Cold flow defects in zinc die casting: prevention criteria using simulation and experimental investigations

Abstract

High-pressure die casting of zinc alloys is increasingly used in the manufacturing of parts with high aesthetic value. These parts must comply with strict requirements on surface quality, which are generally overlooked in traditional mechanical applications. Cold flow defects, which are a primary concern for surface quality, originate from several different causes that have not yet been fully understood. This report investigates the factors that influence cold flow defects and the choices that can lead to an improvement in surface quality. The research method is based on a case study performed at a die casting company. First, an existing process has been analyzed using simulation to explain the causes of cold flow defects observed in production samples. The temperature at the end of the cavity fill has emerged as a key index for the occurrence of defects, which can be controlled by three primary process parameters: injection velocity, temperature of the cooling medium, and lubricant spraying time. These same factors are then assessed using experimental tests on an existing die, where the number of defects in the selected regions of the casting has been evaluated by image processing. The results suggest that the surface quality can be particularly improved by increasing the flow rate of the molten metal through the gates and avoiding excessive flow turbulence in the wide cavity sections. Consequently, the increase in the gate area has been identified as a specific criterion for the die design. These findings have been validated in the redesign of the die and the selection of the process parameters, which have resulted in a significant reduction in the surface defects.

Direct Observation of Filling Process and Porosity Prediction in High Pressure Die Casting

“…The fast shot velocity of 0.88 m/s is relatively low, so the temperature and velocity of aluminum melt is very low where two streams of liquid join. Moreover, the collisions between two streams cause turbulence, and turbulence can promote the formation of droplets, which tend to freeze rapidly [17]. Therefore, the back flow and incoming flow are not closely combined, and the L2 sample is filled with a fairly sparse liquid metal (shown in Figure 7c), which creates a large number of pores.…”

Exploring Casting Defects of AA7075 Alloy in the Gravity Die Casting Simulation of an IC Engine Block

“…This phenomenon has led to an increase in the amounts of cold shuts in the casting. 27 Supporting this hypothesis, the cold shuts mainly occurred in the complex joints of the casting body in the conducted simulations.…”

Unified structural optimization method using topology optimization and genetic algorithms

“…The evaluation process is based on a typical framework of Design for Manufacturing(DFM) by Boothroyd (Boothroyd et al, 2001) that is taken into account in the product design phase. DFM offers a quantitative criterion for the manufac turability evaluation of products and vouches for cost efficiency of the production, and has been successfully applied in product design (Joshi and Ravi, 2010), (Armillotta et al, 2016), (Selvaraj et al, 2009), (Gupta and Nau, 1995). However, the manufacturing evaluation approaches in DFM, based on intuition of the engineers and lumped parameters, cannot be directly applied to the topology optimization process where the structural configuration is implicitly defined by the topo logical variables.…”