diecast
Diecast (English High Pressure Die Casting (HPDC)) is a casting for series or mass production. Cast alloys with a low melting point are usually used for this.
In die casting, the liquid melt is pressed under high pressure of approx. 10 to 200 MPa and at a very high mold filling speed of up to 12 m / s into a die casting mold ( casting mold , cavity ), where it then solidifies. The special thing about the die casting process is that with a permanent mold, i.e. H. without a model . As a result, the mold is only produced once for a series of identical components , but with a significantly higher production cost. In this way, a high output is achieved, especially in a hot chamber die casting machine in which the casting container and the casting piston are constantly in the melt. In the case of alloys with a higher melting point, the cold chamber pressure casting process is used; the casting set is located outside of the metallic melt.
Die casting competes with injection molding with plastics . Metallic materials have advantages in individual cases that secure the market for die-cast articles.
The most commonly used materials are
- Aluminum ( die-cast aluminum )
- Zinc ( die-cast zinc )
- Magnesium ( magnesium die-cast )
- Silicon tombac
properties
Die-cast parts have smooth, clean surfaces and edges. In addition, this method allows for thinner walls than other methods. With zinc, for example, the parts can have a wall thickness of 1 mm and with aluminum of 1.4 mm, in exceptional cases even less than 1 mm.
The achievable tolerances are ± 0.05 to ± 0.15 mm, so that one speaks of an exact or finished casting. For large cast parts, however, slightly larger tolerances are required.
It is possible to use parts made of other materials, such as B. to pour in sockets, threaded bolts or pins. Internal threads are produced directly with rotating steel cores, which can then be removed again later. In contrast to other processes, the casting is called “shot” here. It is possible to carry out up to 1000 shots per hour (depending on the machine size). Depending on the casting material, the mold can stand up to 2,000,000 shots (zinc alloys).
With aluminum, service lives of approx. 80,000 to 200,000 shots are achieved. For a long time, die-cast aluminum was not considered suitable for fusion welding and heat treatment. In the meantime, extensive studies have made it possible to manufacture die-cast aluminum that is suitable for fusion welding and heat treatment. However, for these properties it is necessary to consider the requirements of the process in the design. In addition, however, die-cast aluminum can be welded very well using friction welding processes such as friction stir welding .
Die casting machine
Die casting machines usually consist of a mold clamping unit which is used to open and close the die casting mold. Components of the mold clamping unit (also called the closing part) are:
- a fixed machine plate (also called a fixed plate ) to hold the fixed casting mold half and, in the cold chamber process, the casting set
- a movable machine plate (also mobile plate ) to accommodate the hydraulic ejector and the movable mold half
- the four guide pillars on which the movable machine plate is guided
- the toggle system (consisting of a cylinder plate (also called crosshead), short and long lever and cross head)
- the lock cylinder
The liquid metal is pressed from the casting chamber into the mold with a casting piston. Depending on the type of machine used, a distinction is made between the warm and cold chamber processes. The casting piston is driven by the casting unit's drive. The drive piston is usually acted upon by a hydraulic pressure accumulator. In individual cases the casting piston is driven electrically (as of 2006).
Periphery
The peripheral units work in the vicinity of the die casting machine. These are process engineering devices that are necessary for the automatic process to run.
- Mold spray machine or manipulator for mold spray or fixed nozzles
- Heating and cooling devices
- Extraction devices
- Dosing oven or ladle
Vacuum-assisted die casting process
The vacuum-assisted die-casting process with forced ventilation enables workpieces with little or no gas inclusions. The cavity and the filling chamber are evacuated at the start of casting, so that the air contained therein and the gases formed are sucked out during casting and so fewer or no air inclusions can form in the melt.
Redensification
A similar effect is achieved here through high compression in the phase before the final solidification of the workpiece. Pores and air pockets are significantly reduced. Since during the solidification of the melt, i. In other words, when there is a change in volume from liquid to solid, volume deficits inevitably occur in the interior of thick-walled components. To compensate for this effect of liquid shrinkage, the so-called squeeze process can be used to compress the solidifying melt. For this purpose, the pulpy melt is compressed by means of a "squeeze pin" in the die casting mold in areas where a volume deficit is expected.
Manufacturer of die casting machines
European companies that manufacture die-casting machines include IDRA, COLOSIO, OMS Presse, ITALPRESSE, Müller Weingarten (on December 1, 2007, Oskar Frech GmbH + Co. KG took over the die casting technology division from its former competitor Müller Weingarten AG ), Oskar Frech or Buhler .
Die casting mold and tool
The die- casting tools required in the die-casting process are manufactured in the tool shop or in the mold shop. Special materials or high-strength hot-work steels in accordance with DIN EN ISO 4957 are usually used as the material for the casting molds. A selection is shown in the following table.
| Short material name | Material number |
|---|---|
| 32CrMoV 12-28 | 1.2365 |
| X37CrMoV 5-1 | 1.2343 |
| X40CrMoV 5-1 | 1.2344 |
Phase sequence
In die casting, a pre-tempered (100 to 300 degrees Celsius) permanent mold (two or more parts) is filled with molten metal under pressure at high speed. The actual casting process can be divided into three phases.
The pre-filling phase is used to convey the melt in the casting chamber up to the gate. Due to the relatively low speed of the piston (0.05–0.7 m / s), the air compressed in the sprue system during the first phase can escape through ventilation channels and through the molding plane.
During the mold filling phase, the casting piston presses the melt into the mold at a very high speed (0.4–6 m / s). The mold filling time is extremely short at 5–60 ms. It is therefore practically impossible to vent the mold.
In the holding pressure phase at the end of the mold filling, a very high static final pressure is built up. The air trapped when the mold is filled is compressed and the cast part is replenished.
Casting pressure
Different casting materials each require a different casting pressure. Aluminum and magnesium alloys are cast at 300–1200 bar, zinc at 130–250 bar and brass at 300–1000 bar. The strength of a workpiece is greater at high casting pressure.
The cross section of the drive piston and the operating pressure of the pressure accumulator cannot be changed. In order to achieve a certain casting pressure , the cross-section of the casting piston is varied. The following applies
- (Balance of forces)
With a circular piston cross-section, the following also applies to the drive piston diameter and the casting piston diameter
By rearranging this equation, the required casting piston diameter is obtained for the desired casting pressure
Shrinkage behavior of aluminum-silicon melts
If you pour liquid aluminum at about 700 ° C into a mold cavity in such a way that the mold is exactly filled, then the volume of the metal decreases to room temperature due to its contraction by a total of about 1.3%. Of this total, 0.05% is due to the liquid contraction, 0.75% to the solidification and 0.5% to the solid contraction.
This volume deficit of 1.3%, which is not much smaller with aluminum alloys than with pure metal, becomes noticeable in the solidifying cast part in the form of volume defects. In addition to normal shrinkage, these defects occur partly as sink marks , partly as macro- voids or as shrinkage pores.
The shrinkage cannot take place as unhindered in a die casting mold as it is in a sand casting mold, for example. In foundry technology, the term "shrinkage" means, in addition to a process, a dimension, namely the percentage dimensional deviation of the cooled casting from the dimensions of the mold at operating temperature. Whether shrinkage can take place freely or with hindrance depends almost exclusively on the geometric shape of the cast part. It is therefore possible that the shrinkage rate of 0.5% that is generally used today can be used for many dimensions on one and the same cast part, while 0.4% could still be too much for other areas. The mold dimensions are to be determined taking into account a shrinkage drawing of the die-casting alloys concerned.
| Die cast alloy | Shrinkage in% |
|---|---|
| Aluminum alloys | 0.5-0.7 |
| Magnesium alloys | 0.5-0.8 |
| Zinc alloys | 0.4-0.6 |
Influence of the alloying elements in aluminum casting alloys
Silicon
structure
With 12.5% silicon, aluminum forms a eutectic1 that melts at 577 ° C. In the binary system there is no connection between aluminum and silicon. The solubility of silicon in solid aluminum is 1.65% in equilibrium at 577 ° C., drops to 0.22% at 300 ° C. and even further at lower temperatures. If an alloy has more silicon than the solubility corresponds, the structure contains not only aluminum mixed crystals but also silicon crystals. If the cooling rate is not particularly fast, the silicon solidifies in the eutectic mixture in the form of angular crystals, needles and plates. These forms of training are made possible by small additions of z. B. sodium, strontium, antimony or phosphorus influenced. Die casting causes similar effects due to the rapid solidification.
Eutectic1 is a mixture of alloying elements, which of all possible compositions has the lowest melting point. The eutectic point, e.g. E.g .: an Al-Si alloy, is 12.5% and 577 ° C.
Casting properties
The higher the Si content, the more advantageous it has on the flow and mold filling capacity . As the Si content decreases, the flowability deteriorates.
Influence of silicon on the feeding behavior
With an increasing Si content, over 11%, the macro-void volume increases sharply. The incidence volume runs in the opposite direction to the macro-void volume. With less than 9% Si, suction cavities tend to occur in thick-walled parts. The feeding options for macro-blowholes are limited in die casting (thick-walled cast parts lying from the gate cause some problems, since material accumulations are filled last and solidify last). In hypoeutectic alloys, coarse grain becomes noticeable as a result of overheating from suction points.
A near-eutectic melt (around 11% Si) causes stronger macro-voids. Eutectic and near-eutectic Al-Si melts, which lead to a “granular” or “refined” structure, solidify with the formation of a peripheral shell (exogenous solidification), so that these melts are not susceptible to suction holes.
iron
In die casting, a higher iron content has a positive effect (e.g., alloy EN AC-AlSi9Cu3 (Fe) max. 1.2%), as it reduces the tendency of the metal to stick to the mold surface. Iron normally has no influence on the casting properties within the tolerance limits. Unintentional increases due to improper melting or working methods in the foundry can lead to embrittlement of the cast parts and undesired formation of cavities, hot cracks or sink marks. At low holding and pouring temperatures, gravity segregations often occur, which collect on the floor of the furnace. The segregation products consist of iron, manganese and silicon. The hardness of segregation products is 500–1000 Vickers .
copper
Copper additives reduce the solidification shrinkage. This has the consequence that copper-containing Al alloys allow pressure-tight castings more easily. Cu additives have a positive influence on strength and machinability. Copper reduces the corrosion resistance.
manganese
Manganese addition of a few tenths of a% reduces the disadvantageous influence of iron on elongation and impact resistance. If, however, higher levels of iron and manganese occur, these can occur under unfavorable melting conditions, e.g. B. by low holding temperatures, lead to hard segregation.
magnesium
If silicon, copper or zinc are present at the same time, magnesium leads to an improvement in machinability due to an increase in hardness. Magnesium does not have a negative influence on the corrosion behavior. Magnesium is also becoming more and more interesting in the automotive industry, especially because of its lower weight.
nickel
The most important advantage of the addition of nickel is the increase in heat resistance. Pistons and cylinder heads in particular are the main areas of application for nickel-containing Al alloys.
zinc
Different zinc content within the tolerance limits are generally without influence. In die casting, the Zn content together with magnesium is occasionally reduced in parts prone to hot cracks.
titanium
Titanium is mainly used in aluminum alloys as a grain refiner up to max. 0.15% added. In sand and chill casting alloys, it is a grain-refining alloy component.
lead
In the solid state, lead is undissolved in the form of fine drops. Within the tolerance limits (<0.1%), Pb does not noticeably influence the alloy properties.
tin
With a content of more than 0.02%, tin separates along the grain boundaries and has a very negative effect on hot cracking behavior if the normal tolerance is exceeded - especially in die casting.
Comparison of die casting and injection molding
The investment costs for casting machines and the high production costs for the mold are roughly comparable. In both processes, high numbers of items must justify these high investment costs. After that, the raw material costs have priority.
Die-cast metal components have a significantly higher flexural strength (rigidity) than injection-molded ones; it can be up to 20 times higher. The workpieces produced in die casting can still be used at higher temperatures (aluminum up to approx. 450 ° C). In the case of injection-molded materials, the strength and rigidity decrease sharply at higher temperatures (from 100 ° C); the only exceptions are expensive special plastics. Another advantage is that when the components (with the exception of zinc ) are stressed, there is no creep, as is the case with many injection-molded workpieces. Die-cast parts have a significantly better structural strength , that is, a machined surface is a surface wherein the surface of a plastic part substantially more easily deformed. Furthermore, some injection molded plastics change shape under climatic conditions. Die-cast materials have electromagnetic shielding and are resistant to organic solutions. In addition, recycling is possible without any loss of quality.
Injection molded components are cheaper when using standard materials. Color can also be used for injection molding. Another advantage is the lower weight compared to die-cast workpieces. The latter also have poorer corrosion properties. Furthermore, the production of die-cast metal components is more complex and “complicated” geometries cannot be implemented in some cases.