Refractory Metal Inserts and Cores for Die Casting

Where a steel core cannot carry the heat, the die casting industry has a second family of materials to reach for. Tungsten base and molybdenum base alloys conduct heat several times faster than steel and expand at about half the rate, and that combination is what gives them their resistance to thermal fatigue, to soldering and to erosion. This page sets out the two families, their composition, how they are made, what they will and will not tolerate, and the one nickel base family that works elsewhere but has failed in pressure die casting.

Why a refractory metal comes up at all

The attraction is not strength. It is the two thermal properties together. The thermal conductivity of these alloys is several times higher than that of steel, so heat is carried away from the face faster, and their thermal expansion is about half, so the face moves less against the constraint of the holder. The combined effect leads to good resistance to thermal fatigue, to soldering and to erosion. Their use runs from cores in aluminium die casting right through to complete inserts in dies for the higher melting point brass and ferrous casting alloys. The high thermal conductivity also means that for most core applications water cooling is not required. They are more costly than hot work steels, and the source is clear that they would not be recommended where a steel part is already functioning satisfactorily.

The tungsten base alloys

The tungsten base alloy developed by the Mallory Company in America in the 1960s is known as Anviloy. In Britain it is marketed by Johnson Matthey Metals and known as Matthey 4000. Belonging to a family generally referred to as tungsten base high density composites, these alloys are made by powder metallurgy because of the high melting points involved. Intimate mixtures of fine powders are compacted, either isostatically or in closed dies, and a sintering treatment at about 1500 C then densifies the structure through the interaction of the nickel and iron liquid phase with the tungsten and molybdenum. What comes out is a uniform structure of tungsten grains surrounded by a nickel, iron and molybdenum matrix, with an alloy density above 90 percent. No heat treatment is necessary for these alloys. The lower tungsten content of the 4100 alloy gives it additional ductility.

Nominal composition of the two tungsten base high density composites, in weight percent. The lower tungsten content of the 4100 alloy confers additional ductility.

The molybdenum base alloys

The two molybdenum alloys in the table below contain over 99 percent molybdenum and were developed by the Climax Molybdenum Company. The Mo 0.5 Ti grade has a slightly lower strength than TZM. Production involves compaction of metal powder, sintering and casting under vacuum. There is no phase change to work with, so heat treatment is unnecessary. Hardness and strength are developed instead by strain hardening, by solid solution hardening from the alloying elements dissolving in the molybdenum, and by dispersion hardening from the precipitation of complex carbides. At about 650 C their conductivity is about eight times that of steel, and at 1000 C it is about four times.

Nominal composition of the two molybdenum base alloys, in weight percent. Both contain over 99 percent molybdenum, and in both the balance of the analysis is molybdenum.

The two families against H13

The properties that make these materials worth their cost are set out below against the hot work steel they would replace. Read them together. Neither family is hardened or tempered, so there is no heat treatment to control and no distortion to allow for, and the tungsten alloys hold a tensile strength comparable with H13 up to about 650 C and exceed it above that. The molybdenum alloys are stronger and harder than H13 at 650 C.

Refractory metal die materials compared with H13 hot work steel. The conductivity and expansion columns are relative, as published in the source.

What to watch when you fit them

A refractory alloy part sits inside a steel holder, and the two do not move together. Because the thermal expansion of the refractory alloy differs from that of the steel around it, clearances need to be adjusted to keep the die operating smoothly. To get the full benefit of the high heat transfer rate, the back face of the insert or core and its matching face in the steel holder must be clean and smooth, so that the two make firm contact. These alloys are also more notch sensitive than die steel, so sharp inside corners and notches should be avoided and generous radii provided wherever possible. On machining, tungsten base alloys can be cut with tungsten carbide tipped tools, but tungsten is removed only slowly by spark machining and with high electrode wear, so conventional machining is used as much as possible. Welding of molybdenum base die inserts should be reduced or avoided altogether.

The nickel base family that works elsewhere but not here

The Nimonic alloys were developed during the Second World War for jet aircraft. They are based on the electric resistance wire composition of 80 percent nickel and 20 percent chromium, with other elements added. Nimonic 75 carries about 0.4 percent titanium and 0.1 percent carbon, and in Nimonic 80a and others in the series precipitation hardening takes place through a complex nickel, aluminium and titanium phase. Nimonic 75 and 80a are used in permanent mould casting of aluminium bronze for cores and die inserts. In pressure die casting the record is the opposite. Every test with the Nimonics in pressure die casting dies has failed to show worthwhile results, largely because of grain boundary attack.

Where to go next

The part of the die these materials are fitted into is covered on the die casting die component materials page, and the cores and inserts they replace on the core and insert selection page. The steel grades chosen by casting metal are on the die casting die steel page, and the H13 the tables above are measured against on the H13 tool steel page. The thermal properties of the hot work grades are set out on the hot work thermal constants page, and the properties of the molybdenum bearing hot work grades on the H21 page. How a die softens and cracks in service is on the thermal fatigue cracking page, the cooling design on the die cooling line design page, the mould life question for the lower pressure processes on the permanent mould life page, and the broad failure picture on the die wear and failure page. The choice of steel by die casting process is on the high pressure die casting selection page.

Before you use this as a material specification

This page is a reference summary of a published source and it is not an Aobo Steel specification. The compositions and the property statements are reproduced from the source, which describes grades sold under several trade names. Refractory metal parts are made to supplier specification and are outside the tool steel range we stock. Final specification is confirmed on the job.

Source, Die Casting Metallurgy, A. Kaye and A. Street, Butterworths Monographs in Materials, 1982, Chapter 25, Pressure die casting dies.