GB2066853A - Zinc-based alloys - Google Patents

Abstract

A Zn-based alloy containing Al and Mn, in which the Al and Mn contents, on a graph of % Al versus % Mn lie below a curve consisting of two straight lines joining the points having the following coordinates: (0.025% Mn, 1.7% Al), (0.3% Mn, 0.7% Al), (0.8% Mn, 0.4% Al). The alloy optionally contains 0.5-1.5% Cr and/or 0.01-0.06% Mg. The alloy has good creep resistance and low aggressivity to steel. <IMAGE>

Description

SPECIFICATION Zinc-based alloys The present invention relates to creep-resistant zinc-based alloys. In the following description all percentages are by weight.

The field of use of zinc-based alloys nowadays covers numerous applications in widely varying industrial sectors, the zinc alloys being rolled, formed, or cast. As a result of the incorporation of one or more alloying elements in zinc, even in very small quantities, it is possible to obtain alloys having very different characteristics, which make them suitable for a particularly wide range of applications.

It is well known that the addition of manganese to zinc increases the creep resistance of the alloy obtained. However, it is not possible to use this binary alloy in die casting machines, given its aggressivity in the liquid state with respect to steel. In order to obviate this drawback, it has been proposed to include a certain amount of aluminium in this alloy, which has the effect of decreasing this aggressivity.

It has also been observed that the simultaneous presence of Mn and Al in the zinc leads in some cases to alloys having a marked heterogeneity, which alloys are therefore unacceptable. In other words, the decrease in aggressivity due to the presence of aluminium is accompanied by the appearance, in certain cases, of a heterogenous structure and even of Al-Mn compounds, which are undesirable from the point of view of resistance to creep at high temperatures.

What is desired is a zinc-based alloy having both a good creep resistance in the solid state, and a low aggressivity (in the liquid state) to steel.

The present invention provides zinc-based alloys containing aluminium and manganese in proportions such that their contents, shown in a graph giving their Al contents in percentages on the Y-axis, and their Mn contents in percentages on the X-axis, are located in the zone located below the curve constituted by the two straight lines, one of which connects the coordinate point "0.025% (Mn) - 1.7% (Al)" to the coordinate point "0.3% (Mn) -0.7% (Al)" and the other of which connects the coordinate point "0.3% (Mn) - 0.7% (Al)" to the coordinate point "0.8% (Mn) - 0.4% (Al)", the balance being constituted by zinc and its normal impuritics. Preference is given, however, to alloys whose manganese content is lower than 0.3% and whose aluminium content is lower than 0.8%.

The accompanying drawing illustrates the above-mentioned graph, showing the said curve.

It has been observed that the Zn-Al alloys which satisfy the above conditions have a good resistance to creep at high temperatures and a low aggressivity with respect to the steel of die casting machines, and they do not have a heterogeneous structure.

Alloys in accordance with the present invention have properties which include, with an acceptable tolerance, values which are satisfactory both for resistance to creep at high temperatures and for low aggressivity in respect of the steel of the tools with which the alloy will come into contact when molten.

By way of comparison, the properties of various zinc alloys, having various contents of different alloying elements are considered below.

1) Alloy containing 0.75% Al and 0.25% Mn: good resistance to creep at high temperatures (194 H at 9 kg/mm2 at 1 000C); low aggressivity with respect to steel.

2) Zamak 5 (4% Al -- 1% Cu - 0.4% Mg): poor resistance to creep at high temperatures (4h at 9 kg/mm2 at 1000C).

3) Ilzro 14 (1.25% Cu - 0.17% Ti): good resistance to creep at high temperatures (250h at 9 kg/mm2 at 1 000C); very aggressive in the liquid state with respect to steel and therefore unsuitable for die casting.

In order to evaluate the aggressivity of the alloys with respect to steel, the weight loss of a testpiece of USN 2343 steel (tool steel - 0.38% C -- 1% Si - 0.4% Mn - 5.3% Cr - 1.1% Mo 0.4% V) was evaluated, this test-piece being immersed for 24 hours in different molten alloy baths at the temperature of use of the alloy in die casting (approximately liquidus temperature + 250C).

Results of the Weight Loss Tests Zamak 5: 4100C 1.60 x 106g/mm2 Zamak 5: 4350C 7.9 x 10-6g/mm2 llzro 14: 4450C 7.4 x 10-3 g/mm2 Zn-0.75%Al-0.25%Mn: 4500C 3.1 x 10-5g/mm2 Zn - 1% Al - 0.07% Mn: 4450C 2.6 x 10-5 g/mm2 Zn0.09% Al0.5% Mn: 4450C 3.2 x 10-4 g/mm2.

In accordance with this test, Zn - 0.75% Al - 0.25% Mn caused a weight loss only 20 times greater than that of Zamak but still more than 200 times better than llzro 14, which we consider to be acceptable for use in a die casting machine.

It has also been observed that the addition of 0.5% to 1.5% of Cu and 0.01% of Mg, not only does not modify the aggressivity of the Zn-Al-Mn alloys, and in particular the alloy Zn - 0.75% Al - 0.25% Mn, but also has a known beneficial effect. The beneficial effect of these elements in the zinc alloys is well known, in particular in respect of their hardening effect: increase in tensile strength, toughness and hardness.

The addition of 1% Cu causes an increase of approximately 20% in toughness and approximately 35% in tensile strength. In addition, it is evident that the mechanical properties of an alloy cast under pressure depend on the casting conditions. By way of example, we cite the following properties, for casting conditions corresponding to a mould temperature of 600 C, a molten bath temperature of 4450 C, and test-piece corresponding to the ASTM test-piece (Zn - 0.75% Al - 0.25% Mn % Cu - 0.04% Mg): Tensile strength: 246 MiPa Elastic limit (0.2% proof stress): 1 95 MPa Elongation at fracture: 4.7% Vickers hardness: 109 Toughness: 48 J/cm2 The effect of the Mg is similar to that of the copper, but its effect is obviously less in an alloy already containing Cu than in an alloy which does not contain Cu. In an alloy which already contains Cu, the expected effect of the addition of Mg is a decrease in sensitivity to intercrystalline corrosion, rather than an improvement in mechanical properties.

Claims (3)

1. A zinc-based alloy containing aluminium and manganese, the aluminium and manganese contents, when represented on a graph of wt.% Al on a vertical axis versus wt.% Mn on a horizontal axis, lying in the zone located below a curve consisting of the straight line connecting the points having the coordinates (0.025% Mn, 1.7% Al) and (0.3% Mn, 0.7% Al) and the straight line connecting the points having the coordinates (0.3% Mn, 0.7% Al) and (0.8% Mn, 0.4% Al), optionally 0.5 to 1.5 wt.% Cu, and optionally 0.01 to 0.06 wt.% Mg, the balance being zinc and normal impurities.

2. An alloy as claimed in claim 1, in which the manganese content is lower than 0.3 wt.% and the aluminium content is lower than 0.8 wt.%.

3. An alloy having the following composition by weight: 0.75% Al, 0.25% Mn, 1% Cu, 0.04% Mg, balance zinc.