Fracture Analysis of a high pressure die casting die Teksid Aluminum s.r.l. =

After a brief presentation of the production plant, the high pressure die casting process is described in a more detailed way. Deeply investigating into the details of the process the movement of the die are explained, especially the strokes of the radial carriers, to let the reader understand what are the function of the broken piece, analyzed in this thesis, inside the cycle. The first approach to the fracture analysis consists into a visual inspection, immediately comes out that there is a localized deterioration of the surface of the fillet, concentrated on the right side. From this damaged area a specimen was obtained and inspected in a more accurate way at SEM (Scanning Electron Microscopy) to investigate the morphology of the fillet damaged surface and of the fracture surface. Analyzing the fracture surface helps to determine the kind of breaking mechanism happened, presumably fragile fracture but with some kind of progressive behavior. No clear marks of fatigue are present. From the broken scrap five tensile specimens were machined and used to perform a tensile test to better characterize the material and the mechanical properties of the radial carrier. Data obtained are consistent with values reported by steel producers. Once studied the broken part, two methods are used to define the load acting on it during standard operations. The first indirect method was based on understanding the deformation of the piston moving the carrier by using a strain gage. The first approach failed due to noise affections and low deformability of the piston rod. The second more direct way of understanding the loading cycle was to use a manometer mounted on the hydraulic piston circuit to understand the pressure values variation during the cycle. Material, loads and geometry, after some simplifications are used to build a virtual model to be numerically tested on Ansys Mechanical. With a FEM method stress, strain, life and safety factor are evaluated confirming the localization of stress on the right side of the fillet, the same damaged area on the real piece. At this point, some design improvements are again tested. The focus of design improvements was on reducing the notch effect by enlarging the fillet radius and on trying to spread the stress among all the fillet surface in a more homogeneous way. Design modifications returned good results, demonstrating their effectiveness. In conclusion the root causes of this fracture were the notch effect due to the small radius of the fillet and the asymmetries of the external loads and of the resistant material, that have contributed on concentrating the stress in a localized area of the fillet.