Die Casting of Aluminum Alloys: Part Two

Sumário:

Aluminum die casting alloys are lightweight, offer good corrosion resistance, ease of casting, good mechanical properties and dimensional stability.
A comparison of nine parameters of die-casting versus stamping, forging, sand casting, permanent mold casting and plastic molding is made in this article.

The Most Frequently Used Die Castings Techniques

Gravity Casting constitutes of a classical mould for gravity die casting. Cores (inner parts of the mould) are generally made of bonded sand. Gravity die casting is suitable for mass production and for fully mechanized casting.

In the process of High Pressure Die Casting, the liquid metal is injected at high speed and high pressure into a metal mould. This equipment consists of two vertical platens on which bolsters are located which hold the die halves. One platen is fixed and the other can move so that the die can be opened and closed. A measured amount of metal is poured into the shot sleeve and then introduced into the mould cavity using a hydraulically-driven piston. Once the metal has solidified, the die is opened and the casting removed.

In this process, special precautions must be taken to avoid too many gas inclusions which cause blistering during subsequent heat-treatment or welding of the casting product. Both the machine and its dies are very expensive, and for this reason pressure die casting is economical only for high-volume production.

In Low Pressure Die Casting the die is filled from a pressurized crucible below, and pressures of up to 0.7 bar are usual. Low-pressure die casting is especially suited to the production of components that are symmetric about an axis of rotation. Light automotive wheels are normally manufactured by this technique.

The principle of Vacuum Die Casting is the same as low-pressure die casting. The pressure inside the die is decreased by a vacuum pump and the difference of pressure forces the liquid metal to enter the die. This transfer is less turbulent than by other casting techniques so that gas inclusions can be very limited. As a consequence, this new technique is specially aimed to components which can subsequently be heat-treated.

In Squeeze Casting or Squeeze Forming, liquid metal is introduced into an open die, just as in a closed die forging process, and the dies are then closed. During the final stages of closure, the liquid is displaced into the further parts of the die.

No great fluidity requirements are demanded of the liquid, since the displacements are small. Thus forging alloys, which generally have poor fluidities which normally precludes the casting route, can be cast by this process.

A Comparison of Die-Casting with Other Technologies

The comparison of nine parameters of die-casting versus stamping, forging, sand casting, permanent mold casting and plastic molding shows the following.

Compared with forgings, die casting can be more complex in shape and have shapes not forgeable, can have thinner sections, be held to closer dimensions, and have coring not feasible in forging.

Compared with plastic injection moldings, die casting are stronger, stiffer, more stable dimensionally, more heat resistant, and are far superior to plastics on a properties/coat basis. They help prevent radio frequency and electromagnetic emissions. For chrome plating, die castings are much superior to plastics.

Die castings have high permanence under load compared to plastics, are completely resistant to ultra-violet rays, weathering, and stress-cracking in the presence of various reagents. Manufacturing cycles for producing die castings are much faster than for injection moldings.

Compared to extrusions, die casting can be produced faster and closer to net shape. Features and depressions on the sides, tops and bottoms can be created in one operation. There is less waste using die casting than extrusion. Holes can be cast in place to save additional machining cost.

Compared with stampings, one die casting can often replace several parts. Die casting frequently require fewer assembly operations, can be held within closer dimensional limits, can have almost any desired variation in section thickness, involve less waste in scrap, and are producible in more complex shapes. Die castings can be made in many shapes not producible in stamped form.

Compared with screw machine products, die castings are produced more rapidly, involve much less waste in scrap, can be made into shapes that are difficult or impossible to produce from bar or tubular stock, and may require fewer operations.

Compared with sand castings, die castings require much less machining, can be made with thinner walls, can have all or nearly all holes cored to size, can be held within much closer dimensional limits, and are produced more rapidly in dies which make thousands of die castings without replacement. Die castings do not require new cores for each casting, are easily provided with inserts die cast in place, have smoother surfaces and involve much less labor cost per casting.

Compared with permanent mold castings, die castings can be made to closer dimensional limits and with thinner sections and holes can be cored to near net shape. Die castings are produced at higher rates with less manual labor, have smoother surfaces, and usually cost less per die casting.

The Properties of Die-Casing Alloys

Aluminum die casting alloys are lightweight, offer good corrosion resistance, ease of casting, good mechanical properties and dimensional stability. Although a variety of aluminum alloys can be die cast from primary or recycled metal, designers frequently select common standard alloys listed below. Special alloys for special applications are available but their use usually involves significant cost premiums.

A360 -- Selected for best corrosion resistance and pressure tightness.
A380 -- The most common and cost effective of all die casting alloys, provides a good combination combination of utility and cost.
A383 and A384 -- These alloys are a modification of 380. Both provide better die filling but with a moderate sacrifice in mechanical properties such as toughness.
A390 -- Selected for special applications where high strength, fluidity and wear-resistance/bearing properties are required.
A413 (A13) -- Used for maximum pressure tightness and fluidity.

The most frequent failures of dies can generally be divided into four basic groups: heat checking, corner cracking, sharp radii or sharp edges cracking, and cracking due to wear or erosion. It is generally agreed that one of the principal causes of termination of die life is heat checking, which occurs through a process of crack initiation and propagation induced by the thermal stress fatiguing of a die surface.

Some of the factors that affect die failures may be controlled to some extent by the die-casting experts (designers, manufacturers and operators). These factors include:

  • design,
  • materials selection,
  • heat treatment,
  • finishing operations, and
  • handling and use.

In the process of the die-casting the primary source of loading is cyclic variation of the temperature; the influence of other loads is relatively insignificant. Therefore in the first stage a solution of the problem should be in changing of the position of heating and/or cooling channels, i.e. their closer shifting to the working surface of the die, so the higher and more homogeneous heating should be reached.

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