EUROGUSS 2020 Talent Award Copper Alloys - Plunger Tips for HPDC Industry
What are the effects of Nb and Zr additions on the mechanical properties of a Cu-Ni-Co-Cr-Si alloy with different heat treatments? A study shows how alloys with optimal properties were obtained and plunger tips for HPDC were successfully manufactured.
Conventional die casting is a network-shaped manufacturing process that uses a permanent metal mold that produces components that vary in weight from a few grams to almost 25 kg being one of the most productive processes. Traditionally, die casting is not used to produce large products; previous studies, however, have shown that very large products can be produced, such as a car door, frame or transmission housing. There are two basic conventional high-pressure die casting (HPDC) processes - the hot chamber process and the cold chamber process. Hot chamber machines run faster cycle times, this varies from less than one second for small components that weight some grams, to thirty seconds for castings of several kilograms. Cold chamber die casting is traditionally used for low melting temperature metals, such as lead or zinc alloys. Metals with higher melting temperatures, including aluminum alloys, cause rapid degradation of the metal injection system, significantly damaging the chamber and the piston used for the molten metal injection process.
Plunger Tips Used in The HPDC Industry
Plunger tips used in HPDC must meet certain characteristics in terms of design and mechanical properties, especially their hardness, thermal diffusion and wear resistance. Plungers used in the HPDC industry have undergone a considerable change since the 1960s, where they began manufacturing with cast iron alloys that experienced a useful life of 300 to 500 shots, and by the beginning of the 1980s, the alloy began to be implemented Copper-Beryllium (4,000 to 7,000 shots), as illustrated in figure 1. Copper is an element that can be alloyed with a variety of chemical elements to be used in various industrial applications, to provide solutions to problems of wear resistance, corrosion, and thermal conductivity. To improve the tribological properties in copper alloys and most cases, the alloys are modified with grain modifying elements and different heat treatments.
Currently, the plungers used in aluminum injection machines use a copper-beryllium head with good wear resistance. Some commercial Cu-Be alloys are C17510 and C17530, both alloys with an average hardness of 85 HRB. but beryllium it is been considered as a toxic element causing health problems to workers. Technological history in this area must be part of scientific progress, and new technologies must also be applied to help to evolve towards greater efficiency and productivity and improve production quality; taking technological advances as a reference, the basis of this research is to develop new beryllium-free copper alloys with the same or better mechanical and tribological properties to be used in HPDC.
Experiment with Different Heat Treatments
The modified alloys were melted and degassed with nitrogen at a temperature of 1300 °C in an INDUTHERM TF 4000 induction furnace and cast at a temperature of 1250 °C in a permanent H13 metal mold. Casting bar casted were cut, and these sections were given different heat treatments in an electric furnace, so the first sample was the as cast (AC), then a solution heat treatment (TS), two aging heat treatments (TA1 and TA2) were given. Time and temperatures of the different heat treatments are shown in table 1.
Optical and electronic microscopy characterization was made, and phase identification was done. Table 2 shows the composition of the two alloys studied, measured using optical emission spectrometry. The experimental process sequence for obtaining a plunger is shown in figure 2. Rockwell B hardness measurements were made with 100 kg load and a ball of 1.58 mm for 7 seconds. Each sample was tested ten times according to standard ASTM E18 and IRAM – IAS U 500 – 105.
The wear tests were conducted by a pin-on-disk tribometer according to ASTM G99, on the surface previously polished on the specimens with dry sliding conditions, tests lasted 45 min at constant velocity of 0.12 m/s and an applied load of deadweight of 30 N, the material of the pin was a H13 steel balls with a diameter of 11 mm. The mass loss was obtained by a geometric method computing the data measured in coordinate microscopy.
Positive Effects on the Microstructure and Macrostructure
The alloys studied, with the addition of modified elements and heat treatments increase the strengthening of the properties, have shown interesting results, giving positive effects on the microstructure and macrostructure, see figure 3 and 4. The benefits are reflected in obtaining optimum values of hardness, increasing between 10 % and 15 % in HRB hardness, increasing wear resistance with low rates of loss volume material, mainly in the alloy containing Nb. These results are attributed to grain refinement, dendritic coherence and new precipitated phases caused by the addition of Nb, Zr, and aging heat treatments. Nb and Zr promote a higher density of nucleation sites, columnar region is growing to the center until reaching the equiaxial regions which nucleate in the liquid in the center of the sample. The transition from columnar to equiaxed occurs in the near the wall of the H13 mold, after solution heat treatment and subsequently aging heat treatments some precipitates start to grow in the grain boundary for both alloys, see figure 4.
Crystal Size (XRD), Hardness and Wear Analysis
In general terms, for the set of results obtained by the different tests, the size of grains obtained by XRD can be correlated, applying the Debye Scherrer equation, presenting the values in figure 5, where A1 alloy experienced the best performance in terms of crystal refinement, also presented the best hardness results (reaching an increase between 10 % and 15 % compared to the commercial alloy C17530), and the lower wear rate, as shown in figure 6 and 7. Optimization has been achieved thanks to the addition of the grain modifying elements and the heat treatments applied.
Copper Alloys with Optimal
The conclusions of the research can be summarized as
- Alloys with optimal properties were obtained and plunger tips for HPDC were successfully manufactured, experiencing a good performance in a casting shop.
- The optimization of the mechanical properties was achieved satisfactorily, due to the addition of Nb, Zr, and the heat treatments applied, where the alloy with Niobium is the one that confers the best hardness and wear resistance properties.
- The mechanical properties were improved due to recrystallization, grain refinement, interdendritic coherence (Ni and Si segregated to the interdendritic regions) and the origin of new precipitated phases in the copper matrix.