Investigation the Effects of Process Parameters on Product Quality in Cold Chamber High Pressure Die Casting of Magnesium Alloys

Transcription

1 Investigation the Effects of Process Parameters on Product Quality in Cold Chamber High Pressure Die Casting of Magnesium Alloys Ali Serdar Vanli, Yildiz Technical University, Turkey Anil Akdogan, Yildiz Technical University, Turkey Huseyin Sonmez, Yildiz Technical University, Turkey Abstract: Magnesium alloys can be used for lots of sectors with its suitable characteristics especially preferred for automotive and aerospace industry because of its light weight properties. This material is slight just like the polymers but also exhibits a unique strength feature as the other metals. High pressure die casting of magnesium alloys has been the fastest grown up and the most globally developed section of the magnesium industry. Besides, in die casting industry the cold chamber process has been favored for its unique ability to transform the injected molten magnesium into an accurately dimensioned form in the shortest possible cycle time. Although high pressure die casting is a very precision process; there are lots of factors which are effective on the product quality and mechanical properties. The process parameters; casting and die temperatures, filling time, plunger and gate velocities, injection and intensification pressures, spearhead of these factors. This paper investigates the present works about the effects of process parameters on the product quality in literature. For obtain qualified casting products which have good mechanical properties and low porosity contents; the casting process should be made under high injection and intensification pressures, with low casting and high die temperatures and with minimum flow velocities. Keywords: Magnesium alloys, cold chamber die casting, process parameters, product quality. 1. Introduction Magnesium is an ultra-light structural metal, its density (1.73 gm/cm 3 ) is two-thirds that of aluminum. As a result, during the recent years, cast magnesium alloys have received considerable attention from the automotive industry for the components such as instrument panels, steering wheels, seat frames, doorframes and power train components due to potential of fuel efficiencies and lower emission levels. Magnesium alloy castings are also used in the microelectronics and telecommunications industries for disk drives, DVD chassis, and cell phones, etc. (Department of Trade and Industry 2004; Lee, Gokhale, Patel, and Evans, 2006). High pressure die casting of magnesium is a proven manufacturing process by which products are being produced in volume, from tiny electronic parts to truck transmission cases. High pressure casting is a repetitive process where identical parts are cast at high production rates by injecting molten metal under pressure into a steel die. A die casting die or mould is a closed vessel into which molten metal is injected under high pressure and temperature and then rapidly cooled until the solidified part is sufficiently rigid to permit ejection from the mould (Brungs, 1997; Friedrich and Mordike, 2006). High-pressure die casting, has the unique ability to transform the injected molten magnesium into an accurately dimensioned and smoothly finished form in the shortest possible cycle time. With similarities to the plastic injection molding process, die casting can make parts to the final shape, often with no additional machining or other operations required. Unlike plastics,

2 however, magnesium die castings can generally be produced to tighter and dimensionally stable specifications. A designer can plan for the execution of a product form as a net-shape die casting with far greater freedom and flexibility than with any other metal forming process (ASM Specialty Handbook, 1999). In cold chamber type of high pressure die casting machine (Fig. 1) the molten metal is transferred into the cold chamber cylinder through a port or pouring slot. A hydraulically operated plunger, advancing forward, seals the port forcing metal into the locked die at high pressures. Injection pressures range from 30 to over 100 MPa. In the cold chamber machine, more molten metal is poured into the chamber than is needed to fill the die cavity. This helps sustain sufficient pressure to pack the cavity solidly with casting alloy. Excess metal is ejected along with the casting and is part of the complete shot (Vinarcik, 2003; Friedrich and Mordike, 2006). Figure 1. Cold chamber high pressure die casting machine. The size of the required high pressure die casting machine is mainly determined by the projected area A proj of the casting including runner, biscuit and gating system and the maximum pressure onto the melt p sol.the necessary locking force F lock can be calculated by Eqs. (1) and (2) (Fig. 2). Machine and process characteristics, typical for the cold-chamber die casting process for magnesium are shown in Table 1 (Friedrich and Mordike, 2006). F e = A proj p sol (1) F lock = F e (2)

3 Figure 2. Calculation of the necessary locking force of the high pressure die casting machine. Table 1. Process characteristics of the cold chamber die casting process. Holding furnace Separated from pressure die casting machine Locking force 1 MN 45 MN Plate dimensions max mm 2 Projected area of the entire magnesium casting max. 1.0 m 2 Pressure during solidification max. 120 MPa Casting weight 50 g 40 kg Wall thickness mm 2. Effects of Process Parameters High-pressure die-casting is the preferred manufacturing process for the Mg-alloy components used for automotive as well as for numerous other applications. The highpressure die-casting process often leads to formation of significant amount of defect formation in the Mg-alloy castings. Micro-porosity, macro-segregation and other casting defects adversely affect the fracture related mechanical properties (such as ductility, toughness, and fatigue resistance) of the cast Mg-alloys. Therefore, a systematic study of the correlations between important process parameters of the high-pressure die-casting process and the defect generation in the Mg-alloys is of considerable practical interest. The molten metal temperature, fullness of shot sleeve, cavity fill time, piston rate in pre-fill (slow shot rate) and fill (fast shot rate) phases, gate velocity, intensification, gate dimensions, die temperature, vent area, casting geometry, and die and plunger lubrications are some of the important parameters that affect the porosity, segregation and mechanical properties in the high-pressure die-castings of the Mg-alloys (ASM Handbook Volume 2, 1996) On the Porosity Distribution The previous investigations show that more research is necessary to understand the effects of high-pressure die-casting process parameters on the volume fractions of gas and shrinkage type porosities, the pore size distributions and spatial variations of porosity volume fraction and sizes. Accordingly, in this contribution the effects of gate velocity (injection rate), intensification and melt temperature on these porosity attributes in model plate castings of Mg-alloys should be on focused. The effects of each of these three parameters should be deconvoluted by keeping all the other parameters and process conditions constant. In the near

4 future, the computer simulation algorithms are likely to advance to an extent where detailed quantitative predictions of porosity distributions may be possible. Some experimental parametric studies have been carried out to quantitatively characterize the effects of intensification pressure, gate velocity, and melt temperature on the porosity distributions in the high-pressure die-cast Mg-alloy. The effects of these process parameters on amount of total porosity, gas porosity, shrinkage porosity, and pore size distributions are studied by some researchers. The experimental data lead to the following important observations and conclusions. Application of intensification pressure significantly reduces the total amount of porosity primarily via reduction in the gas porosity. A decrease in the gate velocity decreases the total amount of porosity predominantly via a decrease in the gas porosity and a small extent of decrease in the shrinkage porosity. The lower gate velocity uniformly decreases the number density and area fraction of gas pores of all sizes (small and large). A decrease in the melt temperature also reduces the total amount of porosity primarily via reduction in the gas porosity. The lower melt temperature reduces the number density and area fraction of the gas pores (Lee, Gokhale, Patel, and Evans, 2006) On the Segregation Formation Segregation on the cast surface results from various ways, and it may be divided into two parts, inverse segregation and inverse surface macrosegregation. Surface macrosegregation is different from the classical inverse segregation in that it is accompanied by exudation of solute enriched liquid to the cast surface. Further, the solute segregation by exudations is significantly larger than that associated with classical inverse segregation. As the solidification begins at the surface of a casting, a local contraction occurs on solidification, which can draw the solute rich liquid present in the interdendritic channels to the surface of a casting producing high concentration of solute elements/impurities at the casting surface. This interdendritic liquid flow is driven by the pressure drop due to the air gap formation, and then highly segregated liquid layer forms at the cast surface. This type of segregation is called inverse surface macrosegregation. Inverse surface macrosegregation has been experimentally observed in numerous directionally cast Al and Cu alloys. In general, it has been known that the difference between the heat transfer coefficient for chill contact and for air gap, casting speed, cooling rate, metallostatic pressure, solidification shrinkage, and dendrite coarsening affect the formation of inverse surface macrosegregation. Numerous theoretical models have also been proposed for inverse macrosegregation. Inverse surface macrosegregation can significantly increase the amounts of brittle eutectic constituents (such as intermetallic compounds) at the cast surfaces that may facilitate the crack initiation and thereby adversely affect the fracture related properties such as ductility, toughness, and fatigue resistance of the cast components. In high-pressure die cast Mg-alloy castings, the presence of the inverse surface macrosegregation leads to an early initiation of fatigue cracks, and consequently, it adversely affects the fatigue life. Therefore, it is of interest to identify (and avoid) the process conditions that can lead to such segregation in the cast components (ASM Handbook Volume 15, 1998; Lee, Patel and Gokhale, 2005) On the Mechanical Properties There is a certain correlation between process parameters and mechanical properties namely yield strength, ultimate strength, and elongation. The increased plunger pressure results in increased yield and ultimate strength, while elongation was decreased for all the selected alloys. The relationships between the solidification rates in terms of solidification time versus

5 tensile yield strength are that reduced solidification rate result in increased grain diameter and consequently reduced yield point (Aghion, Moscovitch and Arnon, 2007). 4. Conclusions Although high pressure die casting is a very precision process; there are lots of factors which are effective on the product quality and mechanical properties. The process parameters; casting and die temperatures, filling time, plunger and gate velocities, injection and intensification pressures, spearhead of these factors. This paper investigates the present works about the effects of process parameters on the product quality in literature. For obtain qualified casting products which have good mechanical properties and low porosity contents; the casting process should be made under high injection and intensification pressures, with low casting and high die temperatures and with minimum flow velocities. References ASM Handbook Volume 2 (1996), Properties and Selection: Nonferrous Alloys and Special Purpose Materials, ASM International Handbook Committee, United States of America. ASM Handbook Volume 15 (1998), Casting, ASM International Handbook Committee, United States of America. ASM Specialty Handbook (1999), Magnesium and Magnesium Alloys, ASM International Handbook Committee, United States of America. Brungs, D. (1997), Light Weight Design with Light Metal Castings (Technical Report), Materials & Design vol. 18, Department of Trade and Industry (2004), Magnesium Alloys and Processing Technologies for Light Weight Transport Applications, Global Watch Mission Report, United Kingdom. E. Aghion, N. Moscovitch and A. Arnon (2007), The Correlation Between Wall Thickness and Properties of HPDC Magnesium Alloys, Materials Science and Engineering vol. A El-Mahallawy, N.A., Taha, M.A., Pokora, E. and Klein, F. (1998), On the Influence of Process Variables on the Thermal Conditions and Properties of High Pressure Die-Cast Magnesium Alloys, Journal of Materials Processing Technology vol. 73, Friedrich, H.E. and Mordike, B.L. (2006), Magnesium Technology: Metallurgy, Design Data, Applications, Springer, Berlin.

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