CN111032897A

CN111032897A - Method of forming cast aluminum alloy

Info

Publication number: CN111032897A
Application number: CN201880052663.4A
Authority: CN
Inventors: 冀守勋; 董曦曦; 张亦杰
Original assignee: Brunel University
Current assignee: Brunel University; Brunel University London
Priority date: 2017-08-14
Filing date: 2018-07-31
Publication date: 2020-04-17
Also published as: US20200190634A1; GB201713005D0; EP3669011A1; WO2019034837A1

Abstract

An Al-Si-Mg casting providing enhanced mechanical properties for structural applications, comprising (1) an alloy optimized 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 less than 0.15 wt.% each of Cu, Fe, Zn and other impurity elements, and the balance Al; (2) optimized melt processing with appropriate melting, modification, degassing and grain refinement; (3) a suitable type of grain refiner added to the aluminum melt in an optimized amount and method, and (4) an optimized heat treatment process. When used to make shaped aluminum alloy castings by the gravity casting process, the castings have achieved a 0.2% offset yield strength of greater than 310MPa, an ultimate tensile strength of greater than 365MPa, and an elongation of greater than 10%.

Description

Method of forming cast aluminum alloy

Technical Field

The present invention relates generally to a method of forming a cast aluminum alloy, and in particular to an aluminum casting for enhanced mechanical properties in structural applications. When used to make shaped aluminum alloy castings by the gravity casting process, the castings have achieved an offset yield strength of 0.2% (offset yield strength) of greater than 310MPa, an ultimate tensile strength (ultimate tensile strength) of greater than 365MPa, and an elongation of greater than 10%. More specifically, the casting characteristics inherent in aluminum casting alloy compositions that do not contain the Cu element add superior strength and ductility properties.

Background

Aluminum alloys have been successfully used in various structural applications due to their lower density, high strength and modulus of elasticity, fatigue resistance, and ease of manufacture. This is particularly important to the transportation industry, as savings in structural weight become more critical as fuel consumption and air pollution concerns are leading edge to technical issues. For example, automotive manufacturers are currently using aluminum of unprecedented tonnage. This rising trend in aluminum usage is expected to continue for years.

Most aluminum alloys can be classified into the category of wrought (zero) aluminum alloys and cast aluminum alloys. Wrought aluminium alloys are typically processed by: the initial billet is plastically deformed or cold worked into the final desired shape by rolling, extrusion, forging, and/or drawing. Cast aluminum alloys are very different from wrought aluminum alloys because cast aluminum alloys are ultimately used in the geometry of the original mold. Thus, many of the beneficial processing steps used to produce wrought aluminum are not practical for casting. Alloy design goals, microstructure, processing steps and strengthening mechanisms are major aspects for producing castings with enhanced mechanical properties.

Among cast aluminum alloys, the Al — Si system is one of the most popular materials in casting manufacture because of its excellent castability, high specific strength and toughness, and good fatigue resistance and corrosion resistance, etc. A commonly used high performance Aluminum casting alloy is Aluminum Association (Aluminum Association) alloy 356/357, which has a nominal composition of 7.0 wt.% Si and 0.3 to 0.45 wt.% Mg with minor amounts of Ti, Mn, Fe, Be, and Cu. In aluminum casting alloy systems, the mechanical properties in the highest strength temper are highest. However, with the development of the automotive, aerospace, and military industries, conventional grades of Al-Si alloys have not been able to meet the needs of a variety of automotive and aerospace products, although existing alloys are generally able to meet the higher requirements of tensile strength and elongation of Al-Si alloys in many critical load bearing structures. Therefore, the development of high strength castings based on new Al — Si alloys is an urgent need. There is much work in developing new materials and technologies.

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

EP1347066 discloses a high strength aluminium 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. It is also disclosed that the high strength cast aluminum alloy is obtained by: casting a high-strength aluminum alloy for casting, which contains 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 of Al and inevitable impurities; and the alloy thus cast was subjected to T6 treatment.

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

EP2865772 discloses an aluminium casting alloy comprising: 7 to 9 weight percent silicon, 0.6 to 1 weight percent iron, 0.7 to 1.5 weight percent copper, 0.05 to 0.5 weight percent manganese, 0.1 to 3 weight percent zinc, 0.05 to 0.5 weight percent magnesium, 0.01 to 0.15 weight percent titanium, 0.01 to 0.1 weight percent chromium, 0.01 to 0.1 weight percent nickel, 0.01 to 0.1 weight percent lead, and 0.01 to 0.1 weight percent tin.

WO2004104240 discloses a high strength, heat resistant, ductile cast aluminum alloy (alsi7mg0.25zr, or alsi7mg0.25hf) and (ahi si6mg0.25zr, or ahi si6mg0.25hf) comprising: si: 6.5 to 7.5 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 Al to 100 wt%. The invention relates to the use thereof for workpieces or parts thereof with increased thermal load, such as cylinder heads.

Although the properties of cast Al-Si-Mg alloys are highest and higher amounts of Si provide excellent casting characteristics that are most important for producing complex shapes. However, useful Al-Si alloys typically provide an Ultimate Tensile Strength (UTS) at the 330MPa level, a yield strength at the 250MPa level, and an elongation at the 5% level. The addition of Cu is undesirable because it is detrimental to corrosion resistance. Therefore, it is highly desirable to develop castings with yield strengths in excess of 300MPa and UTS in excess of 350MPa and elongations in excess of 9%. This will not only reduce the cost of producing components by casting alloys, but it is also a significant advantage if new innovative cast Al-Si alloy compositions with properties far superior to those developed so far can be used. More importantly, the casting characteristics inherent in the aluminum casting alloy composition without the Cu element preferably combine excellent strength and ductility properties.

The general principle of the present invention is to disclose a novel casting alloy that contains all the beneficial properties required for casting: for example, excellent flowability and castability, combined with advantageous mechanical properties. The end users of such alloys are quite wide and varied and include electric rotors, structural elements, engine blocks, cylinder heads, gear boxes, air conditioners, commercial machines, industrial equipment, aerospace housings, gear pumps, bearing blocks, engine blocks, joints for connecting tubular structures, wheels, aircraft fittings, flywheel castings, machine tool parts, gear blocks, general automotive castings, marine structures, pressure tight applications (pressure tight applications), recreational equipment, linkages, and a variety of other applications. Along with the aforementioned applications that have been identified with respect to conventional castings, the new Al-Si alloys may encourage the use of castings in new innovative design scenarios that have previously not been achievable with conventional casting alloys.

According to a first aspect of the present invention, there is provided a method of forming a cast aluminium alloy, the method comprising the steps of:

(i) providing an aluminum alloy comprising:

8.5 to 12.5% by weight of Si,

0.4 to 1.0 wt.% of Mg,

up to 0.2% by weight of Ti,

0.05 to 0.25% by weight of Mn,

0.002 to 0.04 wt.% Sr,

0.001 to 0.1% by weight of B,

and less than 0.15 wt% each of other impurity elements of Cu, Fe, Zn, the balance being Al;

(ii) melting the alloy;

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

(iv) cleaning the alloy melt by adding 25% Na2SiF6 and 75% C2Cl6 refining agent in an amount of 0.01 to 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, wherein the boron content is at most 0.1 wt% of B;

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

(vii) solution heat treatment at 520 ℃ to 545 ℃ for a time of 2h to 12 h; and

(viii) the ageing heat treatment is carried out at a temperature of 170 ℃ to 200 ℃ for a time of 2h to 8 h.

The composition of the cast alloy may have 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 may further comprise: ti, TiB in the range of 0.001 to 1.0 wt.% either alone or in combination with each other₂、AlB₂B, Be, Zr, Y, V, Nb, individually or in combination with each other in the range of 0.001 wt.% to about 0.10 wt.% of a chemical modifier such as Na and Sr, and in the range of 0.01 wt.% to about 0.30 wt.% of a phase refiner such as P, with the balance Al and incidental impurities.

Optimization processes for melt processing may include appropriate melting, degassing, and grain refinement. Melt at least one hourAfter homogenization, an Al-10 wt.% Sr master alloy is added to the melt to a preferred content of not less than 120ppm and not more than 200ppm for modification and refinement of the eutectic silicon phase. At least 15 minutes after the addition of Sr, the molten metal is degassed for at least 10 minutes using nitrogen, argon or chlorine or a mixture thereof which is injected into the melt by means of a rotating degassing impeller at a speed of at least 150 rpm. The degassing process includes introducing at least one of nitrogen, argon, or chlorine gas or a mixture thereof into the alloy melt to remove dissolved hydrogen from the melt to a level below 2mL/100 g. It is preferred that the dissolved hydrogen in the melt can be reduced to a level below 0.7mL/100g, even more preferably below 0.2mL/100 g. The TiB-containing master alloy is then added to the melt as a grain refiner. The refining process mainly comprises the following steps: up to 0.3 wt.% of a grain refiner is added to the aluminum alloy melt, which comprises a TiB-containing master alloy for refining primary aluminum phase, the TiB-containing master alloy being at least one of Al-5Ti1B, Al-3Ti1B, Al-1Ti3B or Al-3Ti3B alloy. An alternative method is to add 25% Na in an amount of 0.5 to 0.8% by weight with optimum refining effect₂SiF₆+75％C₂Cl₆Refining the agent and using a rotary degassing unit. The amount of grain refiner may preferably be at a level of up to 0.2 wt%. After degassing, the upper surface of the melt is covered with a commercial granular flux, and the melt is then held for 10 to 15 minutes, after which the melt is ready for casting, and the preferred casting temperature is 700 to 720 ℃.

One embodiment may include an optimization process for heat treating castings made from the developed aluminum alloys. The heat treatment according to practice involves the following steps: solution heat treating at a temperature near the solidus temperature of the given alloy; quenching into water or other suitable medium, and aging at a temperature in the range of ambient temperature to about 300 ℃. Alternatively, a multi-stage solid solution process and a multi-stage aging process may be employed. The solutionizing is carried out at a temperature of 520 ℃ to 545 ℃, preferably 530 ℃ to 540 ℃, and more preferably 535 ℃ to 540 ℃. The solutionizing time at a more preferred solutionizing temperature of 540 ℃ is 2h to 12h, preferably 8h to 10h, as indicated in fig. 1. The ageing is carried out at a temperature of from 170 ℃ to 200 ℃, preferably from 170 ℃ to 190 ℃ and more preferably 170 ℃ or 190 ℃. The ageing time at the more preferred ageing temperature is from 2h to 8h, preferably from 7 to 8h at 170 ℃ or from 3 to 4h at 190 ℃, as indicated in figure 2. The optimized heat treatment process for the alloy is as follows: solutionizing at 540 ℃ for 8 to 10h, then aging at 170 ℃ for 7 to 8h or at 190 ℃ for 3 to 4h, followed by quenching into water or other suitable medium, as indicated in fig. 3.

The alloy and method of making Al-Si-Mg castings preferably provide enhanced mechanical properties for structural applications, including (1) optimization with the following alloys: 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 less than 0.15 wt.% each of Cu, Fe, Zn and other impurity elements, and the balance Al and incidental impurities; (2) optimized melt processing with appropriate melting, degassing and grain refinement; (3) a suitable type of grain refiner added to the aluminum melt in an optimized amount and method, and (4) an optimized heat treatment process.

The alloy and method of making 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 less than 0.15 wt.% each of Cu, Fe, Zn and other impurity elements, and the balance Al and incidental impurities.

The alloy and method of making Al-Si-Mg castings preferably contain less than 0.05 wt.% Cu.

The alloy and method of making Al-Si-Mg castings preferably contain less than 0.12 wt.% Fe.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of a suitable process for producing the melt by degassing and grain refinement.

The alloy and method of making Al-Si-Mg castings preferably consists essentially of degassing in which a gas comprising at least one of nitrogen, argon or chlorine or mixtures thereof is introduced into the alloy melt to remove dissolved hydrogen from the melt to a level below 2mL/100g melt.

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

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

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of: the aluminum melt is cleaned by pumping a solid flux into the aluminum melt, which may be combined with a degassing process.

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of: the aluminum melt is cleaned by pumping a chemical gas flux into the aluminum melt, which may be integrated with the degassing process.

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of: by adding 25% Na in an amount of 0.5 to 0.8 wt%₂SiF₆+75％C₂Cl₆Refining the agent and cleaning the aluminium melt using a rotary degassing unit.

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of: up to 0.3 wt.% of a grain refiner is added to the aluminium alloy melt, which comprises a Sr-containing master alloy for modifying and refining the eutectic silicon phase and a TiB-containing master alloy for refining the primary aluminium phase.

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of: up to 0.2 wt.% of a grain refiner is added to an aluminium alloy melt, which comprises a Sr-containing master alloy for modifying and refining eutectic silicon phases and a TiB-containing master alloy for refining primary aluminium phases.

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of: the primary aluminum phase is refined by adding a TiB-containing master alloy which is at least one of Al-5Ti1B, Al-3Ti1B, Al-1Ti3B or Al-3Ti3B alloy.

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of: the primary aluminum phase is refined by adding a TiB containing master alloy, preferably Al3Ti3B, Al1Ti3B or other B-rich AlTiB master alloys.

The alloy and method of manufacture of the Al-Si-Mg casting preferably consists essentially of at least one heat treatment selected from solution, annealing and aging.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one solid solution at a temperature of 520 ℃ to 545 ℃.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one solid solution at a temperature of preferably 530 ℃ to 540 ℃.

The alloy and method of making Al-Si-Mg castings preferably consists essentially of at least one solid solution at a temperature more preferably from 535 ℃ to 540 ℃.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one solid solution for a time of 2 to 12 hours.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one solid solution for a time of preferably 8 to 10 h.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one ageing at a temperature of 170 ℃ to 200 ℃.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one ageing at a temperature of preferably 170 ℃ to 190 ℃.

The alloy and manufacturing method of the Al-Si-Mg casting preferably consists essentially of at least one aging at a temperature of more preferably 170 ℃ or 190 ℃.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one ageing lasting from 2h to 8 h.

The alloy and the manufacturing method of the Al-Si-Mg casting preferably consist essentially of at least one aging preferably at 170 ℃ for 7 to 8h or at 190 ℃ for 3 to 4 h.

The alloy and method of making an Al-Si-Mg casting that provides enhanced mechanical properties for structural applications preferably comprises: (1) the following alloys were used for optimization: 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 less than 0.15 wt.% each of Cu, Fe, Zn and other impurity elements, and the balance Al and incidental impurities; (2) optimized melt processing with appropriate melting, modification, degassing and grain refinement; (3) a suitable type of grain refiner added to the aluminum melt in an optimized amount and method, and (4) an optimized heat treatment process.

All the above and other features and advantages of the invention will be further understood from the following illustrative and non-limiting description of embodiments of the invention, with reference to the accompanying examples and drawings, in which:

FIG. 1 is a graph showing the microhardness of an alloy versus the solutionizing temperature at 540 ℃;

FIG. 2 is a graph showing the microhardness of an alloy after 8 hours of solutionizing at 540 ℃ versus aging time at an aging temperature of 170 ℃; and

FIG. 3 is a graph showing the yield strength of an alloy after 6 to 14 hours of solutionizing at 540 ℃ versus aging time at an aging temperature of 170 ℃.

In one embodiment, the alloy system according to the principles of the present invention is a modification of the aluminum association alloy system 3 XX. The improved alloy system generally comprises: si in a range of 8.5 to 12.5 wt% and Mg in a range of 0.3 to 0.7 wt%, and one or more of Ti below 0.2 wt%, Mn below 0.1 wt%, Zn below 0.1 wt%, Sn below 0.05 wt%. In addition, the alloy may further comprise: grain refining additives of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, alone or in combination with each other, in the range of 0.001 to about 0.20 wt.% chemical modifiers such as Na and Sr, alone or in combination with each other, and phase refiners such as P, in the range of 0.01 to about 0.30 wt.%, with the balance Al and incidental impurities.

In another embodiment, an alloy system according to the principles of the present invention is an improvement of the aluminum association alloy system 3XX, the improved alloy system 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, less than 0.05 wt.% Sn, and less than 0.1 wt.% zn, further, the alloy further comprises grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, most preferably 0.1 to 0.5 wt.% of an Al3Ti3 64 master alloy comprising TiB2, alone or in combination with each other in the range of 0.001 to 0.5 wt.%, grain refining additions of Ti, TiB2, AlB2, B, Be, Zr, Y, V, Nb, most preferably 0.1 to 0.5 wt.%, a magnesium 3Ti3 master alloy comprising TiB2 and AlB2, alone or in combination with each other in the range of 0.001 to about 0.10 wt.%, and a magnesium 3 Mg 3, and a magnesium alloy, which, a minor precipitation effect, such as a heat treatment, whereby the improvement of the magnesium alloy's ductility and the magnesium alloy's precipitation, the magnesium alloy's grain refining additions of the alloy's precipitation, such as a magnesium precipitation, a major grain refining addition, a magnesium alloy, a magnesium precipitation, a magnesium alloy, a.

In yet another embodiment, the casting will be cast using conventional methods of pouring the molten alloy mixture into permanent, sand or investment molds, or alternatively, using advanced techniques such as high pressure die casting or squeeze casting to produce near net shape (near net shape) castings. Prior to casting, it is necessary to have proper degassing and grain refinement. The cast Al-Si alloy of the present invention may be an alloy element (among its routes) and an aluminum alloy added to resistance furnace melting of molten aluminum; 0.5 to 0.8 percent (quality)Amount percentage) of the refining agent, with the aid of optimum refining effect, using 25% Na₂SiF₆+75％C₂Cl₆Refining the agent and using a rotary degassing unit. The refiner containing TiB has good refining effect on Al-Si alloy, and the best technology is that 0.2 to 0.3 weight percent of refiner is added at 720 ℃, and the temperature is kept for 8 to 15 minutes; al-10Sr alloy with 0.01-0.02 wt% Sr added has excellent modification effect and is added at 740 deg.c; and (3) heat treatment specification: solid solution is carried out for 8 to 10 hours at 540 ℃, and aging is carried out for 7 to 8 hours at 170 ℃; the alloy has high strength and toughness, a yield strength in excess of 300MPa, and an elongation in excess of 9%.

In yet another embodiment, the casting is subjected to a suitable heat treatment according to practice, which involves the following steps: solution heat treating at a temperature near the solidus temperature of the given alloy; quenching into water or other suitable medium, and aging at a temperature in the range of ambient temperature to about 300 ℃. Alternatively, a multi-stage solid solution process and a multi-stage aging process may be employed. For example, in a two-step process, it comprises: primary aging is performed at low temperatures (e.g., less than about 190 ℃, preferably less than 160 ℃) for short periods of time (e.g., greater than 1 hour but less than 10 hours, preferably about 2 hours), followed by secondary aging at high temperatures (e.g., greater than about 100 ℃, preferably about 170 ℃) for extended periods of time (e.g., greater 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 may be applied to improve the disclosed cast alloys. By using the alloys of the present invention to form near net shape castings, significantly improved properties of the cast alloys can be achieved. For example, alloys embodying the present invention have been shown to have a yield strength (0.2% offset) in excess of 300MPa and an elongation in excess of 10%.

The 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 permanent molds (permanent mold). The alloys also include cast aluminum alloys of the a356 and a357 types. A casting is made by: the appropriate ratios of the different elements were weighed and melted in a 12kg clay-graphite crucible in a resistance furnace. After the melt was sufficiently homogenized, it was degassed during which Ar was blown into the melt for 4 minutes by a commercial rotary degasser set at 350 rpm. It should be mentioned that the Al-10Sr alloy is added with 0.01 to 0.02 wt.% Sr before degassing. The TiB-containing refiner was added at 0.005 wt% B at 720 deg.C before pouring. Thereafter, the melt was poured into boron nitride coated steel molds designed based on the ASTM B108 standard to prepare dog bone shaped tensile specimens. In gravity casting using permanent molds, molten metal is poured into steel molds that have been heated to 400 to 460 ℃. Chemical composition analysis was performed using a fountain-Master Pro as a high performance Optical Emission Spectrometer (OES).

Each of the four castings was solution heat treated at 540 ℃ for 8 hours, quenched into ambient temperature water immediately after removal from the furnace, and allowed to stabilize for several days. The ageing was optimized for each alloy by: vickers hardness measurements were made at selected time intervals over a wide range of temperatures according to the American Society for Testing and Materials (ASTM) standard E92-82. The optimized ageing process is ageing at 170 ℃ for 8 hours or at 190 ℃ for 4 hours. Mechanical properties were further measured according to ASTM B557 standard using an Instron5500 universal electromechanical test system equipped with Bluehill software and a ± 50kN weighing cell (loadcell). All tensile tests were performed at ambient temperature (. about.25 ℃). The gauge length of the extensometer was 50mm and the ramp rate of extension was 1 mm/min. The mechanical properties of the four castings after solution and aging treatment are listed in table 2.

TABLE 1

Table 1 chemical composition (wt%) of the alloy in example 1.

As shown in table 2, the developed alloys labeled GC01 and GC02 exhibited higher strength and elongation relative to the commercially available a356 and a357 alloys. This is particularly surprising given that the a357 alloy is the highest strength alloy in the Al-Si-Mg cast alloy systems to date. Further, the Mg content is higher in the a357 alloy, wherein the Mg content is 0.5 to 0.7 wt.%. The published yield strength values of 357-T6 (source: Metals Handbook desktop edition, American Society for Metals, H.E.Boyer and T.L.Gall, 1985, pp.6.48-6.62) (0.55% Mg, yield strength 295MPa) were about 18% higher than those obtained with alloy 356-T6 (same composition as 357 with 0.35% Mg, yield strength 250 MPa). Clearly, the developed compositions were very effective in overcoming this large performance difference observed at slightly different Mg levels. The strength of the developed alloy may be even higher if the Mg content is adjusted above the 0.60 wt% level. More importantly, the developed alloys show good ductility with elongation higher than 10%. The good ductility of the developed alloys can be attributed to the increased castability and reduced porosity levels relative to the existing commercially available a356 and a357 alloys.

In one variation of the above single-step aging process, a two-step aging process is applied to the alloy consisting of: the initial step was carried out at 150 ℃ for 2 hours, followed by aging at 180 ℃ for 6 hours. As the aging time progresses, the alloy achieves a yield level that exceeds that of aging in a single stage. It is evident that the two-step aging process can further expand the gap between new alloys and existing commercial alloy castings. The mechanical properties of the developed alloys labeled GC01 and GC02 under solution and two-step aging treatments are also listed in table 2.

TABLE 2

Table 2 mechanical properties of the permanent mold cast alloy of example 1 after heat treatment.

Example 2: sand casting

Four alloys of the compositions listed in table 3 were cast into sand molds. The alloys also include cast aluminum alloys of the a356 and a357 types. A casting is made by: the appropriate ratios of the different elements were weighed and melted in a 12kg clay-graphite crucible in a resistance furnace. After the melt was sufficiently homogenized, it was degassed during which Ar was blown into the melt for 4 minutes by a commercial rotary degasser set at 350 rpm. It should be mentioned that the Al-10Sr alloy is added with 0.01 to 0.02 wt.% Sr before degassing. The TiB-containing refiner was added at 0.005 wt% B at 720 deg.C before pouring. Thereafter, the melt was poured into a British standard sand mold to prepare a dog bone-shaped tensile specimen. In gravity casting using a sand mold, molten metal is poured into the sand mold at room temperature. Chemical composition analysis was performed using a fountain-Master Pro as a high performance Optical Emission Spectrometer (OES).

Each of the four castings was solution heat treated at 540 ℃ for 8 hours, quenched into ambient temperature water immediately after removal from the furnace, and allowed to stabilize for several days. The ageing was optimized for each alloy by: vickers hardness measurements were made at selected time intervals over a wide range of temperatures according to the American Society for Testing and Materials (ASTM) standard E92-82. The optimized ageing process is carried out at 170 ℃ for 8 hours or 190 ℃ for 4 hours. Mechanical properties were further measured according to ASTM B557 standard using an Instron5500 universal electromechanical test system equipped with Bluehill software and a ± 50kN weighing cell. All tensile tests were performed at ambient temperature (. about.25 ℃). The gauge length of the extensometer was 50mm and the ramp rate of extension was 1 mm/min. The mechanical properties of the four castings after solution and aging treatment are listed in table 4.

TABLE 3

Table 3 chemical composition of the alloy in example 2.

As shown in table 4, the commercially available a356 alloy showed a yield strength of 230MPa and a UTS of 280MPa at 0.35 wt.% Mg and the commercially available a357 alloy showed a yield strength of 275MPa and a UTS of 300MPa at 0.5 wt.% Mg, while the developed alloys with 0.5 wt.% Mg, labeled SC01 and SC02, showed a significant increase in strength, yield strength above 295MPa and UTS above 325MPa compared to the commercially available a356 and a357 alloys. This is particularly surprising given that the a357 alloy is the highest strength alloy in the Al-Si-Mg cast alloy systems to date. More importantly, the Mg content is higher in the a357 alloy, with Mg content from 0.5 wt% to 0.7 wt%, and the a357 alloy achieves higher strength at higher Mg content but elongation significantly reduced to below 3% compared to the a356 alloy, whereas the developed alloy achieves higher strength without significant reduction in elongation compared to the a356 alloy. Clearly, the developed compositions were very effective in overcoming this large performance difference observed at slightly different Mg levels. The strength of the developed alloy may be even higher if the Mg content is adjusted above the 0.60 wt% level.

In one variation of the above single-step aging process, a two-step aging process is applied to the alloy consisting of: the initial step was carried out at 150 ℃ for 2 hours, followed by aging at 180 ℃ for 6 hours. As the aging time progresses, the alloy achieves a yield level that exceeds that of aging in a single stage. It is evident that the two-step aging process can further expand the gap between new alloys and existing commercial alloy castings. The mechanical properties of the developed alloys labeled SC01 and SC02 under solution and two-step aging treatments are also listed in table 4.

TABLE 4

Table 4 mechanical properties of the sand cast alloy of example 2 after heat treatment.

All optional and preferred features and improvements of the described embodiments and the dependent claims are applicable to all aspects of the invention taught herein. Furthermore, individual features of the dependent claims as well as all optional and preferred features and refinements of the described embodiments are combinable and interchangeable with each other.

The disclosure in uk patent application No. 1713005.5, to which priority is claimed in this application, and in the abstract of this application is incorporated herein by reference.

Claims

1. A method of forming a cast aluminum alloy, the method comprising the steps of:

(i) providing an aluminum alloy comprising:

8.5 to 12.5% by weight of Si,

0.4 to 1.0 wt.% of Mg,

up to 0.2% by weight of Ti,

0.05 to 0.25% by weight of Mn,

0.002 to 0.04 wt.% Sr,

0.001 to 0.1% by weight of B,

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

(ii) melting the alloy;

2. The method of claim 1, wherein the alloy of step (i) comprises:

8.5 to 10.0% by weight of Si,

0.45 to 0.65% by weight of Mg,

0.1 to 0.15% by weight of Ti,

less than 0.15 wt.% Mn,

0.008 to 0.02% by weight of Sr,

0.004 to 0.04 wt% of B,

and less than 0.15 wt.% each of Cu, Fe, Zn, and the balance Al and incidental impurities.

3. The method of any preceding claim, wherein the dissolved hydrogen in step (iii) is reduced to a level below 0.2mL/100g melt.

4. The method of any preceding claim, wherein the grain refiner of step (v) comprises up to 3.5 wt% ai 3Ti3B or ai 1Ti 3B.

5. A method according to any preceding claim wherein the B content of the grain refiner of step (v) is 0.004 wt% to 0.04 wt%.

6. The process according to any preceding claim, wherein in step (vi), the amount of Sr is from 0.008 to 0.02 wt%.

7. The method of any preceding claim, wherein the solution heat treatment of step (vii) is carried out at a temperature of 535 to 540 ℃ for a time of 8 to 10 hours.

8. The method of any preceding claim, wherein the ageing heat treatment of step (viii) is carried out at a temperature of about 170 ℃ for a time of 7 to 8 hours.

9. The process as claimed in claims 1 to 7, wherein the ageing heat treatment of step (viii) is carried out at a temperature of 180 to 190 ℃ for a time of 2 to 5 h.

10. An aluminum alloy, comprising: 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, with the balance being Al and incidental impurities.

11. The aluminum alloy of claim 10, further comprising: ti, TiB in the range of 0.001 to 1.0 wt.% either alone or in combination with each other₂、AlB₂B, Be, Zr, Y, V, Nb, chemical modifiers such as Na and Sr, in the range of 0.001 wt% to about 0.10 wt% and phase refiners such as P, alone or in combination with each other, in the range of 0.01 wt% to 0.30 wt%.