ALUMINIUM CASTING ALLOY

Description

FIELD OF INVENTION

The invention relates to the field of metallurgy, namely, to aluminium-based alloys, and can be used in the production of castings of complex shapes by casting into a metal mould, mainly by injection moulding.

PRIOR ART

Complex castings are typically made from thermally non-hardenable and hardenable alloys, mainly based on Al—Si and Al—Mg systems. Castings produced from Al—Si base alloys with magnesium and/or copper additives, intended for highly critical components, are usually used after thermal processing to T7, T6, and T5 states to enhance their strength properties.

Well-known thermally non-hardenable Al—Si base alloys, such as A413.2 or AlSi11 alloys, are noted for their high castability and good corrosion resistance. Disadvantages of this group of alloys include low level of strength properties, in particular, yield strength typically in the as-cast state does not exceed 80 MPa. A higher level of strength properties of castings in the as-cast state is provided by the addition of copper, in particular, alloys of the AA383.1 or AlSi12Cu2 type are known. Disadvantages of these alloys include a significant reduction in corrosion resistance and a poor elongation of no more than 1-2%.

There are also known thermally non-hardenable casting alloys based on the Al—Mg system, such as AMg6L, AMg5K, AMg5Mz (GOST1583), Magsimal® 59 (Rheinfelden Alloys), among others, distinguished by satisfactory castability, good corrosion resistance and decent strength properties and elongation. High linear shrinkage and insufficiently good tightness of thin-walled castings should be highlighted among the disadvantages of the alloys of this system.

The combination of a high level of strength properties, elongation and corrosion resistance is implemented in alloys of the Al—Si system with an addition of 0.2-0.5 wt. % magnesium. Alloys of the AK9 type (GOST 1583), Silafont® 36 (Rheinfelden Alloys), Trimal® 37 (Trimet), etc. are particularly known. Quenching significantly complicates the technological cycle of obtaining castings, since it can cause warping of castings (especially when using quenching in water), changes in overall dimensions and the appearance of cracks.

A casting alloy of the Al—Ni—Mn system is as known intended for the production of structural components for automotive and aerospace applications, as well as an alternative to branded silumins, developed by Alcoa and disclosed in patent U.S. Pat. No. 6,783,730 B2 (publication date: Aug. 31, 2004). This alloy can be used to produce castings with a good combination of casting and mechanical properties in the case of (wt. %) 2-6% Ni, 1-3% Mn, 1% Fe, less than 1% silicon, as well as in the case of other unavoidable impurities. The disadvantages of the proposed alloy include the fact that the high level of casting and mechanical properties is ensured by using high-purity aluminium grades and with a high nickel content, thus significantly increasing the cost of the castings produced. Besides, the material proposed is thermally non-hardenable in the entire concentration range, thus limiting its use, while the corrosion resistance of castings is significantly reduced in the area of high nickel concentration.

Cast aluminium alloys based on Al—Ni and Al—Ni—Mn systems and a method of producing cast parts from them are known, which are described in Alcoa invention U.S. Pat. No. 8,349,462 (published on Jan. 8, 2013) and application EP2011055318 of Rheinfelden Alloys GmbH & Co. KG. The invention proposes alloy compositions for casting applications. Common in the proposed inventions is a high nickel content of 1-6%, which determines the main disadvantage-a significant decrease in corrosion resistance. With relatively poor nickel and manganese content, casting alloys show a low level of strength properties.

There is a known material based on the Al—Ni—Mn system proposed by the National University of Science and Technology ‘MISIS’ (Moscow Institute of Steel and Alloys) and disclosed in patent RU 2478131 (publication date: Mar. 27, 2013). The material contains (wt. %): 1.5-2.5 Ni, 0.3-0.7 Fe, 1-2 Mn, 0.02-0.2 Zr, 0.02-0.12 Sc and 0.002-0.1 Ce. Castings obtained from the alloy after annealing (without using the quenching operation) are characterised by an ultimate tensile strength of at least 250 MPa with an elongation of at least 4%. The first disadvantage of this alloy is its increased tendency to form concentrated porosity, which makes it difficult to achieve high-quality, relatively large castings. The second disadvantage is the need to use higher casting temperatures, which cannot always be achieved in the conditions of foundries.

There is a known material based on the Al—Ca system proposed by the National University of Science and Technology ‘MISIS’ (Moscow Institute of Steel and Alloys) and disclosed in patent RU 2660492. Material for use in the as-cast condition contains (wt. %): 5.4-6.4% calcium, 0.3-0.6% silicon and 0.8-1.2% iron. The disadvantages of the proposed alloy include low elongation that does not exceed 2.6%, thus limiting the use of the material in critical cast parts.

The alloy most closely analogous to the proposed alloy is invented by the Institute of Lightweight Materials and Technology, disclosed in claim RU 2745595. The material for use in the as-cast state contains, wt. %: 1.5-5.1% calcium, 0.1-1.8% zinc, up to 1.0% silicon and up to 0.7% iron. Disadvantages of the proposed alloy include poor yield strength in the as-cast state due to poor solubility of alloying elements, except for zinc, in solid solution and as a consequence insufficient solid-solution hardening.

Invention Disclosure

The invention is intended to provide a new aluminium casting alloy for producing castings mainly by high pressure, but not limited thereto, to be used without heat treatment, with good casting processability, good mechanical properties, including yield strength of no less than 100 MPa and high corrosion resistance.

The key application is casting for automotive engineering, electronic enclosures, etc. Parts for critical applications can be manufactured from this material.

This invention is technically aimed at ensuring high strength properties while maintaining plasticity, castability and high corrosion resistance.

The technical result is achieved by using an aluminium-based casting alloy containing calcium, silicon, iron, zinc, magnesium and, optionally, at least one element from the group including copper, manganese, chromium, titanium, and zirconium, with concentrations of alloying elements by weight, as specified below.

Optionally, the alloy contains at least one alloying element from the group:

The rest is aluminium and unavoidable impurities.

For a particular implementation of the invention, magnesium is arranged in an aluminium matrix and copper is bonded to calcium and forms a eutectic phase, thus providing enhanced strength properties without compromising ductility.

The alloy is applied for producing castings that exhibit the following tensile properties in the as-cast state: yield strength of no less than 100 MPa.

Various modifications and improvements are permitted without going beyond the scope of the disclosure of the invention as described and claimed.

SUMMARY OF THE INVENTION

The concentrations (wt. %) of calcium (2.0-5.2), silicon (0.05-0.8), iron (0.05-1.0), zinc (0.01-5.0) and copper (optionally 0.01-1.4) are limited to the specified limit to ensure forming a structure representing an aluminium solid solution and the corresponding eutectic phases containing calcium and the following elements: silicon, iron, zinc and optionally copper.

Calcium, silicon, iron, zinc, and optionally copper, influence the overall amount of eutectic phase in the alloy. At the minimum content (as specified) of calcium, silicon, iron, zinc and optionally copper, the eutectic content is approximately 2.5 vol. %.

The presence (wt. %) of magnesium (0.01-2.0) and, optionally, at least one element from the group including manganese (0.01-1.5), chromium (0.01-0.2), titanium (0.01-0.2), and zirconium (0.01-0.2) allow, when combined with the above elements (calcium, silicon, iron, zinc and, if present, copper), to form a structure representing an aluminium solid solution as a primary crystallising phase and a eutectic that contains at least one alloying element including manganese, chromium, titanium and zirconium.

Magnesium and, optionally, at least one of the elements including manganese, chromium, titanium and zirconium within the specified limits can enhance hardening through dissolution in the aluminium solid solution (solid solution hardening), while also increasing the crystallisation interval, adversely affecting casting properties.

Research has unexpectedly shown that the suitable combination of the eutectic amount regarding the considered concentration range of alloying elements, where all eutectic phases are related to calcium and aluminium solid solution alloying in combination with crystallisation interval up to 50° C. provides acceptable casting properties and hardening. The presence of magnesium and silicon contributes to the dispersion of eutectic phases containing calcium. A typical structure of a casting in the as-cast state (HPDC casting) is provided in FIG. 1.

FIG. 1 illustrates a typical alloy structure in the as-cast state, featuring the primary aluminium solid solution and the eutectic phases. The structure in the as-cast state is represented by an aluminium solid solution containing zinc, magnesium and eutectic phase particles that include compounds of aluminium, calcium with zinc, aluminium, calcium with iron and aluminium, calcium with silicon, in relation to the presence of certain elements in the alloy. In case of additional alloying with copper, manganese, chromium, titanium and zirconium, the structure in the as-cast state is qualitatively similar and consists of an aluminium solid solution containing zinc, magnesium manganese, chromium, titanium and zirconium, as well as eutectic phase particles with compounds of aluminium, calcium with zinc, aluminium, calcium with iron, aluminium, calcium with silicon and aluminium, calcium with copper.

The impact of alloying elements is detailed below.

Calcium content below 2.0 wt. % results in poor casting properties, failing to ensure the binding of elements such as silicon, iron, zinc and optionally copper with calcium. Calcium content above 5.2 wt. % results in the formation of coarse inclusions of the primary phase Al₄Ca, thus reducing mechanical properties.

Silicon content between 0.05 and 0.8 wt. % with calcium provides good elongation in the as-cast state, as silicon aids in dispersing the eutectic. At a silicon content above 0.8 wt. %, coarse intermetallics containing silicon form, consequently reducing mechanical properties. Below 0.05 wt. % silicon is not enough to form a eutectic with favourable morphology, resulting in inadequate elongation in the as-cast state.

Iron content between 0.05 and 1.0 wt. % with calcium enhances casting properties while maintaining an acceptable elongation level. Iron content below 0.05 wt. % worsens the alloy casting processability, as manifested by increased adhesion of the casting to moulds. Iron content above 1.0 wt. % forms coarse intermetallics of crystallisation origin containing iron and calcium, thereby reducing mechanical properties.

Zinc content between 0.01 and 5.0 wt. % enhances corrosion resistance and boosts casting properties. At zinc content below 0.01 wt. %: no beneficial impact of zinc on strength properties observed. From 0.01 wt. % onwards, there is a modification effect manifested as a change in the calcium-containing eutectic morphology. Zinc content above 5.0 wt. % forms coarse crystallisation-origin phases containing zinc and calcium, adversely affecting the alloy's mechanical properties.

Copper content (optionally) between 0.01 and 1.4 wt. % enhances strength properties without compromising casting performance and maintains corrosion resistance at an acceptable level. Satisfactory corrosion resistance with copper content is maintained by binding copper in a phase with calcium. At copper content below 0.01 wt. %: no positive impact of copper on mechanical or other properties observed. At low copper content starting from 0.01 wt. %, there is modification effect changing the morphology of eutectic phases containing calcium by forming phases with copper and calcium.

Magnesium content between 0.01 and 2.0 wt. % enhances the strength properties in the as-cast state. At magnesium content exceeding 2.0%, the crystallisation interval is significantly widened, thus unacceptably worsening casting properties, in particular, the hot-tearing tendency. At magnesium content below 0.01 wt. %: no positive impact on strength properties together with other elements within the specified chemical composition observed.

Manganese content between 0.01 and 1.5 wt. % positively influences strength properties together with other elements within the specified chemical composition through solid solution hardening. Manganese content above 1.5 wt. % forms coarse crystallisation-origin phases, reducing mechanical properties.

Chromium content between 0.01 and 0.2 wt. % facilitates solid solution hardening in the as-cast state. With higher content, primary crystals of Al—Cr phase are significantly more likely to form, reducing mechanical properties.

Titanium content between 0.01 and 0.2 wt. % aids in modifying primary precipitates of the aluminium solid solution during crystallisation. A higher titanium content in the structure may result in the appearance of primary crystals to reduce the overall level of mechanical properties, while a lower titanium content will not achieve the positive effect of this element. When titanium is added as a multi-component Al—Ti—B or Al—Ti—C, boron or carbon may be found in the alloy proportionally to their content in the master alloy. Boron and carbon, as independent elements, have no significant effect on the mechanical and casting properties for the range in question.

Zirconium content between 0.01 and 0.2 wt. % facilitates solid solution hardening in the as-cast state. Larger quantities require casting temperatures to rise above typical levels, thus reducing the durability of casting moulds and enhancing the hot cracking tendency.

The structure may contain up to 0.3 vol. % of primary crystals including manganese, chromium, zirconium or titanium, thus reducing the casting adhesion to the mould walls.

The invention embodiments were supported by the following methods.

Phase composition, in particular, the number of eutectic phases, the number of primary crystals was quantitatively assessed in at least one of the two following ways: 1) by Thermo-calc calculation; 2) metallographically.

The crystallisation interval was assessed by at least one of two methods: 1) by Thermo-calc calculation; 2) by trial in coordinates along with the construction of the cooling curve in temperature-time coordinates, and the crystallisation interval value as the difference between the liquidus temperature and the solidus temperature.

The technical result was confirmed in laboratory conditions, where the specified alloy compositions were prepared and examined. Alloys were prepared in either an induction or resistance furnace in graphite crucibles using primary aluminium with a minimum content of 99.8 wt. % and 99.99 wt. %, zinc no less than 99.90 wt. %, copper no less than 99.9 wt. % and magnesium no less than 99.9 wt. % (the purity of the base metals is specified for use in the melt), along with master alloys: AlCa10, AlFe10, AlMn20, AlSi10, AlTi5, AlCr10, AlZr10. Other elements and unavoidable impurities in the alloy did not totally exceed 0.05 wt. % found in primary aluminium and master alloys and not regulated for melt preparation.

For determining mechanical properties and structure analysis, alloys were crystallised in a metal mould, ‘separately cast cylindrical sample’, with an operating part diameter of 10 mm and a mould temperature of up to 150° C. Casting properties of the alloys were determined by hot-tearing tendency by using the ‘ring sample’, with the best indicator as the ring having the minimum wall thickness at a constant outer diameter of 40 mm, crystallised without cracking in the row of 3, 7 and 10 mm. The mechanical properties were evaluated under uniaxial tension of separately cast samples in the as-cast state. The test speed was 10 mm/min, with a working part length of 50 mm, as per GOST 1583-93. Adhesion was tested based on the material's ability to be separated from the surface of the metal mould without mechanical impact.

EMBODIMENT

Example 1

To verify and confirm the claimed chemical composition in laboratory conditions, alloys were prepared according to the chemical composition outlined in Table 1. Crystallisation interval and hot cracking tendency analysis results are provided in Table 2. The results of the mechanical tests are presented in Table 3.

The results given in Tables 2 and 3 show that compositions 2-5 and 8-26 according to the claimed concentration range provide an acceptable level of hot cracking resistance. Compositions 1, 6, 7 are not applicable because composition 1 is more likely to adhere to the mould walls. Composition 6 is distinguished by a high hot cracking tendency and composition 7 by the realisation of an unsatisfactory structure with unacceptable primary crystals containing calcium, iron, silicon, and zinc, significantly reducing the elongation.

Example 2

Suitability of the alloy for high-pressure die casting was confirmed by casting 3 mm thick plates with dimensions of 70×150 mm and cutting tensile specimens from these plates. The chemical composition of the alloys is shown in Table 4. The mechanical properties of the alloys are shown in Table 5. A typical structure of the alloy 31 composition is shown in FIG. 1.

The results presented in Tables 4 and 5 show that the alloy provides a good balance of strength and ductility in high-pressure die casting.