High pressure die casting (HPDC) dies are nowadays manufactured with high quality forged steels. Cavities are made by electrical discharge machining (EDM) or by high speed milling. The average life of an aluminium HPDC die is about 125.000 injections. Refrigeration circuits have simple configurations, because they are produced by drilling the die with straight holes. They are limitations in the distances and diameters of holes. Sensors are placed where the geometry of the die permits an easy machining. In order to obtain complex figures, several rapid prototyping methods have been developed. However, there is a limitation in the life of the dies produced by this technique, from several parts to thousands. A new method to obtain semifinished high pressure die casting dies in a steel of higher mechanical properties and with the refrigeration circuits and sensors embedded into it is described in this paper. The method consists in producing a molten steel alloy with micro-nano-special ceramic particles inserted in it and casting the composite material in sand moulds of the desired geometry. The resultant solidified near-net shape die with the cooling tubes and sensors embedded into it. A use-life and a productivity about 50% and 10% higher are obtained.
In the last years HPDC cast parts have developed a quick development due to the high automation, the new die materials, and sensoring and construction methods and also by the advanced control and foundry systems.
HPDC are generally made in H11 or H13 steel alloys by high speed machining combined with electrical discharge machining (EDM). Dies are expensive, due to the expensive tooling and the high energy and man power demanded in these operations.
Rapid prototyping processes are coming more and more employed in HPDC moulds, but they are employed normally for first series, because the properties obtained with those parts produced by this technology cannot obtain the same life time than with standard fabricated dies. According to bibliography [
There are also several indirect metal part production methods [
The steel sand casting technique has been also appointed as a possible way to construct cavities and dies [
One of the main mechanisms that define the total life of a die is the thermal fatigue cracking [
Between them, the metal matrix composites have the potential to increase the thermal fatigue property [
This work deals mainly with the development of a foundry process to produce HPDC dies with customised internal refrigeration and high resistance new steel. The objective is to obtain an economical process to produce advanced HPDC dies.
The base material for the tests has been chosen from the most employed steels used by HPDC die makers. The alloy 1.2344 (H13) was finally elected.
Based on the previous experience, in order to reinforce the base material, a Fe(TiMo)C master-alloy produced by self-propagating high temperature synthesis (SHS) has been chosen to be added to liquid steel.
To compare the new developed material properties in function of different tempering temperatures, some test bars were casted, heat-treated, and machined. An industrial induction furnace was used to prepare 300 kg of molten steel from selected scraps. When the scraps were melted at 1650°C, the steel was dedrossed and a 7.5% in weight of Fe(TiMo)C master alloy was added until a good dissolution was obtained. Test bars were poured in sand moulds at 1.550°C. After the solidification, parts were demoulded and separated from runners and overflows. Test wars were quenched at 890°C for 2 hours and tempered at temperatures between 460 and 600°C for 2 hours. After sand blasting, parts were machined in a high speed milling machine.
For the mechanical characterization, standard EN-10002.1, EN-10045.1, and EN-10003.1 bars were machined to test tensile, resilience, and hardness properties, respectively. Wearing properties were also checked by pin on disk test. Thermal test bars have a dimension of 150 × 50 × 50 mm with internal channels of 20 mm of width, 5 mm of depth, and 110 mm length (see Figure
(a) Thermal fatigue test bar, (b) thermal fatigue test machine.
The two cast materials were quenched at 890°C and tempered at different temperatures (see Table
Temper temperatures and materials.
460°C | 580°C | 600°C | |
---|---|---|---|
Base alloy | ● | ● | |
Reinforced alloy | ● | ● | ● |
In order to define a method to obtain directly from casting the cooling circuits a 125 mm radius and 300 mm length cylinder cavity to be produced into a chemically bounded sand mould was designed with placements for tubes. In this sand mould tubes with different diameters and materials were tested, with and without internal sand as protective material for the tubes, and also a continuous gas flow was introduced to cool the tube during casting solidification. Definitively, standard AISI 316 stainless steel tubes were chosen for the internal cooling circuits, due to the low mismatch in composition between base alloy and AISI 316. On the other hand, zirconia sand was selected as inside filler for the cooling tubes due to its high refractoriness.
We can observe in Table
Tube sizes, fillers, and positions.
Ref. | Outside diameter (mm) | Inside diameter |
Filler | Tube position |
---|---|---|---|---|
1 | 10 | 7 | Gas | Vertical |
2 | 18 | 9 | Sand | Vertical |
3 | 17 | 11 | Sand | Vertical |
4 | 8 | 5.4 | — | Horizontal |
5 | 21.5 | 16 | — | Horizontal |
Die sand mould with placements for tubes.
After pouring and subsequent solidification of the liquid composite material, casting part was cut and inspected to determinate if the tubes resisted the molten metal.
Optical microscopy has been used to determinate the structure of different alloys after heat treatments and also to determinate the coherency between the alloy and the tube material, in order to understand the obtained properties.
In order to test the materials in real production, a design and a cooling optimisation of real parts were made (see Figure
(a) Cooling optimisation, (b) sand mould pattern.
Internal tubes were cold deformed to obtain the desired forms and also in few cases arc welding technique was employed to obtain complex shapes.
In order to determinate a robust process, different cooling tubes in size and position and with and without internal insulating material (zirconia) defined in Table
We can observe in Figure
Placement of cooling tubes in the test sand mould.
H13 base alloy was poured at 1560°C (see Figure
(a) Sand mould with poured metal, (b) study of metal penetration.
We can observe in Figure
The obtained results are resumed in Table
Resistance to metal penetration in tubes.
Ref. | Tube filling | Tube position | Resistant to metal penetration |
---|---|---|---|
1 | Gas | Vertical | No |
2 | Sand | Vertical | Yes |
3 | Sand | Vertical | Yes |
4 | — | Horizontal | No |
5 | — | Horizontal | No |
With these results the external diameter of cooling tubes was set to 10 mm, the internal diameter to 7 mm, and zirconia sand as tube filler.
In order to define the real behaviour of the developed process, dies and cores for industrial validation were defined. Die sand mould patterns were constructed and internal cooling tubes conformed. The mould patterns are 5 mm oversized in comparison with finished die, due to the necessity of having enough material to machine the exterior of the casting to obtain the desired quality and dimensions.
Demonstrators were tested with a H13 base steel having the composition shown in Table
Base alloy composition (% in weight).
Element | C | Mn | Si | Cr | Ni | Mo |
|
||||||
% | 0.22 | 0.80 | 1.00 | 3.00 | 0.70 | 0.35 |
The Fe(TiMo)C master-alloy was produced by powder metallurgy and promoting a SHS reaction. The powders of Ti (48%, 100
SHS produced Fe (TiMo) C master-alloy.
Using an insulation furnace, 250 Kg of H13 base alloy was melted at 1650°C. The master-alloy Fe(TiMo)C was added to molten steel in a 7.5% concentration in weight. When the master-alloy was dissolved in the base alloy, the composite material (steel + ceramic particles) was cleaned and poured at 1660–1665°C in the sand moulds.
Once the castings are solidified, cooled, and demoulded as we can see in Figure
Example of cast HPDC parts with internal cooling circuits.
(a) Annealing furnace, (b) shot blasted part with superficial defects.
When the part it’s machined is possible to observe the external skin that has been in contact with sand mould, which it’s very hard because it contains very hard oxides (see Figure
(a) Partial machined part with contact skin, (b) finished HPDC core.
The thermal fatigue bars were used to determinate the process parameters and properties of the obtained parts. To determinate the most suitable temper temperature, the conventional base alloy and the reinforced alloy were tempered at different temperatures. In Figure
In Figure
In order to determinate the optimum temper temperature, we can observe in Figure
Indeed, pin on disk tests have been performed with the alloys, showing that when we increase the tempering temperature, the wearing speed is increased. Also, the reinforced materials have a much better behaviour to wearing than base alloys, as shown in Figure
With the previous results, the reinforced material with a temper temperature of 580°C has been elected to be tested in tensile test and to be employed in validation test. There is not a big difference between a temperature of 580°C or 600°C, because there is a balance between the obtained properties.
Table
Ultimate and tensile strength for base and reinforced alloy.
UTS |
YS |
Elongation | |
---|---|---|---|
Base alloy (tempering temp. 580°C) | 1.050 | 775 | 19 |
|
|||
Reinforced alloy (tempering temp. 580°C) | 1.070 | 980 | 11 |
As much as the main properties that influence the life of a die are the thermal shock resistance and the hardness, the two alloys were tested during 100.000 cycles in the thermal fatigue test machine. Test bars were analysed under magnetic fields in order to determinate the severity of the cracks generated by thermal shocks. The reinforced alloy did not show any crack after 100.000 cycles. We can observe in Figure
Thermal stress produced cracks observed (a) under magnetic particles, (b) after sand blasting.
The steel microstructure is based on a tempered martensitic matrix with inserted and dispersed ceramic carbide particles, as it can be seen in Figure
(a) Micrograph without attack, (b) attacked micrograph.
In Figure
(a) Micrograph without attack, (b) attacked micrograph.
The best treated material was used to obtain real die casting pieces with a sampling die. Production and quality rates and total die life was compared with standard materials and cooling circuits. For that, validation tests were made at Fiasa with a comparable die with 4 identical cavities. Three cores were made with base alloy and another was made with the reinforced alloy (Figure
The minimum distance between the surface of the mould and the refrigeration tube was established in only 10 mm, where at least 25 mm is required in standard dies (see Figure
Reinforced alloy core with the internal cooling place at 10 mm and 20 mm from surface.
A thermal camera was employed in order to determinate the different core external temperature, maintaining the same water flow in all the cores. We can see in Table
External temperature of cores.
Reinf. alloy core 1 | Base alloy core 2 | Base alloy core 3 | Base alloy core 4 | |
---|---|---|---|---|
Temperature (°C) | 140 | 178 | 180 | 180 |
After only 3.505 injections, the cores were extracted and compared, in order to determinate the performance of the new alloy. In Figure
(a) Base alloy core, (b) reinforced alloy core.
Some heat cracks due to high thermal stresses were detected in the core area where the distance between the cooling circuit was nearer to the surface of the core (see Figure
Heat cracks with the internal cooling circuit shape.
In order to study the cracks and the interphase between the internal cooling tubes and the reinforced alloy, the part was divided in 6 different areas (as shown in Figure
Heat cracks with the internal cooling circuit shape.
In Figure
Heat cracks with the internal cooling circuit shape.
Also we can see there is a metallurgical continuity between the tube and the reinforced alloy material, as shown in Figure
Interphase between tube and reinforced material.
(a) Base alloy cast part, (b) reinforced alloy cast part.
Hardness variation in function of temper temperature.
Hardness variations in function of temper temperature.
Hardness and resilience variations in function of temper temperature.
Pin on disk wearing speed comparison.
Micro-hardness (HV10) values were measured in order to determinate a possible temper action in the core by repetitive injections. We can see in Table
Micro-hardness of reinforced alloy core.
Ref. 1 | Ref. 2 | Ref. 3 | Ref. 4 | Ref. 5 | Ref. 6 |
---|---|---|---|---|---|
320 | 308 | 317 | 319 | 318 | 317 |
The average micro-hardness before testing the core in the foundry was 329 HV10. So there are not big differences between the samples before and after working.
The injected parts quality was compared also after 3.505 injections. A clear difference was determined in the injected part. The part obtained with the unreinforced material show a poorer surface quality, with cracks in the surface and thicker over material in the eroded area, in coincident with the eroded and cracks areas showed in Figures
There are several methods to construct dies, but nowadays it is necessary large machining operations in order to obtain the desired shapes and internal cooling circuits with several constraints in the design. Rapid prototyping process cannot afford by the moment the same results than machining dies from 3D forged hot work steels.
The SHS process allows developing a ceramic Fe(TiMo)C master alloy that is dispersed in the molten steel and after solidification gives a martensitic matrix reinforced with particles of carbides (TiMo)C well distributed.
This composite material allows increasing the UTS, YS, hardness, and wearing, with a decrease in elongation and to impact. Thermal fatigue is also increased with the new reinforced alloy. These properties are obtained producing the near-net shape dies by pouring the liquid material into sand moulds, what permits reducing drastically the machining operations and costs, reducing delivery times in about 2 weeks.
The use of stainless steel tubes filled with zirconia sand allows developing customized cooling circuits, reducing the necessities of machining operations to construct the cooling circuits, given a better thermal control. Also a good continuity between the tube and the reinforced alloy is observed.
Verification tests of new reinforced alloy on casting pieces show an increase on the life time, a decrease in core temperature in working conditions, but also a limit in the distance from the cooling circuits to the surface of the part, due to heat cracking.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work has been partially funded by the Basque government through the Project nos. IG-2010/0000118 and IG-2011/0000678 and CDTI through the Project CDTI IDI-20100385. The authors are also much grateful to the personnel of Industrias Lebario and Fundiciones Inyectadas Alavesas, S.A., companies that have collaborated in aspects related to the design, test, and analysis of results.