Magnesium Alloy Castings

Extrait:

Magnesium alloy castings can be produced by nearly all of the conventional casting methods, namely, sand, permanent, and semi permanent mold and shell, investment, and die-casting. The choice of a casting method for a particular part depends upon factors such as the configuration of the proposed design, the application, the properties required, the total number of castings required, and the properties of the alloy. Magnesium castings of all types have found use in many commercial applications, especially where their lightness and rigidity are a major advantage, such as for chain saw bodies, computer components, camera bodies, and certain portable tools and equipment. Magnesium alloy sand castings are used extensively in aerospace components.

Magnesium alloy castings can be produced by nearly all of the conventional casting methods, namely, sand, permanent, and semi permanent mold and shell, investment, and die-casting. The choice of a casting method for a particular part depends upon factors such as the configuration of the proposed design, the application, the properties required, the total number of castings required, and the properties of the alloy. Magnesium castings of all types have found use in many commercial applications, especially where their lightness and rigidity are a major advantage, such as for chain saw bodies, computer components, camera bodies, and certain portable tools and equipment. Magnesium alloy sand castings are used extensively in aerospace components.

Designation system of magnesium alloy castings

The method of codification used to designate magnesium alloy castings is taken from ASTM Standard Practice B 275 (Table 1). It gives an immediate, approximate idea of the chemical composition of an alloy, with letters representing the main constituents and figures representing the percentages of these constituents.

Generally speaking magnesium alloy designation consists of three parts:

Part 1: Indicates the two principal alloying elements and consists of two code letters representing the two main alloying elements arranged in order of decreasing percentage (or alphabetically if percentages elements and arranged in are equal). These letters are the following: A-Aluminum, E-Rare Earth, H-Thorium, K-Zirconium, M-Manganese, Q-Silver, S-Silicon, T-Tin and Z-Zinc.

Part 2: Indicates the amounts of the two principal elements and consists of two whole numbers corresponding to alphabet.

Part 3: Distinguishes alloys with the same percentages of the two principal alloying elements. It consists of the one of the following letters: A-First compositions, B-Second compositions, C-Third composition registered ASTM, D-High-purity and E-High corrosion resistance.

As an example, consider the three alloys AZ91A, AZ91B, and AZ91C. In these designations:

  • A represents aluminum, the alloying element specified in the greatest amount
  • Z represents zinc, the alloying element specified in the second greatest amount
  • 9 indicates that the rounded mean aluminum percentage lies between 8.6 and 9.4
  • A as the final letter in the first example indicates that this is the first alloy whose composition qualified assignment of the designation AZ91
  • The final serial letters B and C in the second and third examples signify alloys subsequently developed whose specified compositions differ slightly from the first and from one another but do not differ sufficiently to effect a change in the basic designation.

Casting processes

Sand Casting. Magnesium alloy sand castings are used in aerospace applications because they offer a clear weight advantage over aluminum and other materials. A considerable amount of research and development on these alloys has resulted in some spectacular improvements in general properties compared with the earlier AZ types.

Although there has been, and still is, a large volume of castings for aerospace applications being produced in the older, conventional AZ-type alloys, the trend is toward the production of a greater proportion of aerospace castings in the newer zirconium types.

Although the magnesium-aluminum and magnesium-aluminum-zinc alloys are generally easy to cast, they are limited in certain respects. They exhibit microshrinkage when sand-cast, and they are not suitable for applications in which temperatures of over 95°C are experienced. The magnesium rare earth-zirconium alloys were developed to overcome these limitations. Sand castings in the EZ33A alloy do in fact show excellent pressure tightness. The greater tendency of the zirconium-containing alloys to oxidize is overcome by the use of specially developed melting processes.

The two magnesium-zinc-zirconium alloys originally developed, ZK51A and ZK61A, exhibit high mechanical properties, but suffer from hot-shortness cracking and are nonweldable.

For normal, fairly moderate temperature applications (up to 160°C), the two alloys ZE41A and EZ33A are finding the greatest use. They are very castable and can be used to make very satisfactory castings of considerable complexity. In addition, they have the advantage of requiring only a T5 heat treatment (that is, precipitation treatment).

When a demand arose in some aerospace engine applications for the retention of high mechanical properties at higher elevated temperatures (up to 205°C), thorium was substituted for the rare earth metal content in alloys of the ZE and EZ type, giving rise to the alloys of the type ZH62A and HZ32. Not only were there substantial improvements in mechanical properties at elevated temperatures in these alloys, but good castability and welding characteristics also were retained. The thorium-containing alloys, however, exhibited a greater tendency for oxidation, requiring greater care in meltdown and pouring.

A further development aimed at improving both room-temperature and elevated-temperature mechanical properties produced an alloy designated QE22A. In this alloy, silver replaced some of the zinc, and the high mechanical properties were obtained by grain-refinement with zirconium and by a heat treatment to the full T6 condition (that is, solution heat treated, water quenched, and precipitation aged). However, problems were experienced with both of these alloys. The use of thorium has become increasingly unpopular environmentally, and the price of silver has become very unstable in recent years. Hence, there has been a considerable amount of research and development work on alternative alloy types.

The most recent alloy emerging from this research was an alloy containing about 5.0% Y in combination with other rare earth metals (that is, WE54A), replacing both thorium and silver. This alloy has better elevated-temperature properties and a corrosion resistance almost as good as the high-purity magnesium-aluminum-zinc types (AZ91C). The alloys used for investment casting are very similar to those used for the sand casting process.

Permanent Mold Casting. In general, the alloys that are normally sand cast are also suitable for permanent mold casting. The exception to this are the alloys magnesium-zinc-zirconium type which exhibit strong hot-shortness tendencies and are consequently unsuitable for processing by this method.

Die Casting. The alloys from which die castings are normally made are mainly of the magnesium-aluminum-zinc type. Two versions of this alloy from which die castings have been made for a many years are AZ9lA and AZ9IB. The only difference between these two versions is the higher allowable copper impurity in the AZ91B.

The most important feature of magnesium castings, which gives rise to their preferred use compared with other metals and materials, is their lightweight. Because of this, magnesium castings have found considerable use since World War II in aircraft and aerospace applications, both military and commercial. More recently, as a result of a general requirement for lighter weight automobiles to conserve energy, there has been a growing use of magnesium in the automotive field, principally as die-castings.

Magnesium, however, has other important casting advantages over other metals:

  • It is an abundantly available metal
  • It is easier to machine than aluminum
  • It can be machined much faster than aluminum, preferably dry.
In the die casting process, it can be cast up to four times faster than aluminum. Die lives are considerably longer than with the aluminum alloys, because much less welding onto the die surfaces takes place. When protected correctly, particularly against galvanic effects, it behaves in a very satisfactory manner. Modern casting methods and the application of protective coatings currently available ensure long life for well-designed components.

Today`s state-of-the-art technology makes it possible to produce parts of considerable complexity having thin-wall sections. The end product has a high degree of stability as well as being light in weight.

Furnaces for melting and holding molten magnesium casting alloys are generally the indirectly heated crucible type, of a design similar to those employed for the aluminum casting alloys. The different chemical and physical properties of the magnesium alloys in comparison to aluminum alloys, however, necessitate the use of different crucible materials and refractory linings and the modification of process equipment design.

When magnesium becomes molten, it tends to oxidize and explode, unless care is taken to protect the molten metal surface against oxidation. Molten magnesium alloys behave differently from aluminum alloys, which tend to form a continuous, impervious oxide skin on the molten bath, limiting further oxidation. Magnesium alloys, on the other hand, form a loose, permeable oxide coating on the molten metal surface. This allows oxygen to pass through and support burning below the oxide at the surface. Protection of the molten alloy using either a flux or a protective gas cover to exclude oxygen is therefore necessary.

Molten magnesium does not attack iron in the same way as molten aluminum, and the metal can therefore be melted and held at temperature in crucibles fabricated from ferrous materials. It is common practice, therefore, especially with larger castings, to melt and process the molten magnesium alloy and to pour the casting from the same steel crucible.

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