Magnesium Alloy - Hot Extrusion

Hot Extrusion

Technical difficulties in the removal of chloride inclusions from the manufactured metal delayed progress in the development and engineering uses of magnesium and its alloys in the period when a great advance was being made in the application of aluminium base alloys. With the eventual solution of this problem the way became clear, and an impetus was given to the discovery and exploitation of new alloys, which, once again, has been reinforced by the requirements of the aircraft and nuclear power industries in the current period.

Magnesium's particular merits are similar to those of aluminium alloys: low specific gravity with satisfactory strength. Magnesium provides advantages over aluminum, in being of even lower density (approx. I8) that aluminum (about 2.8). Mechanical properties of magnesium alloys, however, are below those of the strongest of the aluminium alloys.

A particular attraction of magnesium alloys lies in their extraordinary good machining properties, in which respect they are superior even to screwing brass. There is, perhaps, no group of alloys where extrusion is more important than it is to these—since the comparatively coarse-grained structure of the cast material makes most of them too susceptible to cracking to work by other means until sufficient deformation has been imparted to refine the grain. Therefore, except for one or two soft alloys, machining is invariably a preliminary step beforeo other shaping processes.

Not much pure magnesium is extruded, for it has somewhat poor properties, especially as regards its proof stress. The alloying elements of chief concern at present are aluminium, zinc, cerium and zirconium; manganese is usually also present since, though it has little effect on the strength, it has a valuable function in improving corrosion resistance. One important binary alloy, containing up to 2.0 per cent manganese, is used extensively for the manufacture of rolled sheet. It is comparatively soft and easier to extrude than other alloys, and is also one of the few that can be rolled directly without pre-extrusion. In the UK, extrusions are made from billets of 2.87-12 in. dia. On presses varying in power over the range 600-3500 tons; normal maximum pressures on the billet are 30-50 tons/sq. in the U.S the Dow chemical company have recently installed a 13.200 ton press capable of handling billets up to 32 in. Extrusion technique is generally similar to that for aluminum base alloys but, according to Wilkinson and fox, die design requires special consideration and, in their opinion, should incorporate short bearing lengths and sharp die entries. Tube extrusion in alloys AM503, ZW2, and ZW3 is now made with bridge dies. (The aluminium-bearing alloys do not weld satisfactorily.) In contrast to the previous practice of using bored billets, mandrel piercing is now used in the extrusion of large diameter tubes in ZW3 alloy.

The stiffness of the alloys towards extrusion is increased in proportion to the amount of hardening elements they contain, and the temperature employed is generally higher the greater the quantity of these. Billet temperatures are also affected by the size of the sections, being higher for heavy reductions, but are usually in the range 250-450 C. Container temperatures should be identical with, or only slightly higher than billet temperature. Pre-heating of the billets must be carried out uniformly to promote as far as possible a homogeneous structure by absorption of compounds, such as Mg4Al, present in the alloys.

Fox points out and this is also applicable to aluminium alloys. The initial structure of the billetis important, and casting methods that lead to fine grain are worthwhile. In coarse material, larger particles of the compounds are present that are less readily dissolved, and tend to cause a solution gradient. In magnesium alloys, this causes internal stress, since solution is accompanied by a small contraction, and it can also influence the evenness of response to later heat treatment.

The binary magnesium-manganese alloy (AM505) is readily extruded at low pressures in the temperature range 250 to 350 C., the actual temperature used depending upon the reduction and billet length rather than the properties desired, which are relatively insensitive to extrusion conditions. Good surface condition of the extrusion is achieved only with high speeds, of the order of 50–100 ft. per minute.

With the aluminium and zinc containing alloys, and particularly those with the higher aluminium contents such as AZM and AZ855 difficulties arise at high speeds due to hot-shortness. Under conditions approaching equilibrium magnesium is capable of dissolving about 12 per cent aluminium, but in cast billets 4-5 per cent usually represents the limit of solubility. Alloys containing 6 per cent Al or more therefore contain Mg4Al3, which forms a eutectic melting at 435 C. The extrusion temperature may vary from 250 to 400 C, but at the higher values speeds are restricted to about 12 ft. per minute. Continuous casting improves the homogeneity of these alloys and water cooling of the dies or taper heating of the billets further facilities their extrusion.

Introduction of the magnesium-zinc-zirconium alloys, ZW2 and ZW3, represents a considerable advance in magnesium alloy technology for a number of reasons. They are high strength, but, since they do not contain aluminium, the cast billet contains only small quantities of the second phase. Since the solidus temperature is raised by about 100 C., the risk of hotshortness at relatively high extrusion speeds is much reduced. However, the mechanical properties are sensitive to billet preheating time, temperature and extrusion speed, Long preheating times and high temperatures and speeds produces properties similar to those in older aluminium-containing alloys, Heating times must be short and temperatures and speeds low to produce high properties. Increasing zinc content to 5 or 6 per cent, as in the American alloy ZK60 and ZK61, reduces sensitivity to extrusion speed in respect of mechanical properties.

Alloying of zirconium-bearing materials has been a major problem in their development. It is usual to add the zirconium from a salt—and careful control can produce good results. Dominion Magnesium Limited in Canada have developed a method adding in the conventional manner through a master alloy.

Explanation for the low extrusion rates necessary to successfully extrude some magnesium alloys does not lie outside reasons put forward for other metals. Altwicker considers that the most significant cause is connected. With the degree of recovery from crystal deformation, which is less compete when work is applied quickly, causing higher stresses and the exhausting of the capacity for slip in the crystals. This is worthy of consideration, for the speed of re-crystallization varies from one metal to another, and according to temperature. It is also a fact that a metal worked in what is considered its working range can frequently be made to show marked work hardening if quenched immediately after deformation—showing that temporary loss of plasticity can easily accompany rapid working.