Case Studies

Producing large ductile iron axle housings can be a challenging task. The part shown above was well into production when a shrinkage defect was noticed during machining. After several attempts to find a solution, the foundry contacted MAGMA to evaluate the part design.

The initial simulations identified areas within the casting which were prone to defects, correlating to the real samples. Reviewing the results, the root cause was established - feeding paths were closing too soon.

After redesigning the risers to solve this problem, the pouring weight had been reduced by 13 kg, in turn decreasing the filling time by 2,5 s. The riser contact area was reduced by 25 % for easier removal and lower cleaning costs. The re-design also reduced the total solidification time by 11 minutes, which allowed a significant production rate increase.

The foundry approached MAGMA simply looking for a solution to the casting defect and came away with much more than they anticipated.

An HPDC client in India who is a large manufacturer of motorcycles and scooters for both domestic and export markets, experienced porosity problems with a cast component from one of their suppliers after machining the casting on the top side.

Simulation indicated that a vertical rib directly under this feature was responsible for allowing metal to very quickly fill through the vertical ribs and swirl around the circumference of the shaft before entrapping air in the thick wall sectional.

To solve the problem, the middle ingate was modified by placing an “island” in front of it to divert the melt flow away. Further simulations showed that this island did divert the flow, but that wasn’t enough. Only changing the design itself in this area led to a reduction in metal speeds flowing along the vertical wall.

The communication between foundry and designer, and the changes agreed upon by both sides, completely solved the problem.

Cast iron engine blocks can be found in more than half of all passenger cars. Their performance and weight are, among other criteria, dependent on their geometry and the casting process, especially defects like porosity and residual stresses.

Only by using manufacturing process simulation, is it possible to determine the distribution of residual stresses in castings and to consider these properties in the design optimization process. Residual stresses develop during solidification and cooling of the casting due to uneven cooling rates in the different wall thicknesses.

Tensile stresses, those stresses that are induced by “pulling”, may be critical during the manufacturing process, if their value exceeds the material’s tensile strength. By reaching this value, cracking occurs and the part fails. As a rule, tensile residual stresses are lower than this threshold value and the casting does not fail during the casting process.

Nevertheless, even if tensile stresses are below the critical value, the casting may fail during assembly, or even worse during driving.

Compressive stresses are the opposite of tensile stresses and are those stresses that are induced by pushing. It is unusual that compressive stresses are critical. One can try to use compressive stresses as a pre-loading, like in pre-tensioned concrete.

The biggest influences on residual stresses in castings are the design (80%) and the casting process itself (20%). To avoid problems, a manufacturing process simulation can be performed at an early stage of the design process, where geometric or casting process modifications may be made to decrease the level of tensile stresses. This approach leads to an optimized design and casting process of the engine block.

Before start of production, many real castings are required for testing under realistic conditions. As with many of these prototype castings, the aluminum steering knuckle shown here was produced as a sand casting – a suitable process for low volume castings. The finite element analysis and durability calculations for a cyclic loading of the component point out the most critical locations. The failure of the prototype casting in a test run would lead to design changes: heavier cross-sections to better withstand the load.

However, the comparison of two virtual castings – the sand cast prototype and the low pressure die cast series casting – clearly shows different microstructures. Particularly the durability relevant dendrite arm spacing in the critical casting sections differ by 100 % for the two castings!

Knowing this, the designer can exploit the higher durability of the die cast series casting, even if the “real” prototypes would mislead him into moving in the wrong direction.

The original gating system of a front fork component led to problems that were analyzed using MAGMASOFT   ®. The original design of the gating system of the front fork led to several problems that were detected by simulation. The top ingate was filled too quickly, and this caused casting defects due to turbulence and a large amount of entrapped air. The filling pattern also led to a hotspot and a shrinkage defect in the bottom of the casting. In addition, the yield was only about 49 %.

Several types of gating systems were simulated to analyze and solve these problems. The improved gating system shows little turbulence during filling, the hotspot and shrinkage problem in the bottom of the casting are also eliminated. Through the new leaner runner bar, the yield is improved by around 18.5%. The higher yield means a faster filling time, and thinner walls lead to a shorter solidification time. Both factors taken together mean a reduction in cycle time about 20 s. As an added benefit, the yield and tensile strengths of the components were tested and through these changes the mechanical properties were increased by around 10 %.

The surface of a die casting suffered from peeling or had two-tone discolorations. Sometimes a second buffing was required to eliminate these surface problems.

Simulation indicated that the die was being overheated to more than 450° C in several locations. This led to soldering of the part to the die, which was the reason for the surface defects. Based on these results different measures were taken. In particular the designers were encouraged to revise their design by increasing the wall thickness in the critical location by 0.5 - 0.65 mm.

Using knowledge about thermally induced casting stresses in design and manufacturing is gaining acceptance in industry. These stresses are the root cause of many quality issues in castings, such as distortion, cracking, or reduced lifetime during operation.

Casting stresses are changed significantly during the manufacturing process. As-cast residual stresses may be uncritical, but trimming or raw part machining causes a redistribution of residual stresses and may already lead to stress concentrations which result in cracks.

Subsequent heat treatment reduces the stresses but may lead to further time and temperature dependent casting distortion. Due to high cooling rates during quenching, substantial stresses develop, which are only partially reduced by a subsequent tempering process and final machining.

Casting process and stress simulation in MAGMASOFT   ® is capable of addressing each of these process steps and predicts the impact of the process and the component’s design on the stress and strain levels. The finished part stresses can be exported to be used as an input for performance simulations.