Aluminium casting alloy

Description

[0001] The invention relates to an aluminium casting alloy for casting operations, in particular die-casting operations. Further the invention relates to the application of the aluminium casting alloy in particular into cast products for automotive components.

[0002] Conventional aluminium casting alloys have many attractive properties, such as high ultimate tensile strength (>180 MPa) and high elongation at fracture (>9 %) with moderate 0.2% yield strength (>120 MPa). However, there is a demand for aluminium casting alloys for manufacturing cast product which combine improved mechanical properties, in particular high elongation at fracture, with a good corrosion resistance, such as resistance to pitting.

[0003] Some disclosures of aluminium casting alloys found in the prior art literature will be mentioned below.

[0004] EP-A-0918095 discloses a structural component made of an aluminium die-casting alloy, consisting of, in weight percent:

Si	< 0.5
Fe	< 1.0
Mn	0.1 to 1.6
Mg	< 5.0
Ti	< 0.3
Zn	<0.1
Sc	0.05 to 0.4 and optional Zr 0.1 to 0.4
balance aluminium and impurities.

By the addition of the very expensive Sc in a range of 0.05 to 0.4% and optionally in combination with Zr in a range of 0.1 to 0.4% and the requirement of an heat treatment in the range of 230 to 350°C following the die-casting of the structural component a yield strength of about 120 MPa, a tensile strength of 180 MPa and an elongation at fracture of 16% is obtained.

[0005] EP-A-0918096 discloses a structural component made of an aluminium die-casting alloy, consisting of, in weight percent:

Si	< 1.4
Fe	< 0.8
Mn	0.1 to 1.6
Mg	< 5.0
Ti	< 0.2
Zn	< 0.1
V	0.05 to 0.3
balance aluminium and impurities.

By the addition of the expensive V in a range of 0.05 to 0.3 and the requirement of an heat treatment in the range of 200 to 400°C following the die-casting of the structural component an yield strength of above 120 MPa, a tensile strength of more than 180 MPa and an elongation at fracture of more than 10% is obtained.

[0006] EP-A-0908527 discloses an aluminium casting alloy, in particular suitable as a die-casting alloy, consisting of, in weight percent:-

Mg	2.0 - 3.3
Si	0.15 - 0.35
Mn	0.2 - 1.0
Fe	< 0.20
Cu	< 0.05
Cr	< 0.05
Zn	< 0.10
Be	< 0.003
Ti	< 0.20
Ce	< 0.80
balance aluminium and impurities.

This casting alloy is capable of achieving a yield strength of more than 100 MPa and an elongation of more than 14%. Further the die-sticking of the alloy in a die-casting operation can be reduced by replacing part of the Mn by more expensive Ce.

[0007] WO-A-00/17410 discloses an aluminium die-casting alloy, consisting of, in weight percent:-

Mg	2.5 - 4.0
Mn	1.0 - 2.0
Fe	< 0.60, preferably 0.25 - 0.60
Si	< 0.45, preferably 0.20 - 0.45
Cu	<0.10
Zn	<0.10
Be	< 0.03
balance aluminium and impurities.

This aluminium die-casting alloy does not suffer from die-sticking and cast products are capable of achieving a yield strength of at least 117 MPa and an elongation of at least 18%.

[0008] US-A-4,605,448 discloses an aluminium wrought alloy for use in manufacturing both can body parts and can ends, the aluminium wrought alloy having a composition, in weight percent:-

Mg	0.50 - 1.25
Mn	0.30 - 1.50
Si	0.52 - 1.00
balance aluminium and impurities.

Can stock material and which is being rolled and otherwise thermo-mechanically treated is not within the technical field of the present invention.

[0009] It is an object of this invention to provide an aluminium casting alloy that can be used in a variety of casting operations.

[0010] It is another object of this invention to provide an aluminium casting alloy ideally suited for use in die-casting operations, in particular in high-pressure die casting operations.

[0011] It is also an object of this invention to provide improved cast products and components manufactured from an improved aluminium casting alloy cast members that ideally are suited for automotive applications.

[0012] It is also an object of this invention to provide an aluminium casting alloy for making cast products having in at least the as-cast condition the following minimum mechanical properties: 0.2% yield strength (YS) of at least 120 MPa, a tensile strength (UTS) of at least 180 MPa and an elongation at fracture of at least 9%.

[0013] It is also an object of this invention to provide an aluminium casting alloy for making cast products having in the as-cast condition the following minimum mechanical properties: 0.2% yield strength of at least 120 MPa, a tensile strength of at least 180 MPa and an elongation at fracture of at least 9 %, which minimum mechanical properties can be achieved without the addition of expensive alloying elements such as Sc, V and Ce.

[0014] According to the invention there is provided an aluminium casting alloy having the following composition, in weight percent:-

Mg	1.0 - 2.6
Si	0.5 - 2.0
Mn	0.9 - 1.4
Fe	< 0.50
Cu	< 1.0
Zn	< 0.30
Ti	< 0.20
Be	< 0.003
balance aluminium and inevitable impurities.

[0015] By the invention cast products or cast bodies can be provided having high strength in combination with high elongation at fracture. In addition these products have a good corrosion resistance, in particular resistance to pitting corrosion and stress corrosion, and can be welded using known welding techniques for this type of casting alloys. It has been found also that alloys of the present invention have a good castability, in particular in die-casting operations, and no soldering occurs when using the casting alloy. The aluminium casting alloy according to the invention is capable of achieving in the as-cast condition an 0.2% yield strength of more than 120 MPa, in combination with a tensile strength of more than 180 MPa and an elongation at fracture of more than 9%, which mechanical properties are being achieved without the addition of expensive alloying elements such as Sc, V and Ce.

[0016] The invention also consists in products made from the aluminium casting alloy set out above. Typical examples of such cast products are die-cast, in particular high pressure die-cast, safety components, vehicle wheels, steering wheels, steering columns, airbag modules/cans, brake drums and frame members for a vehicle such as frame members for automobiles and trains. The aluminium casting alloy is particularly suited for manufacturing products having load and impact requirements where properties of high strength and high elongation at fracture are desirable.

[0017] The present aluminium casting alloy is environmentally friendly and is readily recyclable because it does not contaminate the wrought alloy stream of recycled materials. The aluminium alloy is typically solidified into ingot-derived stock by continuous casting or semi-continuous casting into a shape suitable for remelting for casting, which shape is typically an ingot billet.

[0018] It is believed that the improved properties available with the invention, particularly improved strength levels and a high elongation at fracture in combination with good casting characteristics, results from the combined additions of Mg, Si, and optionally Cu in the given ranges. The aluminium casting alloy is therefore ideally suited for the improved post casting processing. i.e. the elimination of conventional high temperature solution heat treating and optionally ageing at room temperature or elevated temperature, while providing complexly shaped cast products with improved dimensional stability and mechanical properties.

[0019] The reasons for the limitations of the alloying elements of the aluminium casting alloy according to the present invention are described below. All composition percentages are by weight.

[0020] In an embodiment of the casting alloy according to the invention the following levels for the Mg, Si and Cu are selected:-

Mg	1.0 - 1.6
Si	0.5 - 1.0
Cu	0.5 - 1.0, and preferably 0.5 - 0.75

In this embodiment high strength levels are achieved due to the high Cu levels. As a result of this high Cu level the corrosion resistance, such as resistance to pitting corrosion, is somewhat reduced but still acceptable dependent on the application environment of the cast product. In this embodiment also the highest elongation at fracture levels are reached in the as-cast condition.

[0021] In another embodiment of the casting alloy according to the invention the following levels for the Mg, Si and Cu are selected:

Mg	1.5 - 2.6, and preferably 1.7 - 2.4
Si	0.5 - 1.0
Cu	<0.50, and preferably < 0.30, and more preferably < 0.10.

In this embodiment still very good tensile levels and elongation at fracture levels are achieved in the as-cast condition and a significant improvement in corrosion resistance, such as the resistance to pitting corrosion.

[0022] In another embodiment of the casting alloy according to the invention the following levels for the Mg, Si and Cu are selected:-

Mg	1.5 - 2.6, and preferably 1.7 - 2.4
Si	0.9 - 1.5
Cu	0.5 - 1.0, and preferably 0.5 - 0.75

In this embodiment the highest strength levels are achieved in the as-cast condition due to the high levels of Mg, Si and Cu.

[0023] Mn is an important alloying element for all embodiments of the aluminium casting alloy according to the invention. The Mn level should be in the range of 0.9 to 1.4%. A more preferred Mn level is in the range of 0.9 to 1.3, and more preferably in the range of 1.0 to 1.3 as a compromise in the achievable strength levels and casting behaviour of the aluminium alloy.

[0024] Fe is a known element in aluminium casting alloys and may be present in a range of up to 0.5 %. At higher levels Fe may form undesirable large compounds with Mn in the holding furnaces typically employed in casting operations. When higher fracture toughness and/or ductility is desired a suitable maximum for the Fe content is 0.4 %, and more preferably 0.3 %, and most preferably 0.2 %.

[0025] Zn is an impurity element which can be tolerated in an amount of up to 0.30 %. A more preferred upper limit for the Zn is 0.10 %.

[0026] Ti is important as a grain refiner during solidification of both cast products and welded joint produced using the alloy of the invention. A preferred maximum for Ti addition is 0.2 %, and a more preferred range is of 0.01 to 0.14 %.

[0027] Be may be added to magnesium containing casting alloys to prevent oxidation of the magnesium in the aluminium alloy, the amount added varying with the magnesium content of the alloy. As little as up to 0.003% causes a protective beryllium oxide film to form on the surface. Preferably, the Be level has a maximum of 0.003%, and more preferably is absent without detonating the properties of the cast product with this aluminium casting alloy.

[0028] The balance is aluminium and inevitable impurities. Typically each impurity is present at 0.05 % maximum and the total of impurities is 0.25 % maximum.

[0029] In an embodiment of the aluminium casting alloy according to the invention the aluminium alloy is capable of achieving in the as-cast condition an 0.2% yield strength of more than 140 MPa, and in the best examples of more than 175 MPa, in combination with a tensile strength of more than 230 MPa, preferably more than 260 MPa, and in combination with an elongation at fracture of more than 10%, and in the best examples even more than 14%. By optimising the casting parameters, further improved tensile properties, and in particular in elongation at fracture, can be obtained. Furthermore, improvements in the mechanical properties of the aluminium casting alloy according to the invention can be obtained by heat-treating the cast product or cast body as is conventional in the art, e.g. high temperature solution heat treating followed by cooling and ageing. This further improvement is achieved at the expense of the loss of the earlier advantage that following casting operation no further heat-treatments are required to achieve a desirable level of mechanical properties.

[0030] The aluminium casting alloy in accordance with the invention may be processed by applying various casting techniques. The best results are being achieved when applied via permanent mould casting, die-casting, or squeeze casting. In particular when die-casting processes are applied, including vacuum die-casting processes, the best combination of desirable properties and castability characteristics is being obtained. It is believed that by applying vacuum die-casting the weldability characteristics of the aluminium alloy according to the invention may be further improved. It is to be understood here that die-casting includes high-pressure die-casting operations.

[0031] The invention will now be explained by reference to non-limiting examples.

Example 1.

[0032] On an industrial scale of casting three aluminium alloys according to the invention, see Table 1, have been die-cast on a Mueller-Weingarten cold-chamber-die-casting machine with a locking pressure of 2 MN. The casting parameters varied comprised the preheat temperature of the die (130°C and 210°C) and the back-pressure (500 and 900 bar). The die-cast product consisted of a plate having dimensions 100x150x2 mm.

[0033] From this die-cast plate three tensile specimens have been machined and subsequently tested in the as-cast condition. Alloy no 3 has been subjected also to a heat-treatment of holding the die-cast product for 1000 hours at 150°C. The mechanical properties in the as-cast condition averaged over three specimens tested have been listed in Table 2, where 0.2% YS stands for 0.2% yield strength and UTS for ultimate tensile strength. The aluminium alloy according to the invention showed during the die-casting operation no tendency to die-sticking or soldering. Alloy 3 had after heat-treatment an 0.2% YS of 184 MPa, an UTS of 247 MPa, and an elongation at fracture of 5.0%.

[0034] From the results in Table 2 it can be seen that the aluminium alloy according to the invention results in very high tensile properties and high elongation in the as-cast condition. These surprisingly high properties are achieved without the need for further heat treatments. Further heat treatment may further increase the strength of the cast product. In particular the UTS and the elongation can be improved by increasing the back-pressure in the die-casting operation. Smaller improvements in mechanical properties can be obtained by increasing the die-temperature. Further improvements can be expected by optimising the casting conditions, in particular by applying vacuum (high pressure) die-casting instead of conventional (high pressure) die-casting.

Example 2.

[0035] The 2mm vacuum die-cast product of Example 1 having the composition of Alloy no. 2 of Table 1 has been subjected also to a welding operation, during which in particular the development of porosity has been assessed.

[0036] Various welded joints have been made whereby the 2 mm die-cast plate was put on top of a 1.6 mm gauge AA6016A-wrought sheet such that an overlap was created ("Überlappverbindung"). At the overlap a weld was made by means of automated MIG welding, in a single pass and using 1.2 mm filler wire of AlSi12 (DIN 1732). Following welding the porosity in the welds has been determined using standard metallographic assessment techniques. An important criteria of a large European car manufacturer is that the level of porosity, as assessed by the square area occupied by the pores, must be 8% or less in order to qualify the weld as acceptable. Furthermore the pore size must be smaller then 0.5 times the thinnest welded sheet used, and only the pores that are larger then 0.05 mm should be taken into account.

[0037] In the welds using the die-cast alloy according to the invention the average porosity level was always in the range of 0.5 to 2.0%. No large pore sizes (> 0.8 mm) have been found. This qualifies the aluminium die-casting alloy as being very good weldable.

Table 1.

Alloy	Alloying element, in wt. %, balance aluminium
	Si	Fe	Cu	Mn	Mg	Zn	Ti	Zr
1	1.4	0.23	0.52	1.15	2.5	0.02	0.01	0.004
2	0.95	0.25	0.03	1.20	2.4	0.02	0.005	0.01
3	1.04	0.29	0.01	1.27	1.6	0.15	0.005	0.01

Table 2.

Alloy	Die-casting parameter applied	Mechanical properties in the as-cast condition
		0.2% YS [MPa]	UTS [MPa]	Elongation at fracture [%]
1	130°C/500bar	154	251	10.1
1	210°C/500 bar	156	251	10.5
1	130°C/900 bar	153	256	12.2
1	210°C/900 bar	154	257	14.6
2	130°C/500bar	169	274	9.7
2	210°C/500 bar	180	289	10.0
2	130°C/900 bar	173	275	9.0
2	210°C/900 bar	172	286	10.8
3	130°C/900 bar	131	210	7.0

Example 3.

[0038] Specimens taken from die-cast plates having dimensions of 100x150x4 mm and having the composition of, in weight percent: Si 1.04, Fe 0.32, Mn 1.27, Mg 1.60, Zn 0.15, Zr 0.01, Cu 0.01, balance aluminium and impurities, and have been used for stress corrosion cracking ("SCC") tests in accordance with ASTM G39-90 using four-point loaded specimens. The surface roughness of the specimens were Ra 0.6-0.7µm, applied stress was 80% of the yield strength. The SCC-testing took place in three conditions, namely as-cast, after holding for 1 hour at 190°C, and after holding for 1000 hours at 150°C.

[0039] It has been found that the aluminium casting alloy according to the invention showed no cracks in neither three conditions after been tested for SCC according to ASTM G39-90. This qualifies the aluminium die-casting alloy as having a good corrosion resistance, in particular against stress-corrosion cracking, and which good corrosion resistance enhances its applicability for automotive applications.

Claims

1. An aluminium casting alloy consisting of, in weight percent:-

Mg	1.0 - 2.6
Si	0.5 - 2.0
Mn	0.9 - 1.4
Fe	< 0.50
Cu	< 1.0
Zn	< 0.30
Ti	< 0.20
Be	< 0.003
impurities	each 0.05 max. total 0.25 max.
balance aluminium.

2. An aluminium casting alloy according to claim 1, wherein

Mg	1.0 - 1.6
Si	0.5 - 1.0
Cu	0.5 - 1.0

3. An aluminium casting alloy according to claim 1, wherein

Mg	1.5 - 2.6
Si	0.9 - 1.5
Cu	< 0.5

4. An aluminium casting alloy according to any one of claims 1 to 3, wherein the Mn content is in the range of 0.9 to 1.3, and preferably in the range of 1.0 to 1.3.

5. An aluminium casting alloy according to any one of claims 1 to 4, wherein the aluminium alloy is a die-casting alloy.

6. Aluminium alloy according to claim 5, where an aluminium die-cast product in the as-cast condition has an UTS of at least 230 MPa, and an 0.2% YS of at least 140 MPa, and an elongation of at least 10 %.

7. Use of a die-casting alloy according to claim 5 or 6 for die-casting of safety components.

8. Use of a die-casting alloy according to claim 5 or 6 for die-casting of a frame member of a vehicle.

9. A cast product, preferably a die-cast product, made of an aluminium casting alloy according to any one of claims 1 to 5.

10. A cast product according to claim 9, wherein the aluminium cast product in the as-cast condition has an UTS of at least 230 MPa, and an 0.2% YS of at least 140 MPa, and an elongation of at least 10 %.