged die service life can be succeeded. A reliable die service life prediction allows for accurate estimates of actual die and production cost, reduces undesired machine down times, and helps to achieve good production management during production processes [
2,
9,
10,
11]. Furthermore, thermo-mechanical fatigue (TMF) experiments on die casting require costly equipment, time consuming, and often conducted through thermal testing under the same operating conditions as that for die-casting dies; however, in fact, the temperature and number of cycles are fixed and kept constant [
9,
10,
11,
12]. The damage on the die obtained by TMF experiments is less than that obtained in isothermal fatigue (IF) experiments [
13]. The purpose of this study is to fit a model of the die life by using an experimental data. Experimental procedures were designed to mimic the commercial die casting process. The thermal fatigue process was conducted in a cyclic manner with constant sample temperature during the test [
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15,
16]. Analysis of dies was performed by associating several of the previously mentioned factors that influence the temperature profile and the thermal gradients with the structural state, within the tool. The step was implemented by employing the temperature profiles obtained through a previous 1D thermal analysis and imposing them on the die cavity/casting interface via 2D thermal finite element models (FEMs), respectively. Thermal finite element analysis was conducted using ABAQUS software (Version 6.13.1, ABAQUS, Johnston, RI, USA, 2013), and this analysis was followed by a sequentially coupled structural analysis. The cooling effects of the lubricants were studied through experimental and numerical investigations. The effects of initial die cavity surface temperature and spray fluid density (spray volume per-unit surface area per-unit time as defined by Lee), have been a focus of several previous researchers [
3,
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20].
In this study the effects on service life were investigated through response surface methodology (RSM), employing a three-factor Box-Behnken design. The thermal fatigue cycle parameters were evaluated with three levels for each factor. An experimental investigation was conducted to quantify the relationship between the input factors (as-machined surface roughness, Ra1, wall thickness, and immersion time) and responses (crack length, surface roughness due to thermal fatigue, Ra2, and hardness properties). Moreover, by combining existing and new data on heat transfer in die-casting dies and die material properties with RSM modelling, a method for predicting the onset of heat checking in H13 steel dies has been proposed.