Gary Savage

Thermal and flow modelling of ladling and injection in high pressure die casting process

Air entrapment and premature solidification in the shot sleeve of the cold chamber high pressure die casting process are problems affecting the casting quality. These issues are addressed from several viewpoints, including heat loss during metal ladling, turbulence generation and gas escape during pouring, wave formation and propagation during metal pouring and injection, and heat loss and solid creation in the shot sleeve during metal injection. Measurements and simulations have been conducted for a typical industrial case where the fill ratio of the shot sleeve is about 30%. It has been found that the settling time of the aluminium alloy in the shot sleeve has a profound influence on the air entrapment. The settling time also has a negative effect on the metal quality the solid formation (premature solidification or cold flakes) in the shot sleeve. Fast pouring and quick injection (no settling time) are recommended as a good approach to minimise the air entrapment and cold flakes in the shot sleeve in this particular case. IJCMR/474

Evaluation of castability of high pressure die cast magnesium based alloys

Abstract Alloy characteristics that impact on castability, such as hot tearing and fluidity, are as significant to the success of an alloy development program as the mechanical properties requirements of the final component. However, castability is often overlooked in the alloy development process and this could lead to substantial problems in fabrication. A new die has been developed to test multiple indicators of the high pressure die castability of an alloy. A rating system has been introduced to evaluate several exterior quality factors for high pressure die castings. This is coupled with selected metallographic inspection of the interior defects and the results are combined to compare the relative performance of different alloys. It was found that ZA124 alloy was the most castable of the alloys tested followed by AZ91 with the castability decreasing with decreasing Al content for Mg–Al based alloys.

Manufacture of high pressure die-cast radio frequency filter bodies

Abstract The manufacture of a radio frequency filter box using high pressure die casting (HPDC) is compared to the traditional high speed machining route. This paper describes an industrial exercise that concluded HPDC to be an economical and appropriate method to produce larger volumes of thin-walled telecommunications components. Modifications to the component design were made to make the component suitable for the HPDC process. Development of the die design through simulation modelling is described. The wrought alloy was replaced by near-eutectic Al–Si die casting alloy that was found to give better temperature stability performance. Apart from the economic benefits, HPDC was found to give lower filter efficiency losses through better surface finish. The effects of HPDC process variables, such as intensification pressure and injection piston velocity, on component quality, particularly porosity levels, were investigated. The pressure was analysed in terms of HPDC machine set pressure and the pressure measured in the die cavity by pressure sensors. Porosity was found to decrease with increased pressure and slightly increase with higher casting velocities.

Heat treatment of vacuum high pressure die cast magnesium alloy AZ91

Abstract Alloys produced by high pressure die casting (HPDC) are generally considered non-heat treatable because trapped gas pores tend to expand, causing surface blistering and bulk distortion. In this paper, vacuum assisted HPDC of magnesium alloy AZ91 was used, and the properties were assessed. The specimens produced using vacuum die casting contain less porosity. Little improvement in yield strength by applying vacuum is found, although a small increase in elongation is observed. A conventional heat treatment applied to the vacuum die cast AZ91 shows pronounced precipitation hardening during aging, especially after a prior solution treatment. However, an associated improvement in yield strength after aging is not observed, and this is related to the decreased contribution of the ‘skin’ effect as a result of grain growth.

3D Thermo-Mechanical Modelling of Wheel and Belt Continuous Casting

Wire rod is produced by hot-rolling a bar of metal coming from a wheel/belt continuous casting process. This kind of process, e.g. Properzi, is an elaborate process in which the molten metal is poured in a cooled rotating mould formed by the groove of a wheel and closed by a belt. In order to better understand the heat transfer phenomenon and solidified bar characteristics, depending on process parameters a three dimensional thermo-mechanical model has been developed. The model, based on the finite-element method, calculates the heat transfer coefficient of the air gap at the metal-mould interface as a function of the size of the gap determined by the bar contraction and wheel and belt thermal deformations. The air gap formation due to metal shrinkage and mould deformation is the main factor which determines the heat extraction. Wheel temperature measurements with thermocouple and belt temperature measurements with an infrared system were carried out to verify model results. Attempts were also made to measure a liquid pool profile using doping with copper rich alloy. The model shows the effect of the casting temperature and the rotation speed on the air gap formation and resulting temperature and stress fields. The model can be applied to issues such as maximising wheel and belt life and minimising solidification defects.

Development of Magnesium-Rare Earth Die-Casting Alloys

An overview of the development of a high-performance Mg–RE based alloy, HP2+, is presented, which has a good combination of die-castability and mechanical properties at ambient and elevated temperatures. The original alloy, HP2, was a die-casing version of the sand-cast alloy SC1 developed for powertrain applications. However, HP2 tended to crack substantially, leading to unusable castings due to its high Nd content. It was found that the solidification path of Mg–RE alloys can be engineered to reduce the propensity to hot tearing by changing the mixture of RE elements towards La-rich, which leads to an increase in the amount of eutectic and a reduction of the solidification range. Precipitate-forming RE elements, such as Nd or Y, were optimized for HP2+ to meet the requirement for high temperature creep resistance. Whilst some challenges remain with the commercial application of HP2+, the learnings from the alloy design process can be applied to other alloy development programs.

Performance Evaluation of High-Pressure Die-Cast Magnesium Alloys

Over 90% of the magnesium (Mg) alloys in commercial applications are produced by high-pressure die-casting. This paper presents our efforts in evaluating castability and properties of commercial and near-commercial magnesium alloys to demonstrate how the currently available alloys can be applied to different situations across a range of property space. For high temperature applications, i.e. 175 °C and above, Mg–RE and Mg–Al–Ca based alloys have creep properties at least comparable to aluminium (Al) alloy A380 although these alloys have some challenges with casting or cost. For moderate temperatures, Mg–Al–RE based alloys, especially AE44, are most attractive due to an excellent combination of creep resistance, strength and castability. For automotive structural applications where a good combination of strength and ductility is required, Mg–Al alloys provide the baseline, but Mg–Al–RE based alloys can provide outstanding performance, especially with recent discoveries about its response to age hardening treatments. Therefore, high-pressure die-cast Mg alloys hold great promise for continued growth in automotive applications.

Castability of some magnesium alloys in a novel castability die

This paper reports on the results of the castability of three MRI alloys (153A, 153M and 230D). MRI153A was found to cast best, with castings produced rated with a quality approaching AZ91. MRI230D produced the next best castings, whilst MRI153M showed the worst castability across a range of conditions. However, these alloys showed a tendency to build-up oxide in the melt transfer tube leading to melt transfer problems. This was particularly severe in MRI230D.

Modelling Die Filling in Ultra-Thin Aluminium Castings

This work aims to develop flow and thermal control methods for the high pressure die casting (HPDC) of very thin-walled aluminium components where thicknesses are predominantly less than 1 mm. One specific aim includes developing advanced modelling capability using CFD software to predict the complex structure of the metal flow in the die and the casting solidification. The modelling based on FLOW-3D started initially with a fluidity die study to establish several key parameters in HPDC modelling through experimental validation. A new test casting geometry has been designed in the form of a shallow tray with other features such as changes in curvature, fins and bosses. The casting thickness can be made variable in the die. The experimental work was conducted on a 250-tonne HPDC machine. Initial models of molten metal flow in the die cavity based on a runner design for casting thicknesses between 1.5 mm and 1 mm are presented. The detailed model required a very large mesh of very small elements, and more accurate physical parameters which may not have been previously available.

Technologies to Improve Productivity and Reduce Tooling Costs in High Pressure Die Casting (HPDC)

Globalisation of supply chains for the automotive industry has made it increasingly difficult for developed economies to compete on price alone. In Australia, Nissan Casting has adopted technologies to reduce tooling costs and improve productivity of the die casting machines which use vacuum. Nissan Casting was experiencing cracking of their dies which extended into cooling lines, causing porosity outbreaks and shortened die life. CAST developed a repair technology called CASTrepair™ which is a relatively quick, simple and inexpensive repair technique for cracked cooling lines in HPDC dies. Also Nissan Casting is an extensive user of vacuum and a major issue was identified as unscheduled machine stoppages caused by aluminium blockages in the vacuum valves. The valves were of the mechanical shut off type and when they failed to shut off quickly enough aluminium entered the valve which then had to be changed. CAST developed CASTvac™ which is essentially a three dimensional chill vent with no moving parts required as a means to stop the aluminium entering the vacuum system. CASTvac™ has proven to be very reliable in production and has significantly reduced downtimes and toolroom maintenance at Nissan Casting. These two technologies, CASTrepair™ and CASTvac™, will be explained in detail with the latest developments in the technologies and associated benefits of their adoption.