Influence of B and Nb Additions and Heat Treatments on the Mechanical Properties of Cu-Ni-Co-Cr-Si Alloy for High Pressure Die Casting Applications

During the high pressure die casting process (HPDC), it is necessary to develop new designs and alloys for the copper plungers. In this research, two alloys Cu-9Ni-1Co-1.6Cr-2Si-1.3Fe-0.25B wt.% (A1) and Cu-9Ni-1Co-1.6Cr-2Si-0.1Fe-0.2Nb wt.% (A2) under different heat treatments (HT) were studied. Optical microscopy technique was applied to reveal the regions of dendritic morphology, also lower Secondary Dendrite Arm Spacing (SDAS); and different grain orientations. The results reveal that the solidification sequence is primary Cu dendrites and secondary intermetallics; heat treatments increase the redistribution of alloying elements in the interdendritic regions. During the heat treatments, some precipitates were found in the grain boundary after aging heat treatments for both alloys, which were determined by X-ray diffraction. Hardness test HRB presented a decrease with the solution heat treatment and an increase with the aging heat treatments proposed for both alloys. Finally, the wear resistances for both alloys were compared with a commercial alloy C17530, with decreased A1 with B additions having the best result in the as-cast condition 4.07 × 10−4 mm3/Nm, while for A2 with Nb additions wear resistance increased, with the best result in the one with aging heat treatment 1.69 × 10−4 mm3/Nm while for the C17530 alloy this was 2.74 × 10−4 mm3/Nm.

Low Pressure Carbonitriding of Steel Alloys with Boron and Niobium Additions

Low pressure carbonitriding (LPCN) has the potential to improve impact and fatigue strength, with gears being an example application, through the enrichment of nitrogen in addition to carburizing at higher heat treatment temperatures. In this study, the LPCN response of four different steel alloys is investigated. The influence of unprotected boron is evaluated by comparing the LPCN response of 20MnCr5 with and without boron additions. The influence of Nb microalloying is assessed by comparing the LPCN response of 8620 with and without Nb additions. Low pressure carbonitriding heat treatments were developed to achieve case depths of 0.65 to 0.75 mm in each alloy. The hardness and case microstructure are correlated to bending fatigue response measured with Brugger fatigue specimens, which are designed to simulate the root of a gear tooth.