Magnesium alloy and heat treatment method thereof

Abstract

A magnesium alloy having excellent castability and creep properties has compositions of 1.0-6.0 wt % Zn, 0.5-3.0 wt % Ca, 1.0 wt % or less Zr, 1.0-5.0 wt % at least one lanthanoid, the remainder being Mg and unavoidable impurities. This magnesium alloy undergoes heat treatment of heating the magnesium alloy to 430-470 ° C., quenching, and then heating to 150-250 ° C. for aging. Hot tearing and temperature strength are improved by the addition of an element which is effective for causing eutectic reaction and peritectic reaction with Mg and making Mg particles divied finely.

Description

1. FIELD OF THE INVENTION[0001] The present invention relates to a magnesium (Mg) alloy habing excellent castability for die casting and creep characteristics, which contains Zn, Ca, Zr and at least one rare earth element in predetermined ratios, as the essential components, and a heat treatment method for further improving mechanical properties (particularly hardness, creep strength and fatigue strength) of a magnesium alloy.2. DESCRIPTION OF THE RELATED ART[0002] In recent years, needs for a light-weight material are increasing. In paticular, a magnesium alloy, the smallest density of practical alloys is noted as materials for aircrafts and automobiles.[0003] However, due to poor high-temperature strength of this magnesium alloy or in order to increase heat resistance thereof, expensive elements such as yttrium (Y) or silver (Ag) are added thereto. As a result, such an alloy becomes a very expensive alloy and also hot teating is liable to occur where it is used for high pressure d...

AI Technical Summary

Benefits of technology

[0013] According to a first aspect of the present invention, there is provided a magnesium alloy having excellent high pressure die casting properties and creep properties, comprising: calcium(Ca) in the content range of 0.5 to 3.0 wt %; zinc(Zn) in the content range of 1.0 to 6.0 wt %; zirconium(Zr) in the content range of 1.0 wt % or less (exclusive of 0 wt %); at least one rare earth element in the content range of 1.5 to 2.7 wt %; the remainder being magnesium; and unavoidable impurities.

[0018] As a result of various investigations to solve the above problem, the present inventors have found that casting crack can be suppressed if an element which causes eutectic reaction with Mg and an element which has an effect for finely dividing Mg particles and causes peritectic reaction with Mg are added in combination, to the Mg--Zn--Ca alloy. Through further investigations on these additives, the inventors have come to know that rare earth elements which have a relatively low solubility (Lanthanoids) to Mg, e.g. lanthanum (La), cerium (Ce), praseodymium (Pr) neodymium (Nd) and misch metal, a mixture of those elements are effective in eutectic reaction. They also discovered that zirconium (Zr) is very effective in causing peritectic reaction, which finely divides and spheroidize crystal particles.

[0020] The magnesium alloy of the present invention improves formability of hot tear and high temperature strength of the conventional Mg--Zn--Ca alloy. The decrease of formability of hot tear is considered to be due to that an appropriate increase of eutectic compound, narrowed solidification temperature, and finely dividing a --Mg particles are acted cooperatively. The increase of machaical properties is speculated to be caused by fine a --Mg particles solid solution hardening and precipitation hardening.

Problems solved by technology

However, due to poor high-temperature strength of this magnesium alloy or in order to increase heat resistance thereof, expensive elements such as yttrium (Y) or silver (Ag) are added thereto.

As a result, such an alloy becomes a very expensive alloy and also hot teating is liable to occur where it is used for high pressure die casting.

Thus, the magnesium alloy is used only for very limited purposes.

On the other hand, usual Mg-R.E. based alloys such as EZ33 undergo T5 treatment because it is difficult to complete a solution heat treatment to them conducted.

However, sufficient high temperature strength cannot be imparted to general magnesium alloys which do not contain Ca, such as Ze41 or EZ33.

On the other hand, heat treatment to the magnesium alloys was not sufficiently investigated in JP-A-6-25791 and 6-200348, and failed making such alloys sufficiently exhibit their characteristics.

And, those alloys have the disadvantages that hot tearing tends to occur in die casting, and therefore it is difficult to conduct die casting.

Further, creep properties of those alloys are good as compared with other magnesium alloys, but are poor as compared with the conventional alloys having added thereto expensive elements such as yttrium.

However, the above-mentioned Mg--Zn--Ca alloys in the related arts are liable to cause hot tear, and it is difficult to apply the alloys to high pressure die csting.

If the Zn content is less than 1% by weight with respect to a total alloy weight which is hereinafter refered to as % by weight or wt %, the effect thereof is insufficient, and hot tear is liable to occur.

On the other hand, if the Zn content exceeds 6% by weight, creep properties are deteriorated.

If the Ca content is less than 0.5% by weight, the alloy cannot sufficiently reinforced.

On the other hand, if the Ca content exceeds 3.0% by weight, elongation is decreased and also many hot tearing occur in high pressure die casting.

However, if the Zr content exceeds 1% by weight, a melting poingt of the alloy becomes higher, so that zirconium does not disperse uniformly, which is industrially unfavorable.

If the total content of the rare earth element is less than 1.5% by weight, an effect for suppressing hot tear is small, and on the other hand, if the total content thereof exceeds 5% by weight, the alloy becomes brittle due to increase of the eutectic compound.

Incidentally, if the proportion of aluminum exceeds 2.0% by weight, creep properties may deteriorate.

On the other hand, if the temperature exceeds 470.degree. C., a low temperature melting portion re-melts, and defects are formed.

Further, in order to sufficiently dissolve the compounds in the matrix, 5 hours or more are necessary, and the solubility substantially reaches the dissolution limit in 24 hours.

If the aging temperature is less than 150.degree. C., it takes long time for hardening.

Further, toughness of the alloy decreases as the amount of Ca added increases.

However, if Zn is added in an amount exceeding 6% by weight, a large amount of compounds precipitates in the alloy, resulting in decrease of toughness of the alloy.

However, even if Zr is added in an amount exceeding 1% by weight, it is difficult for Zr to dissolve in the alloy.

If rare earth element to be added exceeds 5% by weight, not only the effect of improving the castability is saturated, but also reaction between elements and a large amount of compounds decreases the toughness of the alloy.

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