METHOD OF FORMING A CAST ALUMINIUM ALLOY

Abstract

Al—Si—Mg castings to provide enhanced mechanical properties for structural applications comprising (1) alloy optimisation with 8.5 to 12.5 wt. % Si, 0.46 to 1.0 wt. % Mg, 0.1 to 0.2 wt. % Ti, 0.05 to 0.25 wt. % Mn, 0.01 to 0.02 wt. % Sr, 0.004 to 0.1 wt. % B and other impurity elements of Cu, Fe, Zn each less than 0.15 wt. % and the balance of Al; (2) optimised melt treatment with appropriate melting, modification, degassing and grain refining; (3) appropriate type of grain refiner with optimised amount and method to add into the aluminium melt, and (4) optimised heat treatment process. When being utilized to make shape aluminium alloy castings with gravity casting process, the castings have been achieved the 0.2% offset yield strength of greater than 310 MPa, the ultimate tensile strength of greater than 365 MPa and the elongation of greater than 10%.

Description

FIELD OF THE INVENTION

The present invention generally relates to a method of forming a cast aluminium alloy, and in particular aluminum castings for enhanced mechanical properties in structural applications. When utilized to make shape aluminium alloy castings with gravity casting process, the castings have achieved a 0.2% offset yield strength of greater than 310 MPa, an ultimate tensile strength of greater than 365 MPa, and an elongation of greater than 10%. More specifically, the excellent strength and ductility properties are superimposed with the casting characteristics inherent for aluminum cast alloy compositions without containing Cu elements.

BACKGROUND OF THE INVENTION

Aluminum alloys have been successfully used in a wide range of structural applications because of their relatively low density, high strength and elastic modulus, fatigue resistance and ease of fabrication. This is particularly important for the transportation industry because structural weight savings are becoming more critical as fuel consumption and air pollution concerns come to the forefront of technological issues. For example, automotive manufacturers are currently using aluminum in unprecedented tonnages. This upward trend of aluminum usage is expected to continue for several years.

The multitude of aluminum alloys can be divided into the categories of wrought and cast aluminum alloys. Wrought aluminum alloys are generally processed by the plastic deformation or cold working of the initial cast billet into final desired shapes through rolling, extruding, forging and/or drawing. Cast aluminum alloys differ greatly from wrought aluminum alloys as cast aluminium alloys are ultimately used in the geometry of the original mould Therefore; many of the beneficial processing steps used to produce wrought aluminum are not practical for use in castings. The alloy design goals, microstructure, processing steps and strengthening mechanisms are the major aspects for making castings with enhanced mechanical properties.

In cast aluminum alloys, Al—Si systems are one of the most popular materials in casting manufacturing because of their excellent casting performance, high specific strength and toughness, and good fatigue resistance and corrosion resistance, etc. A commonly used high performance aluminum casting alloy is Aluminum Association alloy 356/357 with a nominal composition of 7.0 wt. % Si and 0.3 to 0.45 wt. % Mg and minor amounts of Ti, Mn, Fe, Be, and Cu. Mechanical properties in the highest strength temper are among the highest in the aluminum cast alloy systems. However, although the existing alloys are generally capable of satisfying in many critical load-bearing structures, with the development of automotive, aviation, aerospace and military industry, the tensile strength and elongation of Al—Si alloys of a higher requirement, conventional grades of Al—Si alloys have been unable to meet the needs of a number of automotive and aerospace products. Therefore, development of high strength castings based on new Al—Si alloy is an urgent need. There are numerous efforts to develop new materials and technologies.

WO2010003349 discloses a high strength casting aluminium alloy material comprises (in weight %) Cu 2.0-6.0%, Mn 0.05-1.0%, Ti 0.01-0.5%, Cr 0.01-0.2%, Cd 0.01-0.4%, Zr 0.01-0.25%, B 0.005-0.04%, rare earth 0.05-0.3%, and balance aluminium and trace impurities. The alloy has reduced cost.

EP1347066 discloses a high-strength aluminum alloy for casting comprising 3.5 to 4.3% of Cu, 5.0 to 7.5% of Si, 0.10 to 0.25% of Mg, not more than 0.2% of Fe, 0.0004 to 0.0030% of P, 0.005 to 0.0030% of Sr, and the balance comprising Al and unavoidable impurities. A high-strength cast aluminum alloy is also disclosed obtained by: casting a high-strength aluminum alloy for casting comprising 3.5 to 4.3% of Cu, 5.0 to 7.5% of Si, 0.10 to 0.25% of Mg, not more than 0.2% of Fe, 0.0004 to 0.0030% of P, 0.005 to 0.030% of Sr, 0.05 to 0.35% of Ti, and the balance comprising Al and unavoidable impurities; and subjecting the alloy thus cast to a T6 treatment.

WO2015121635 (Brunel University) discloses a high strength cast aluminium alloy for high pressure die casting comprising magnesium silicide 6 to 12 wt. %, magnesium 4 to 10 wt. %, X element from copper (Cu), zinc (Zn), silver (Ag), gold (Au) and Lithium (Li) at 3 to 10 wt. %, manganese 0.1 to 1.2 wt. %, iron max. 1.5 wt. %, titanium or the other grain refining elements from Cr, Nb, and Sc with 0.02 to 0.4 wt. %, and impurity and minor alloying elements at a level of maximum 0.3 wt. % and totally <0.5% of at least one element selected from scandium (Sc), zirconium (Zr), Nickel (Ni), chromium (Cr), niobium (Nb), gadolinium (Gd), calcium (Ca), yttrium (Y), antinomy (Sb), bismuth (Bi), neodymium (Nd), ytterbium (Yb), vanadium (V), chromium (Cr), beryllium (Be) and boron (B) and the remainder aluminium.

EP2865772 discloses an aluminium casting alloy comprising 7-9% by weight of silicon, 0.6-1% by weight of iron, 0.7-1.5% by weight of copper, 0.05-0.5% by weight of manganese, 0.1-3% by weight of zinc, 0.05-0.5% by weight of magnesium, 0.01-0.15% by weight of titanium, 0.01-0.1% by weight of chrome, 0.01-0.1% by weight of nickel, 0.01-0.1% by weight of lead and 0.01-0.1% by weight of tin.

WO2004104240 discloses a high-strength, thermally-resistant, ductile, cast aluminium alloy (AlSi7Mg0.25Zr, or AlSi7Mg0.25Hf) and (Al Si6Mg0.25Zr or Al Si6Mg0.25Hf), comprising Si: 6.5 to 7.5 wt. % and 5.5 to 6.5 wt. %, Mg: 0.20 to 0.32 wt. %, Zr: 0.03 to 0.50 wt. % and/or Hf: 0.03 to 1.50 wt. %, Ti: 0 to 0.20 wt. %, Fe: <0.20 wt. %, Mn: <0.50 wt. %, Cu: <0.05 wt. %, Zn: <0.07 wt. % and made up to 100 wt. % with Al. The invention relates to the use thereof for workpieces or parts thereof with elevated thermal loading, such as a cylinder head.

Despite the property of casting Al—Si—Mg alloys is among the highest and the relatively high amount of Si affords excellent casting characteristics that are paramount to produce complex shapes. However, the available Al—Si alloys generally offer the ultimate tensile strength (UTS) at a level of 330 MPa, the yield strength at a level of 250 MPa, and the elongation at a level of 5%. The addition of Cu is not desirable because of the detrimental of corrosion resistance. Therefore, it would be highly desirable to develop castings with yield strength more than 300 MPa and UTS more than 350 MPa with an elongation more than 9%. This will not only decrease the cost of producing components by casting alloys, but also be clear advantageous if a new, innovative cast Al—Si alloy composition could be used with properties that are far superior to those developed thus far. More importantly, the excellent strength and ductility properties are preferably superimposed with the casting characteristics inherent for aluminum cast alloy compositions without containing Cu elements.

The general principle of the present invention is the disclosure of new types of casting alloys that contain all of the advantageous properties required of castings: e.g., excellent fluidity and castability, combined with the favorable mechanical properties. The end users for such an alloy are quite extensive and varied, and include electrical rotors, structural members, engine bodies, cylinder heads, gear boxes, air conditioners, business machines, industrial equipment, aerospace housings, gears pumps, bearing houses, engine blocks, nodes for connecting tubular structures, wheels, aircraft fittings, flywheel castings, machine tool parts, gear blocks, general automotive castings, marine structures, pressure tight applications, recreational equipment, connecting rods and numerous other applications. Along with the aforementioned applications that have already been established with conventional castings, the new Al—Si alloys may stimulate the use of castings in new, innovative design scenarios that were not previously achievable with conventional casting alloys.

In accordance with a first aspect of the invention, there is provided a method of forming a cast aluminium alloy, including the steps of:

(i) providing an aluminium alloy including
from 8.5 to 12.5 wt. % Si,
from 0.4 to 1.0 wt. % Mg,

up to 0.2 wt. % Ti,

from 0.05 to 0.25 wt. % Mn,
from 0.002 to 0.04 wt. % Sr,
from 0.001 to 0.1 wt. % B
and other impurity elements of Cu, Fe, Zn, each at less than 0.15 wt. % with the balance being Al;
(ii) melting said alloy;
(iii) degassing the alloy melt by introducing into the melt a gas including at least one of nitrogen, argon or chlorine or a mixture thereof in order to reduce dissolved hydrogen in the melt to a level of less than 0.7 mL/100 g melt;
(iv) cleaning the alloy melt by adding 25% Na2SiF6 and 75% C2Cl6 refining agents in an amount from 0.01-0.8 wt. %;
(v) adding a grain refiner in the form of a TiB-containing master alloy, a B-containing master alloy, or an Al—B master alloys with the boron content up to 0.1 wt. % B;
(vi) refining and modifying the eutectic silicon phase by adding from 0.002 to 0.04 wt. % Sr in the form of an Al—Sr master alloys;
(vii) carrying out a solution heat treatment at a temperature from 520° C. to 545° C. for a time from 2 h to 12 h; and
(viii) carrying out an ageing heat treatment at a temperature from 170° C. to 200° C. for a time from 2 h to 8 h.

A composition of a casting alloy may be provided with a primary alloying addition of 8.5 to 12.5 wt. % Si, 0.46 to 1.0 wt. % Mg, 0.1 to 0.2 wt. % Ti, 0.05 to 0.25 wt. % Mn, and less than 0.05 wt. % Sn. In addition, the alloy could further include grain refining additions of Ti, TiB₂, AlB₂, B, Be, Zr, Y, V, Nb, singly or in combination with one another in the range of 0.001 to 1.0 wt. %, chemical modifiers such as Na and Sr, singly or in combination with one another in the range of 0.001 to about 0.10 wt. % and phase refiners such as Pin the range of 0.01 to about 0.30 wt. %, and the balance of Al and incidental impurities.

An optimised process for melt treatment may include appropriate melting, degassing, and grain refining. After at least one hour of homogenisation of the melt, Al-10 wt. % Sr master alloy is added into the melt to the preferred content of no less than 120 ppm and no higher than 200 ppm for the modification and refinement of the eutectic silicon phase. At least 15 minutes after the adding the Sr, the molten metal is degassed using nitrogen, argon or chlorine or their mixtures injected into the melt by means of a rotary degassing impeller at a speed of at least 150 rpm for at least 10 min. The degassing process includes the introduction of at least one of nitrogen, argon or chlorine or their mixtures into the alloy melt to remove the dissolved hydrogen in the melt to a level of less than 2 mL/100 g. It is preferred that the dissolved hydrogen in the melt can be reduced to a level of less than 0.7 mL/100 g, even preferably to a level of less than 0.2 mL/100 g. Then TiB-containing master alloy is added into the melt as grain refiner. The refining process consists essentially of adding up to 0.3 wt. % grain refiners into the aluminium alloy melt, which includes the TiB-containing master alloy for refining primary aluminium phase, which is at least one of Al-5Ti1B, Al-3Ti1B, Al-1Ti3B, or Al-3Ti3B alloys. The alternation method is adding 25% Na₂SiF₆+75% C₂Cl₆ refining agents and with the rotary degassing unit with the use of the best refining effect in an amount of 0.5-0.8 wt. %. The amount of grain refiner can be preferably at a level of up to 0.2 wt. %. After degassing, the top surface of the melt is covered by commercial granular flux, then the melt is held for 10-15 min, thereafter the melt is ready for casting, and the preferred casting temperature is at 700-720° C.

An embodiment may include an optimised process for heat treatment of castings made by the developed aluminium alloys. The heat treatment in accordance with the practice that involves the steps of solution heat treatment at temperatures approaching the solidus temperature of a given alloy; quenching into water or other appropriate media, and ageing at temperatures ranging from ambient to about 300° C. Alternatively, a multiple stages solution process and multiple stages ageing process can be utilized. The solution is conducted at a temperature between 520° C. to 545° C., preferably between 530° C. to 540° C., and more preferably between 535° C. to 540° C. The solution time at the more preferably solution temperature 540° C. is between 2 h to 12 h, preferably between 8 h to 10 h, as indicated in FIG. 1. The ageing is conducted at a temperature between 170° C. to 200° C., preferably between 170° C. to 190° C., and more preferably at 170° C. or 190° C. The ageing time at the more preferably ageing temperature is between 2 h to 8 h, preferably ageing at 170° C. for 7-8 h or ageing at 190° C. for 3-4 h, as indicated in FIG. 2. The optimised heat treatment process for the alloy is solution at 540° C. for 8-10 h, then quenching into water or other appropriate media, after ageing at 170° C. for 7-8 h or ageing at 190° C. for 3-4 h, as indicated in FIG. 3.

The alloy and manufacturing method of Al—Si—Mg castings preferably provide enhanced mechanical properties for structural applications comprising (1) alloy optimisation with 8.5 to 12.5 wt. % Si, 0.46 to 1.0 wt. % Mg, 0.1 to 0.2 wt. % Ti, 0.05 to 0.25 wt. % Mn, 0.01 to 0.02 wt. % Sr, 0.004 to 0.1 wt. % B and other impurity elements of Cu, Fe, Zn each less than 0.15 wt. % and the balance of Al and incidental impurities; (2) optimised melt treatment with appropriate melting, degassing and grain refining; (3) appropriate type of grain refiner with optimised amount and method to add into the aluminium melt, and (4) optimised heat treatment process.

The alloy and manufacturing method of Al—Si—Mg castings preferably comprises: 8.5 to 10.0 wt. % Si, 0.46 to 0.65 wt. % Mg, 0.1 to 0.15 wt. % Ti, less than 0.15 wt. % Mn, 0.012 to 0.018 wt. % Sr, 0.004 to 0.04 wt. % B and other impurity elements of Cu, Fe, Zn each less than 0.15 wt. % and the balance of Al and incidental impurities.

The alloy and manufacturing method of Al—Si—Mg castings preferably comprises less than 0.05 wt. % Cu.

The alloy and manufacturing method of Al—Si—Mg castings, preferably comprises less than 0.12 wt. % Fe.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of an appropriate process for making melt through degassing and grain refining.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of degassing, in which the gas including at least one of nitrogen, argon or chlorine or their mixtures is introduced into the alloy melt to remove the dissolved hydrogen in the melt to a level of less than 2 mL/100 g melt.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of degassing, in which the gas including at least one of nitrogen, argon or chlorine or their mixtures is introduced into the alloy melt to remove the dissolved hydrogen in the melt to a preferred level of less than 0.7 mL/100 g melt.

The Al-alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of degassing, in which the gas including at least one of nitrogen, argon or chlorine or their mixtures is introduced into the alloy melt to remove the dissolved hydrogen in the melt to a more preferred level of less than 0.2 mL/100 g melt.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of cleaning the aluminium melt through pumping solid flux into aluminum melt, which can be associated with degassing process.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of cleaning the aluminium melt through pumping chemical gas flux into aluminum melt, which can be associated with degassing process.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of cleaning the aluminium melt through adding 25% Na₂SiF₆+75% C₂Cl₆ refining agents in an amount of 0.5-0.8 wt. % and with the rotary degassing unit.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of adding up to 0.3 wt. % grain refiners into the aluminium alloy melt, which includes Sr-containing master alloys for modification and refining eutectic silicon phase, and TiB-containing master alloys for refining primary aluminium phase.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of adding up to 0.2 wt. % grain refiners into the aluminium alloy melt, which includes Sr-containing master alloys for modification and refining eutectic silicon phase, and the TiB-containing master alloy for refining primary aluminium phase.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of refining primary aluminium phase by adding TiB-containing master alloys, which is at least one of Al-5Ti1B, Al-3Ti1B, Al-1Ti3B, or Al-3Ti3B alloys.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of refining primary aluminium phase by adding TiB-containing master alloys, which are preferred to be Al3Ti3B, Al1Ti3B, or other B-rich AlTiB master alloys.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one heat treatment from solution, annealing and ageing.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one solution at a temperature between 520° C. to 545° C.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one solution at a temperature preferably between 530° C. to 540° C.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one solution at a temperature more preferably between 535° C. to 540° C.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one solution for a time between 2 h to 12 h.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one solution for a time preferably between 8 h to 10 h.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one ageing at a temperature between 170° C. to 200° C.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one ageing at a temperature preferably between 170° C. to 190° C.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one ageing at the temperature more preferably 170° C. or 190° C.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one ageing for a time between 2 h to 8 h.

The alloy and manufacturing method of Al—Si—Mg castings preferably consists essentially of at least one ageing preferably at 170° C. for 7 h to 8 h or 190° C. for 3 h to 4 h.

The alloy and manufacturing method of Al—Si—Mg castings to provide enhanced mechanical properties for structural applications preferably comprises (1) alloy optimisation with 8.5 to 10.0 wt. % Si, 0.46 to 0.65 wt. % Mg, 0.1 to 0.15 wt. % Ti, less than 0.15 wt. % Mn, 0.012 to 0.018 wt. % Sr, 0.004 to 0.04 wt. % B and other impurity elements of Cu, Fe, Zn each less than 0.15 wt. % and the balance of Al and incidental impurities; (2) optimised melt treatment with appropriate melting, modification, degassing and grain refining; (3) appropriate type of grain refiner with optimised amount and method to add into the aluminium melt, and (4) optimised heat treatment process.

All the above and other characteristics and advantages of the invention will be further understood through the following illustrative and non-limitative description of embodiments thereof, with reference to the appended examples and drawings, in which:

FIG. 1 is a graph showing micro hardness of the alloy versus solution time at a solution temperature of 540° C.;

FIG. 2 is a graph showing micro hardness of the alloy versus ageing time at an ageing temperature of 170° C. after solution at 540° C. for 8 hours; and

FIG. 3 is a graph showing yield strength of the alloy versus ageing time at an ageing temperature of 170° C. after solution at 540° C. for 6-14 hours.

In one embodiment, an alloy system in accordance with the principles of the present invention is a modification of the Aluminum Association's alloy system 3XX. This modified alloy system generally comprises of Si in the range of 8.5 to 12.5 wt. % and Mg in the range of 0.3 to 0.7 wt. %, with one or more of Ti less than 0.2 wt. %, Mn less than 0.1 wt. %, Zn less than 0.1 wt. %, Sn less than 0.05 wt. %. In addition, the alloy could further include grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, singly or in combination with one another in the range of 0.001 to 1.0 wt. %, chemical modifiers such as Na and Sr, singly or in combination with one another in the range of 0.001 to about 0.20 wt. % and phase refiners such as P in the range of 0.01 to about 0.30 wt. %, and the balance of Al and incidental impurities.

In another embodiment, an alloy system in accordance with the principles of the present invention is a modification of the Aluminum Association's alloy system 3XX. This modified alloy system preferably comprises of 8.5 to 10.0 wt. % Si, 0.46 to 0.65 wt. % Mg, 0.1 to 0.15 wt. % Ti, less than 0.15 wt. % Mn, Sn less than 0.05 wt. %, and Zn less than 0.1 wt. %. In addition, the alloy further include grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, singly or in combination with one another in the range of 0.001 to 0.5 wt. %, most preferably grain refining additions of 0.1 to 0.5 wt. % Al3Ti3B master alloy comprising TiB2 and AlB2, chemical modifiers such as Na and Sr, singly or in combination with one another in the range of 0.001 to about 0.10 wt. % and phase refiners such as P in the range of 0.01 to about 0.20 wt. %, and the balance of Al and incidental impurities. In the present invention, the silicon can be used to improve the performance of the alloy casting, improve mobility and reduce hot cracking tendency, reduce shrinkage, improve air tightness. Magnesium's role is to improve its strength and toughness; cast, in addition to a small amount of magnesium dissolved in the α-Al substrate body, mainly exists in the larger size of the Mg2Si phase, therefore, cast magnesium alloy on the mechanical properties of the obvious. The role of magnesium in the alloy is achieved by heat treatment; solution treatment, magnesium dissolved a matrix of precipitated Mg2Si during aging, the alloy strengthening.

Still in another embodiment, the castings will be cast using the conventional method of pouring the molten alloy mixture into a permanent, sand or investment type mold or alternatively cast using advanced techniques such as high pressure die casting or squeeze casting to produce a near net shape cast parts. Prior to casting, it is essential to have a proper degassing and grain refining. The casting Al—Si alloys of the present invention may be resistance furnace smelting, alloying elements above the middle of its way and aluminium alloy added to the molten aluminium; with 25% Na₂SiF₆+75% C₂Cl₆ refining agents and with the rotary degassing unit with the use of the best refining effect in amount of 0.5-0.8% (mass percentage). TiB-containing refiner on Al—Si alloy with good refinement effect, the best technology for the 720° C. adding 0.2-0.3 wt. % of refiner, insulation 8 min-15 min; Al-10Sr alloy has good metamorphism, added at 0.01-0.02 wt. % Sr, was added at the temperature 740° C.; heat treatment specification: 540° C. solid solution for 8-10 hours, 170° C. aging 7-8 hours; alloy having high strength and toughness, yield strength in exceed of 300 MPa and elongation in exceed of 9%.

Still in another embodiment, the casting is subjected to an appropriate heat treatment in accordance with the practice that involves the steps of solution heat treatment at temperatures approaching the solidus temperature of a given alloy; quenching into water or other appropriate media, and ageing at temperatures ranging from ambient to about 300° C.

Alternatively, a multiple stages solution process and multiple stages ageing process can be utilized. For example, in a two-step process, it includes primary ageing at a low temperature (e.g., less than about 190° C., preferably less than 160° C.) for an short period of time (e.g., longer than 1 hours but less than 10 hours, preferably about 2 hours) followed by secondary ageing at a high temperature (e.g., greater than about 100° C., preferably about 170° C.) for an extended period of time (e.g., longer than 2 hours but less than 48 hours, preferably about 8 hours).

Additional processing steps such as hot isostatic pressing, machining, surface modification and shot peening can be applied to further improve the casting alloys disclosed in the present invention. By utilizing the alloys of the present invention to form near net shape cast parts, significantly improved cast alloy properties can be achieved. For example, alloys, which embody the present invention, have been shown to have yield strengths (0.2% offset) in excess of 300 MPa and elongation in excess of 10%.

The present invention will be further described with reference to Examples:

Example 1: Gravity Casting

Four alloys of the compositions listed in Table 1 were cast into a permanent mold. The alloys also include an A356 and an A357 type cast aluminum alloys. The castings were made by weighting different elements with an appropriate ratio and melting them in a 12 kg clay-graphite crucible in an electric resistance furnace. When the melt was fully homogenised, it was subjected to degassing, during which Ar was blown into the melt by a commercial rotatory degasser adjusted at 350 rpm for 4 min. It should be mentioned that Al-10Sr alloy was added at 0.01-0.02 wt. % Sr before degassing. TiB-containing refiner was added at 720° C. with 0.005 wt. % of B and before pouring. Thereafter, the melt was poured into the boron nitride painted steel mould, designed based on ASTM B108 standard, to produce dog-bone shape tensile specimens. In a gravity casting using permanent mold, molten metal was poured into the steel mold which was already heated up to 400-460° C. Chemical composition analysis was carried out using the Foundry-Master Pro which is a high-performing optical emission spectrometer (OES).

Each of the four castings was solution heat treated at 540° C. for 8 hours, immediately quenched into ambient temperature water upon removal from the furnace and allowed to stabilize for several days. Ageing was optimized for each alloy by taking Vickers hardness measurements in accordance with the American Society for Testing and Materials (ASTM) standard E92-82 at selected time intervals for a wide range of temperatures. The optimized ageing process is ageing at 170° C. for 8 hours or ageing at 190° C. for 4 hours. The mechanical properties were further measured in accordance with ASTM B557 standard using an Instron 5500 Universal Electromechanical Testing Systems equipped with Bluehill software and a ±50 kN load cell. All the tensile tests were performed at ambient temperature (˜25° C.). The gauge length of the extensometer was 50 mm and the ramp rate for extension was 1 mm/min. The mechanical properties of the four castings after solution and ageing treatment are listed in Table 2.

TABLE 1
Chemical composition (wt. %) of the alloys in EXAMPLE1
Alloy	Si	Mg	Cu	Fe	Mn	V	Ti	Sr	B	Al
A356	6.99	0.35	0.00	0.11	0.06	0.016	0.14	0.015	0.002	Balance
A357	7.01	0.50	0.00	0.11	0.06	0.017	0.14	0.015	0.002	Balance
GC01	9.21	0.50	0.00	0.11	0.06	0.017	0.14	0.015	0.005	Balance
GC02	9.72	0.50	0.00	0.11	0.07	0.018	0.14	0.015	0.005	Balance

As shown in Table 2, the developed alloys labelled as GC01 and GC02 display higher strengths and elongations over the commercially available A356 and A357 alloys. This is especially surprising given that the A357 alloy is by far the highest strength alloy in the Al—Si—Mg cast alloy system. Moreover, the Mg content is higher in the A357 alloy, in which Mg content is 0.5-0.7 wt. %. Since published yield strength values (source: Metals Handbook Desk Edition. American Society for metals, H. E. Boyer and T. L. Gall, eds., 1985, pp. 6.48-6.62) for 357-T6 (0.55% Mg, yield strength 295 MPa) are about 18% greater than those obtained with alloy 356-T6 (same composition as the 357 with 0.35% Mg, yield strength 250 MPa). Clearly, the developed compositions are very potent in overcoming this large property disparity that is observed with slightly different Mg levels. If the Mg content were adjusted to the 0.60 weight percent level or above, it is likely that the strength of the developed alloys would be even greater. More importantly, the developed alloys show good ductility with an elongation higher than 10%. The good ductility of the developed alloys could be attributed to the increase of castability and the decrease of porosity level over the existing commercially available A356 and A357 alloys.

In a variation on the above-noted single step ageing treatment, a two-step ageing treatment consisting of an initial step of 150° C. for 2 hours followed by ageing at 180° C. for 6 hours was applied to the alloys. As ageing time progresses, the alloys attain yield levels that exceed that of aged at a single stage ageing. It is evident that a two-step ageing treatment could further widen the gap between the new alloys and the existing commercial alloy castings. The mechanical properties of the developed alloys labelled as GC01 and GC02 under solution and two-step ageing treatment are also listed in Table 2.

TABLE 2
Mechanical properties of the permanent mold casting alloys in EXAMPLE1 after heat treatment.
		Yield
		strength	UTS	Elongation
Alloy	Heat treatment	(MPa)	(MPa)	(%)
A356(Al—7Si—0.35Mg)	540° C./8 h + 170° C./8 h	250	310	8.5
A357(Al—7Si—0.5Mg)	540° C./8 h + 170° C./8 h	285	340	5.0
GC01(Al—9.2Si—0.5Mg)	540° C./8 h + 170° C./8 h	312	365	11.2
GC02(Al—9.7Si—0.5Mg)	540° C./8 h + 170° C./8 h	316	370	10.2
GC01(Al—9.2Si—0.5Mg)	540° C./8 h + 150° C./2 h + 80° C./6 h	317	370	10.8
GC02(Al—9.7Si—0.5Mg)	540° C./8 h + 150° C./2 h + 80° C./6 h	321	374	10.1

Example 2: Sand Casting

Four alloys of the compositions listed in Table 3 were cast into a sand mold. The alloys also include an A356 and an A357 type cast aluminum alloys. The castings were made by weighting different elements with an appropriate ratio and melting them in a 12 kg clay-graphite crucible in an electric resistance furnace. When the melt was fully homogenised, it was subjected to degassing, during which Ar was blown into the melt by a commercial rotatory degasser adjusted at 350 rpm for 4 min. It should be mentioned that Al-10Sr alloy was added at 0.01-0.02 wt. % Sr before degassing. TiB-containing refiner was added at 720° C. with 0.005 wt. % of B and before pouring. Thereafter, the melt was poured into the British standard sand mould, to produce dog-bone shape tensile specimens. In a gravity casting using sand mold, molten metal was poured into the sand mold which was at room temperature. Chemical composition analysis was carried out using the Foundry-Master Pro which is a high-performing optical emission spectrometer (OES).

Each of the four castings was solution heat treated at 540° C. for 8 hours, immediately quenched into ambient temperature water upon removal from the furnace and allowed to stabilize for several days. Ageing was optimized for each alloy by taking Vickers hardness measurements in accordance with the American Society for Testing and Materials (ASTM) standard E92-82 at selected time intervals for a wide range of temperatures. The optimized ageing process is at 170° C. for 8 hours or at 190° C. for 4 hours. The mechanical properties were further measured in accordance with ASTM B557 standard using an Instron 5500 Universal Electromechanical Testing Systems equipped with Bluehill software and a ±50 kN load cell. All the tensile tests were performed at ambient temperature (˜25° C.). The gauge length of the extensometer was 50 mm and the ramp rate for extension was 1 mm/min. The mechanical properties of the four castings after solution and ageing treatment are listed in Table 4.

TABLE 3
Chemical composition of the alloys in EXAMPLE2.
Alloy	Si	Mg	Cu	Fe	Mn	V	Ti	Sr	B	Al
A356	7.08	0.35	0.00	0.11	0.06	0.016	0.14	0.015	0.002	Balance
A357	7.05	0.50	0.00	0.11	0.06	0.016	0.14	0.015	0.002	Balance
SC01	9.14	0.50	0.00	0.11	0.06	0.016	0.14	0.015	0.005	Balance
SC02	9.76	0.50	0.00	0.11	0.07	0.016	0.14	0.015	0.005	Balance

As shown in Table 4, the commercially available A356 alloy displays a yield strength of 230 MPa and an UTS of 280 MPa with 0.35 wt. % Mg, and the commercially available A357 alloy shows a yield strength of 275 MPa and an UTS of 300 MPa with 0.5 wt. % Mg, while the developed alloys labelled SC01 and SC02 with 0.5 wt. % Mg display a remarkable increase of strength over the commercially available A356 and A357 alloys, with a yield strength above 295 MPa and an UTS above 325 MPa. This is especially surprising given that the A357 alloy is by far the highest strength alloy in the Al—Si—Mg cast alloy system. More importantly, the Mg content is higher in the A357 alloy, in which Mg content is 0.5-0.7 wt. %, and the A357 alloy achieves higher strength over the A356 alloy at higher Mg content with significant decrease of elongation to be below 3%, while the developed alloys achieve higher strength over the A356 alloy without obvious decrease of elongation. Clearly, the developed compositions are very potent in overcoming this large property disparity that is observed with slightly different Mg levels. If the Mg content were adjusted to the 0.60 weight percent level or above, it is likely that the strength of the developed alloys would be even greater.

In a variation on the above-noted single step ageing treatment, a two-step ageing treatment consisting of an initial step of 150° C. for 2 hours followed by ageing at 180° C. for 6 hours was applied to the alloys. As ageing time progresses, the alloys attain yield levels that exceed that of aged at a single stage ageing. It is evident that a two-step ageing treatment could further widen the gap between the new alloys and the existing commercial alloy castings. The mechanical properties of the developed alloys labelled as SC01 and SC02 under solution and two-step ageing treatment are also listed in Table 4.

TABLE 4
Mechanical properties of the sand mold casting alloys in EXAMPLE2 after heat treatment.
		Yield
		strength	UTS	Elongation
Alloy	Heat treatment	(MPa)	(MPa)	(%)
A356(Al—7Si—0.35Mg)	540° C./8 h + 170° C./8 h	230	280	4.5
A357(Al—7Si—0.5Mg)	540° C./8 h + 170° C./8 h	275	300	3.0
SC01(Al—9.1Si—0.5Mg)	540° C./8 h + 170° C./8 h	300	325	4.5
SC02(Al—9.8 Si—0.5Mg)	540° C./8 h + 170° C./8 h	305	330	4.0
SC01(Al—9.1Si—0.5Mg)	540° C./8 h + 150° C./2 h + 80° C./6 h	305	331	4.5
SC01(Al—9.8Si—0.5Mg)	540° C./8 h + 150° C./2 h + 80° C./6 h	310	335	4.0

All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

The disclosures in UK patent application number 1713005.5, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

Claims

1.-11. (canceled)

12. An aluminium alloy comprising:

From about 8.5 to about 12.5 wt. % Si;

from about 0.46 to about 1.0 wt. % Mg;

from about 0.1 to about 0.2 wt. % Ti;

from about 0.05 to about 0.25 wt. % Mn;

and less than about 0.05 wt. % Sn;

the balance being Al and incidental impurities.

13. An aluminium alloy as claimed in claim 12, further comprising:

grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, singly or in combination with one another in the range from about 0.001 to about 1.0 wt. %;

Na and Sr, singly or in combination, in the range of from about 0.001 to about 0.10 wt. %; and

P in the range from about 0.01 to about 0.30 wt. %.

14. A method of forming a cast aluminium alloy, comprising the steps of:

(i) providing an aluminium alloy comprising from about 8.5 to about 12.5 wt. % Si, from about 0.46 to about 1.0 wt. % Mg, up to about 0.2 wt. % Ti, from about 0.05 to about 0.25 wt. % Mn, from about 0.002 to about 0.04 wt. % Sr, from about 0.001 to about 0.1 wt. % B and other impurity elements of Cu, Fe, Zn, each at less than about 0.15 wt. % with the balance being Al and incidental impurities;

(ii) melting the alloy to yield an alloy melt;

(iii) degassing the alloy melt by introducing into the alloy melt a gas comprising at least one of nitrogen, argon, or chlorine, or a mixture thereof to reduce dissolved hydrogen in the melt to a level of less than about 0.7 mL/100 g alloy melt;

(iv) cleaning the alloy melt by adding about 25% Na2SiF6 and 75% C2Cl6 refining agents in an amount from about 0.01 to about 0.8 wt. %;

(v) adding a grain refiner in the form of a TiB-containing master alloy, a B-containing master alloy, or an Al—B master alloy with a boron content up to about 0.1 wt. % B, to yield an alloy having a eutectic silicon phase;

(vi) refining and modifying the eutectic silicon phase by adding from about 0.002 to about 0.04 wt. % Sr in the form of an Al—Sr master alloy;

(vii) carrying out a solution heat treatment at a temperature from about 520° C. to about 545° C. for a time from about 2 h to about 12 h; and

(viii) carrying out an ageing heat treatment at a temperature from about 170° C. to about 200° C. for a time from about 2 h to about 8 h.

15. A method as claimed in claim 14, wherein the alloy of step (i) comprises:

from about 8.5 to about 10.0 wt. % Si,

from about 0.456 to about 0.65 wt. % Mg,

from about 0.1 to about 0.15 wt. % Ti,

less than about 0.15 wt. % Mn,

from about 0.008 to about 0.02 wt. % Sr,

from about 0.004 to about 0.04 wt. % B

and other impurity elements of Cu, Fe, Zn, each at less than about 0.15 wt. % and the balance of Al and incidental impurities

16. A method as claimed in claim 14, wherein the dissolved hydrogen in step (iii) is reduced to a level of less than about 0.2 mL/100 g alloy melt.

17. A method as claimed in claim 14, wherein the grain refiner of step (v) includes up to about 3.5 wt. % Al3Ti3B or AlTi3B.

18. A method as claimed in claim 14, wherein the grain refiner of step (v) has a B content from about 0.004 to about 0.04 wt. %.

19. A method as claimed in claim 14, wherein in step (vi) the amount of Sr is from about 0.008 to about 0.02 wt. %.

20. A method as claimed in claim 14, wherein the solution heat treatment of step (vii) is carried out at a temperature from about 535° C. to about 540° C. for a time from about 8 h to about 10 h.

21. A method as claimed in claim 14, wherein the ageing heat treatment of step (viii) is carried out at a temperature of about 170° C. from about 7 h to about 8 h.

22. A method as claimed in claim 14, wherein the ageing heat treatment of step (viii) is carried out at a temperature from about 180 to about 190° C. from about 2 h to about 5 h.