Determination of the heat transfer coefficient at the metal–die interface for high pressure die cast AlSi9Cu3Fe
Highlights
► The HTC between the casting and die is critical to predict casting quality. ► A Magmasoft® simulation model was used to simulate the casting die. ► A good fit to the simulation model was established using a peak HTC of 42 kW/m2 K. ► The improved simulation model will improve the accuracy to predict casting defects.
Introduction
High pressure die casting ‘HPDC’ is a widely used process to manufacture non-ferrous castings for the automotive industry.
With HPDC the molten metal is forced into the die cavity under pressure. Due to the high filling speed and rapid solidification rate, this casting process can produce shapes which are more detailed than components manufactured using gravity or low pressure die casting methods. Non-ferrous alloys, mainly aluminium, magnesium and zinc are most commonly cast using this process.
Fig. 1, Fig. 2, Fig. 3 shows a cross section view of an HPDC die. HPDC is ideal for high volume thin walled castings due to the fast cycle times, ranging from seconds to several minutes depending on casting size and wall thickness.
The predominant non-ferrous metal used for HPDC is Aluminium, the most commonly used alloy of which is (AlSi9Cu3Fe) with a chemical composition by (wt%): Cu (3.0–4.0%), Fe (1.3%), Si (7.5–9.5%), Mg (0.1%), Mn (0.5%), Zn (3.0%), Ni (0.5%), Sn (0.35%), others (0.5%), Al (remainder) this alloy has good castability and mechanical properties [1]. The properties of AlSi9Cu3Fe important to casting are provided in Table 1.
HPDC accounts for almost 70% of aluminium components manufactured today [1]. Many aluminium components for the automotive industry are cast using this method, due to the high productivity and near net shape production. Large components such as gearbox housings and engine blocks are typical examples where casting weight can be in excess of 15 kg. Due to the short cycle times, the die is exposed to high temperature fluctuations each and every casting cycle. The HPDC process involves rapid temperature fluctuations on the surface of the die, resulting in steep thermal gradients on and below the die surface [2].
Casting simulation software such as Magmasoft® [3] is commonly used to predict the behaviour of the casting and die throughout the casting process to ensure correct filling and adequate directional solidification of the casting [4]. To ensure accurate prediction from the simulation model, the correct parameters need to be used. The HTC between the casting and the die will determine the rate of heat loss from the molten metal to the die material and therefore influence the solidification characteristics of the casting. As the molten metal solidifies the contact force between the die and the casting will change, causing the HTC to vary considerably [5].
In order to determine the HTC at the metal–die interface, two production series dies were instrumented with thermocouples. The resulting temperature traces from the instrumentation trials were then compared to the Magmasoft® casting simulation models. From this, the HTC at the metal–die interface was adjusted to better correlate the simulation model to the instrumentation trial results. When satisfactory results were obtained from the first comparison with Die 1, a second trial was run with a die of different design, Die 2, to validate the new HTC dataset.
The objective of this research was to get better HTC data for the metal–die interface to improve the accuracy of the simulation model and its ability to predict casting defects.
Section snippets
Die temperature
With HPDC, the die surface is rapidly heated when the molten aluminium is injected into the die cavity. After the casting solidifies, it is ejected and then removed from the die. The die surface is then cooled by die-lube spray (mix of oil and water), used to both lubricate and cool the surface of the die. For HPDC there is no insulating diecoat layer between the casting and the die, which is commonly used for gravity and low pressure permanent mould casting. Therefore, the molten aluminium
Results
Initially the standard Magmasoft® HTC dataset between the die and the casting was used for the simulation analysis of the Die 1 trial, however this gave a poor correlation to the instrumentation trial results. From this, the HTC between the die and the casting was modified. Only the HTC was changed as the thermal properties of the die steel and aluminium were believed to be correct. Initially the HTC value used for the die-metal interface was the default value defined for AlSi9Cu3Fe to the die
Theory
It will be noted from Fig. 19 and Table 2 that the heat transfer coefficient used within Magmasoft® is dependant only on the temperature of the aluminium alloy; the motion of the liquid metal when the die is filled is ignored. Although die filling is brief (typically tens of milliseconds), it is during this period that the motion of the liquid metal relative to the die will cause the greatest heat transfer. As the molten metal is at its greatest temperature during the die filling the
Discussion
The results from the Die 1 trial have given a new HTC range for the metal–die interface, which when used with the simulation model gives a very good correlation with the instrumentation results. When Die 2 was compared to the instrumentation trial results there was yet again a very good correlation between the simulation model and reality, with a maximum temperature difference of only 20 °C seen for both Die 1 and Die 2 comparisons. The peak HTC was of the same order of magnitude as determined
Conlusions
Basing the heat transfer between the casting metal and the die purely on the metal temperature is far from ideal for the work described in this paper. Nevertheless to model die filling and casting solidification this simple approach has proved perfectly adequate. Given this constraint, the authors have determined heat transfer values which allow a much more accurate determination of the instantaneous local metal temperatures for the die during a casting cycle than was previously obtainable.
Acknowledgements
This research was financially supported by Invest Northern Ireland under Start Project ST247. The experimental work was carried out at Ryobi Aluminium Castings (UK) Ltd. Technical support and instrumentation was provided by Queen’s University Belfast.
Nomenclature
- Cp
- Specific heat at constant pressure, J/kg°C
- d
- Diameter, m
- h
- Heat transfer coefficient, W/m2 k
- k
- Thermal conductivity, W/m k
- u
- Velocity, m/s
- μ
- Dynamic viscosity, Pa s
- ρ
- Density, kg/m3
- Pr
- Prandtl number
- Re
- Reynolds number
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