EP3339465B1

EP3339465B1 - Method for solution heat treating with pressure

Info

Publication number: EP3339465B1
Application number: EP17206338.0A
Authority: EP
Inventors: Kevin R. Morasch; Kevin R. Anderson; Raymond J. Donahue; Christopher J. Misorski
Original assignee: Brunswick Corp
Current assignee: Brunswick Corp
Priority date: 2016-12-23
Filing date: 2017-12-11
Publication date: 2020-01-15
Anticipated expiration: 2037-12-11
Also published as: EP3339465A1; JP7054621B2; JP2018103264A; CN108239731A

Description

The present application relates to heat treatment of high pressure die cast objects, and more particularly to the solution heat treatment of high pressure die cast objects with pressure. In particular, the present invention relates to a method of heat treating a high pressure die cast aluminum alloy object with the features of the introductory part of claim 1.
High pressure die casting is a low cost, dimensionally accurate casting process for high volume production of many cast objects, including, but not limited to marine propellers, marine and automotive engine components, vehicle chassis, vehicle closures, vehicle structural bodies, and advanced vehicle bodies. Approximately 70% of all cast aluminum is cast using the high pressure die casting process, primarily using aluminum alloys. In high volume, the high pressure die casting process delivers castings on a cost per pound basis that are lower cost at higher dimensional accuracy than alternative methods such as permanent mold casting or sand casting.
The disadvantage of the high pressure die casting process is that it results in cast objects having comparatively lower strength than permanent mold or sand casting. This lower strength is due to the fact that the high pressure die casting process moves metal in a violent, turbulent fashion, entrapping air in the molten metal during the casting process. This entrapped air becomes problematic because, in order to increase the strength of a high pressure aluminum die cast object, it is preferable to heat treat the cast object after casting. A typical heat treatment to strengthen aluminum alloys is solution heat treatment and artificial aging to achieve the T6 treatment condition. However, heat treatments above 371°C (700°F) are not used for increasing the strength and ductility of high pressure die cast objects because often the air entrained or entrapped in the castings expands during the solution heat treatment at temperatures above 371°C (700°F), creating blisters on and in the cast product. The blisters cause aesthetic issues with surface finish and create large pores that reduce the mechanical properties of the casting. Therefore, conventional high pressure die castings are used in the "as cast" condition. For many low copper content alloys like Aluminum Association alloys 367, 360, or 361 the associated yield strength is about 1172 bar (17 KSI), in the "as cast" condition.
In traditional solution heat treatment, the cast object is placed in an air furnace, fused salt bath, or fluidized sand bed and manipulated with heat to develop a wide range of mechanical properties and achieve a combination of properties unattainable by other means. The high pressure die cast object is heated to a specific temperature to create a super-saturated solution of alloy elements, and the object is then soaked at that temperature for a given time. Subsequently, the die cast object is rapidly quenched and artificially aged at a lower temperature for a given period of time.
During the solution heat treatment portion of the T6 heat treating process, cast objects may be subjected to temperatures up to 538 °C (1000°F) for up to 12 hours. At 538°C (1000°F) a permanent mold or sand cast aluminum alloy cast object will dissolve any magnesium present into solid solution for the given alloy, and will thermally modify the eutectic silicon providing mechanical advantages. Again however, when high pressure die cast objects are subjected to T6 heat treatment conditions, any entrained gas in the casting will volumetrically expand at the increased temperature, increasing the pressure within the pore or defect. This increase in pressure and the result of low mechanical properties of the metal at that elevated temperature creates a situation where the metal plastically deforms leaving a blister defect either internally or at the surface that creates aesthetic and mechanical problems. Prior to the present application, blister defects were prevented by heat treating high pressure die cast objects at very short solution times (e.g. 15 minutes). This short solution time fails to allow for sufficient modification of the eutectic and does not create the mechanical advantages that a longer (e.g. 2-12 hour) treatment creates.
It remains highly desirable to conduct heat treatment of high pressure die cast objects because the heat treatment generally doubles the yield strength. For example, a T6 heat treatment of an aluminum alloy high pressure die cast object will increase from 1172 bar (17 KSI) in the "as cast" condition to approximately 2413 bar (35 KSI) in the T6 heat treated condition, if no blistering defects arise to impair the mechanical properties. Moreover, this dramatic increase in strength will allow a design engineer to redesign a part that typically achieves a 30% reduction in the weight of the part when considering multiple modes of loading and part geometries.
It is known in the art to apply pressure to objects cast using the sand casting or permanent mold casting processes through hot isostatic pressing or HIP. The HIP process involves healing of shrinkage porosity and subsequent improvements in tensile and fatigue properties for sand cast or permanent mold cast aluminum castings. Internal shrinkage porosity results from solidification shrinkage of the alloy and processing variables such as the geometric effects of the mold, or the effects of casting parameters including metal temperature, mold temperature, cooling rate, and pour rate. The HIP procedure involves the use of uniform gas pressure applied at elevated temperatures and subsequent slow cooling to room temperature. The parts are commonly solution heat treated after cooling to room temperature. In the case of aluminum alloys, pressures above 1034 bar (15 KSI) and temperatures around 527°C (980°F) can be used. The applied pressure causes plastic flow in the material and the resulted healing of shrinkage porosities, however, in the HIP process, it is well known that pressures of 690 bar (10 KSI) or less are inadequate for full densification of the material within the time and temperature of limitations for the HIP process. Accordingly, a pressure of 1034 bar (15 KSI) or greater is generally required to realize the advantages of the HIP process.
There are several problems with using the hot isostatic pressing process with high pressure die cast objects. First, the high pressure die casting process requires high pressures, above 1034 bar (15 KSI), and large, expensive pressure vessels to attain that pressure. More significantly, the hot isostatic pressing process is incapable of fixing blistering defects resulting from the high pressure die casting process. In other words, the extensive amount of entrapped air in high pressure die cast aluminum alloy castings cannot be fixed by the hot isostatic pressing process. This well-known lack of effectiveness of HIP processing on high pressure die castings was verified by the inventors in an experiment where a high pressure die cast propeller was subjected to 1034 bar (15 KSI) pressure at 538°C (1000°F) for 4 hours of hot isostatic pressing and allowed to cool to room temperature. The same propeller was then heat treated at 538°C (1000°F) for 4 hours at atmospheric conditions. Blistering defects were still evident after the process showing that the internal defects in the casting were not healed by the HIP process as shown in Figs. 9A and 9B. Thus, it is known that the hot isostatic pressing (HIP) process is incapable of curing blistering defects from subsequent solution heat treatment of cast objects, particularly aluminum alloy cast objects.
In a method of heat treating a high pressure die cast aluminum alloy object according to the starting point of the invention (R.N. LUMLEY, R.G. ODONNEL, D.R. GUNASEGARAM, M. GIVORD: "Heat Treatment of High Pressure Die Castings", METALLURGICAL AND MATERIALS TRANSACTIONS, vol. 38A, 29 August 2010 (2010-08-29), pages 2564-2574). Blistering defects on the high pressure die cast object are prevented by lowering the temperature of solution heat treating to values below 525°C or even below 490°C along with shortening the time of solution heat treating to less than 20 or even 15 minutes. This method was applied in an experimental environment. US3732128 discloses a heat treatment of die castings of Al, Mg or their alloys consisting of heating to 300°C to 600°C in a closed chamber at 100-10000 psi for 1-72 h are rapidly cooled while still under pressure, and are then reheated to 100°C to 250°C for 1 to 72 h under atmospheric pressure. Mechanical properties are improved without the formation of blisters.
It is the object of the present invention to provide a method of heat treating a high pressure die cast aluminum alloy object that avoids or at least reduces blistering effects in another way and in particular applicable in a practical manufacturing environment.
The above-mentioned object is met with a method comprising the features of claim 1. Preferred improvements and modifications of the method according to claim 1 are the subject matter of dependent claims 2 to 8.
In accordance with the present application, it has been found that the application of pressure during the solution heat treatment process, at pressures below conventional hot isostatic pressing (HIP) pressures, results in a beneficial pressure equilibrium within an "as cast" high pressure die cast object where air entrained in the casting due to the high pressure die casting process cannot expand and form blistering defects. Accordingly, the present application discloses an application of external pressure during solution heat treatment of a high pressure die cast object to inhibit the problematic blistering defects that occur during a traditional heat treatment of a high pressure die cast object.
The present application discloses a method of heat treating a high pressure die cast object. The method includes obtaining a high pressure die cast object, and solution heat treating the high pressure die cast object above 371°C (700°F) for 0.5 to 12 hours at a pressure between 35 bar (0.5 KSI) and 2413 bar (35 KSI). Subsequently, the cast object may be quenched and artificially aged to create a high pressure die cast object without blistering defects. The pressure applied during the solution heat treatment step is between 172 bar (2.5 KSI) and 690 bar (10 KSI) or at any pressure or range of pressures therebetween. In an embodiment the pressure applied is between 172 bar (2.5 KSI) and 345 bar (5 KSI) or at any pressure or range of pressures therebetween. The use of external pressure above 172 bar (2.5 KSI) creates a pressure balance during the heat treatment of the high pressure die cast object such that air cannot expand to cause the problematic blistering on the final heat treated object. The use of external pressure between 35 bar (0.5 KSI) and 2413 bar (3.5 KSI) is sufficient to reduce and/or eliminate blistering defects.
In one embodiment, the step of solution heat treating comprises a T6 heat treatment with the application of pressure between 172 bar (2.5 KSI) to 690 bar (10 KSI), or at any pressure or range of pressures between 172 bar (2.5 KSI) and 690 bar (10 KSI). In another embodiment, the solution heat treatment temperature is between 371°C (700°F) and 649°C (1200°F), or at any temperature or range of temperatures therebetween. In another embodiment, the temperature is between 427°C (800°F) and 538°C (1000°F), or at any temperature or range of temperatures therebetween. In another embodiment, the solution heat treatment temperature is at 538°C (1000°F). In one embodiment, the solution heat treatment step is 2 to 8 hours, while in yet another embodiment, the solution heat treatment time with pressure is 4 to 6 hours. It will be recognized that such ranges are exemplary, and the range of time may be at any time within the ranges noted.
The method of heat treating may further include the step of quenching the cast object. The method of heat treating may also include a step of artificially aging the cast object. The step of quenching will typically occur immediately after the cast object is removed from the solution heat treatment pressure vessel. Allowing the cast object to slowly cool to room temperature without cooling is not desirable since the beneficial effects to the microstructure from solution heat treatment may be lost.
In a further preferred embodiment the step of obtaining a high pressure die cast aluminum alloy object includes the steps of casting the object with high pressure in a die casting equipment and removing the object thereafter from the high pressure die casting equipment.
By employing the solution heat treatment and pressure method of the present application, it is contemplated that the yield strength of cast objects may increase by 50% to 100%. This translates into a 15-30% weight reduction on average for structural components. This weight reduction has substantial economic and societal value in terms of energy and CO2 footprint reduction in automotive and other transportation applications where increasing fuel economy is paramount.
Further details for the present invention can be obtained from the following description of preferred and non-limiting embodiments of the invention with reference to the drawings. In the drawings:

Fig. 1: is a perspective view of an exemplary pressure chamber used for the disclosed method of solution heat treating with pressure processing of the present application.
Fig. 2: is a photograph demonstrating blister defects on a solution heat treated high pressure die cast aluminum alloy propeller blade subjected to solution heat treatment at 538°C (1000°F) with no pressure applied.
Figs. 3a-3c: are photographs of a solution heat treated high pressure die cast aluminum alloy propeller blade demonstrating reduction of blistering defects when 35 bar (0.5 KSI) (500 psi) of pressure is applied during solution heat treatment.
Figs. 4a-4c: are photographs of a solution heat treated high pressure die cast aluminum alloy propeller blade demonstrating reduction of blistering defects when 138 bar (2.0 KSI) (2000 psi) of pressure is applied during solution heat treatment.
Figs. 5a - 5b: are photographs of a solution heat treated high pressure die cast aluminum alloy propeller blade demonstrating elimination of blistering defects when 240 bar (3.5 KSI) (3500 psi) of pressure is applied during solution heat treatment.
Figs. 6a - 6b: are photographs demonstrating a solution heat treated high pressure die cast aluminum alloy propeller blade where blistering defects are eliminated through the application of 345 bar (5.0 KSI) (5000 psi) of pressure during solution heat treatment.
Figs. 7a - 7b: are photographs of a solution heat treated high pressure die cast aluminum alloy propeller blade demonstrating elimination of blistering defects when 690 bar (10.0 KSI) (10,000 psi) of pressure is applied during solution heat treatment.
Figs. 8a - 8b: are photographs of a solution heat treated high pressure die cast aluminum alloy propeller blade demonstrating elimination of blistering defects when 1034 bar (15.0 KSI) (15,000 psi) of pressure is applied during solution heat treatment.
Figs. 9a - 9b: are photographs of a high pressure die cast aluminum alloy propeller blade treated by hot isostatic processing (HIP) and subsequently solution heat treated with no pressure.
Figs. 10a - 10: are photographs of a high pressure die-cast aluminum alloy propeller blade treated with 159 bar (2.3 KSI) of pressure during solution heat treatment.

The present application relates to a method of reducing and/or eliminating blistering defects in high pressure die cast metal objects that typically occur during the solution heat treatment of high pressure die cast objects. The present application contemplates that application of pressure between 172 bar (2.5 KSI) and 690 bar (10 KSI), or at any pressure or range of pressures therebetween, will eliminate blistering defects. In one embodiment, the application pressure is lower than the pressure applied during hot isostatic pressing. The application of pressure creates a balance with any air that may be entrained or entrapped in the high pressure die cast object due to the turbulent nature of the high pressure die casting process. By the application of pressure, any air entrained or entrapped in the casting cannot expand, and therefore blisters are reduced and/or eliminated. Accordingly, the increasing internal pressure of entrained air during solution heat treatment is offset as the casting is heated to elevated temperatures with external pressure. If the external pressure and the inherent strength of the material at elevated temperatures is greater than the internal pressure of the entrapped air, blistering will not occur.
The present application contemplates a method of heat treating a high pressure die cast object. In one embodiment, the high pressure die cast object is an aluminum alloy high pressure die cast object, however, the present application may be used for heat treating any high pressured die cast metal object wherein air becomes entrained during the turbulent high pressure die casting process. The method contemplates first obtaining a high pressure die cast object 2. In the embodiment shown in the Figs. 2-8, the high pressure die cast object 2 is a marine propeller, however the present application is applicable for any high pressure die cast object that may be formed using the high pressure die casting method, including but not limited to, vehicle chassis, vehicle closures, structural bodies, and advanced vehicle bodies.
Referring now to Fig. 1, once the high pressure die cast object is obtained, the object is subjected to solution heat treatment with pressure. A pressure vessel 4 having an end closure 6, heating means such as heating elements 8 and workload support 10 may be used for the step of solution heat treating with pressure. However, the pressure vessel 4 may be any certified pressure vessel capable of applying pressure up to 2413 bar (35 KSI) and heat up to 649°C (1200°F). In one embodiment, a certified pressure vessel capable of applying pressure up to 1034 bar (15 KSI) is acceptable, while in other embodiments, certified vessels that have a maximum pressure of 690 bar (10 KSI) or 345 bar (5 KSI) are acceptable. Acceptable pressure vessels for solution heat treatment with pressure in accordance with the present application may be obtained from American Isostatic Presses, Inc. of Columbus, Ohio. The pressure vessel 4 may further include insulation 12 to efficiently solution heat treat the high pressure die cast object 2 at the requisite temperatures and pressures. Additionally, a thermocouple feed through 14 and power feed through 16 may be present to provide for the measurement of heat and pressure. The pressure vessel 4 is connected to a compressor (not shown) to create the necessary pressure during the solution heat treatment process.
The high pressure die cast object 2 is placed within the pressure vessel 4, and the pressure vessel 4 is sealed with end closure 6. The cast object is then solution heat treated to above 371°C (700°F) at a pressure between 172 bar to 390 bar (2.5 KSI to 10 KSI) for 2 to 8 houers. In one embodiment, the temperature is between 371°C (700°F) and 649°C (1200°F) or at any temperature or range of temperatures therebetween. In another embodiment, the temperature is between 427°C (800°F) and 538°C (1000°F), in yet another embodiment, the temperature is at 538°C (1000°F).The pressure may vary, with one embodiment applying pressure between 172 bar (2.5 KSI) and 690 bar (10 KSI), another embodiment applying pressure between 172 bar (2.5 KSI) and 345 bar (5 KSI), and an embodiment where pressure is applied at 345 bar (5 KSI). In one embodiment, the time and temperature comprises a T6 heat treatment. In yet another embodiment, the high pressure die cast object is solution heat treated at 538°C (1000°F) for 4 hours at 345 bar (5 KSI) to achieve a high pressure die cast object devoid of blistering defects.
The gas used to apply pressure through the compressor may be atmospheric gas, an inert gas, or any other gas sufficient to apply the required pressures during solution heat treatment without combusting. In one embodiment, the gas is an inert gas. In another embodiment, the gas used is argon. Once the high pressure die cast is solution heat treated for the desired time, the die cast object is quenched and may optionally be artificially aged. Quenching contemplates rapidly cooling the solution heat treated object directly after removal from the solution heat treatment pressure vessel, and not allowing the object to slowly cool to room temperature. In one embodiment, the cast object is artificially aged for at least 2 hours. However, the length of time and the temperature for artificial aging is generally dictated by the strength and ductility levels desired, as is well-known by those of ordinary skill in the art.
The solution heat treatment with pressure provides for the ability to increase the yield strength of high pressure die cast objects. For example, and without limitation, the typical mechanical properties of high pressure die cast alloy A360.0 in the as cast condition for temperatures up to 371°C (700°F) are demonstrated in Table 1, below.

TABLE 1

Table 1 Typical tensile properties for separately cast test bars of alloys 360.0-F and A360.0-F at elevated temperature

Temperature		Tensile strength		Yield strength(s)		Elongation(b)
°C	F°	MPa	ksi	MPa	ksi	%
360.0 aluminum
24	75	325	47	170	25	3
100	212	305	44	170	25	2
150	300	240	35	165	24	4
205	400	150	22	95	14	8
250	500	85	12	50	7.5	20
315	600	50	7	30	4.5	35
370	700	30	4.5	20	3	40

A360.0 aluminum
24	75	315	46	165	24	5
100	212	295	43	165	24	3
150	300	235	34	160	23	5
205	400	145	21	90	13	14
250	500	75	11	45	6.5	30
315	600	45	6.5	28	4	45
370	700	30	4	15	2.5	45
(a) 0.2% offset, (b) In 50 mm or 2 in.

According to Table 1, at higher temperatures required for solution heat treatment, i.e. above 371°C (700°F), the tensile strength will be less than 276 bar (4 KSI) and the yield strength will be less than 172 bar (2.5 KSI). Thus, at solution heat treatment temperatures, when the yield strength of the die casting alloy is less than the pressure in the entrapped air, the air will expand, creating blistering defects 20, as shown in Fig. 2. As shown in Figs. 5-8, 10a and 10b, by applying an external gas pressure above 172 bar (2.5 KSI) to the cast object, the internal trapped gas cannot expand, and therefore the blistering of the high pressure die cast object can be eliminated. Even at lower pressures from 35 bar (0.5 KSI) to 240 bar (3.5 KSI), blistering defects can be reduced as shown in Figs. 3, 4, 10a and 10b. Since the blistering defects may be eliminated while obtaining the advantages of the solution heat treatment process, the yield strength of the solution heat treated high pressure die cast objects may increase by 50% to 100%. This translates into a 15-30% weight reduction on average for a redesigned component or a substantially higher safety factor on the same geometry component. It is known in the art that a 10% increase in yield strength of an aluminum casting can facilitate a designed weight reduction of 3% on average for the cast object. This is highly important, particularly in vehicle design (whether automobiles, trucks, or marine vehicles, because even a 10% total vehicle weight reduction improves mileage by 5-7%. Accordingly, the present invention provides a significant advance as weight reductions of 15-30% may be obtained.

EXAMPLES

EXAMPLE 1

High pressure die cast aluminum alloy marine propellers were selected as a test sample. Fig. 2 demonstrates a high pressure die cast aluminum alloy marine propeller treated at the T6 heat treatment of 538°C (1000°F) for 4 hours with 0 bar (0 KSI) pressure applied. Numerous blistering defects 20 are demonstrated.
Subsequently, high pressure die cast aluminum alloy marine propellers were subjected to solution heat treatment with pressure. Eighteen (18) high pressure die cast aluminum alloy propellers were solution heat treated with an externally applied gas pressure of 1034 bar (15 KSI), 690 bar (10 KSI) and 345 bar (5 KSI), respectfully (i.e. six (6) samples at each pressure). Each solution heat treatment was at 538°C (1000°F) for 4 hours. The externally applied gas pressure was accomplished through placing the high pressure die cast aluminum alloy marine propellers in a pressure vessel 4, and the pressure was applied using argon. The results are shown in Figs. 6, 7 and 8, wherein an elimination of blister defects 20 was observed.

EXAMPLE 2

High pressure die cast aluminum alloy marine propellers were subjected to solution heat treatment with pressure. Three (3) high pressure die cast aluminum alloy marine propellers were each solution heat treated at 240 bar (3.5 KSI), 138 bar (2.0 KSI), and 35 bar (0.5 KSI) at 538°C (1000°F) for 4 hours. The three propellers solution heat treated at 240 bar (3.5 KSI) demonstrated an elimination of blistering defects as shown in Fig. 5. The three high pressure die cast aluminum alloy marine propellers solution heat treated at 35 bar (0.5 KSI) demonstrated a reduction of blistering defects as shown in Fig. 3. The three high pressure die cast aluminum alloy marine propellers solution heat treated at 138 bar (2.0 KSI) demonstrated a significant reduction of blistering, with one small blister on only 1 of 3 propeller blades, as shown in Fig. 4.
Accordingly, the experiments confirm solution heat treatment with pressures between 35 bar (0.5 KSI) and 2413 bar (35.0 KSI) result in reduction of blister defects on high pressure die cast aluminum marine propellers, and pressures between 240 bar (3.5 KSI) and 1034 bar (15.0 KSI) demonstrate an elimination of blistering defects in high pressure die cast aluminum alloy marine propellers.

EXAMPLE 3

The lack of effectiveness of HIP processing on high pressure die castings was verified by the inventors in an experiment where a high pressure die cast propeller was subjected to 1034 bar (15 KSI) pressure at 538°C (1000°F) for 4 hours of hot isostatic pressing and allowed to cool to room temperature. The same propeller was then heat treated at 538°C (1000°F) for 4 hours at atmospheric conditions. Blistering defects were still evident after the process showing that the internal defects in the casting were not healed by the HIP process as shown in Figs. 9A and 9B.

EXAMPLE 4

In an effort to more accurately define the lowest pressure at which blistering will not occur, two (2) high pressure die cast aluminum alloy marine propellers each having three (3) blades were each solution heat treated at 172 bar (2.5 KSI) and 159 bar (2.3 KSI) at 538°C (1000°F) for 4 hours. The 3-blade propeller solution heat treated at 172 bar (2.5 KSI) demonstrated a total elimination of blistering defects. In contrast, the 3-blade high pressure die cast aluminum alloy marine propeller solution heat treated at 159 bar (2.3 KSI) also demonstrated a significant reduction in blistering, bit did demonstrated one small blister at the root of one of the propeller blades. The diameter of this blister is approximately 1mm in diameter, as shown in Figure 10a. The propeller was subjected to a drop weight impact test to ascertain whether the blister reduced the mechanical ductility of the propeller blade. As shown in Figure 2, the propeller processed at 159 bar (2.3 KSI) after a drop weight impact test showing the very small blister did not substantially reduce ductility or result in fracture of the blade. However, because one (1) very small blister was detected on one (1) blade at 159 bar (2.3 KSI), the lower pressure limit where the invention can be expected to be reliably practiced without any blistering is 172 bar (2.5 KSI).
The highest pressure limit where both blistering will not occur and the invention has utility is not defined by the blistering mechanism itself. Pressures equal to 172 bar (2.5 KSI) and up to 2413 bar (35 KSI) have been demonstrated to eliminate blistering. Instead, the upper limit of pressure where the invention has practical utility is defined by the increasing equipment cost of the pressure vessel and increasing process cycle time that adds to the final product cost. As pressure increases, both the capital cost of the pressure vessel itself increases and the process cycle time and associated cost increases on a non-linear basis. When these capital and process cycle time costs become excessive, it is not financially advantageous to use high pressure diecast articles and the inventive processing method. Instead, other metalworking or casting processes, such as but not limited to forging a wrought blank and machining or ablation casting become more financially preferred. The upper pressure limit of 1034 bar (15 KSI) is defined by the capital equipment and process costs of today's modern pressure vessel technology.
In the above description, certain terms have been used for brevity, clearness and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes only and are intended to be broadly construed. The different systems and methods described herein may be used alone or in combination with other systems and methods. Various alternatives and modifications are possible within the scope of the appended claims. While each of the method claims includes a specific series of steps for accomplishing the claimed method, the scope of this disclosure is not intended to be bound by the literal order or literal content of steps described herein.

Claims

A method of heat treating a high pressure die cast aluminum alloy object, the method comprising: obtaining a high pressure die cast aluminum alloy object, solution heat treating the high pressure die cast aluminum alloy object so that the step of solution heat treating eliminates blistering defects on the high pressure die cast object, and quenching the high pressure die cast aluminum alloy object after said solution heat treating,
characterized in that
the solution heat treating is effected in a solution heat treatment vessel at a temperature above 371°C (700°F) while applying pressure between 172 bar and 690 bar (2.5 KSI and 10 KSI) for 2 to 8 hours, and
quenching is effected after removing the die cast object from the solution heat treatment vessel.
The method of claim 1, characterized in that
the step of solution heat treating comprises applying pressure between 172 bar and 345 bar (2.5 KSI and 5 KSI).
The method of claim 2, characterized in that
the method further comprises subsequently quenching the die cast object and artificially aging the die cast object to effect a T6 heat treatment.
The method according to any one of the preceding claims, characterized in that
the step of solution heat treating comprises solution heat treating the die cast object between 371°C (700°F) and 649°C (1200°F).
The method of claim 4, characterized in that
the step of solution heat treating comprises solution heat treating the die cast object at 538°C (1000°F).
The method according to any one of the preceding claims, characterized in that
the step of solution heat treating comprises solution heat treating the die cast object for 4 to 6 hours.
The method according to any one of the preceding claims, characterized in that
the method of heat treating further comprises the step of artificially aging the die cast object.
The method according to any one of the preceding claims, characterized in that
the step of obtaining the high pressure die cast aluminum alloy object includes the step of casting an aluminum alloy object with high pressure die casting equipment and removing the aluminum alloy object from the high pressure die casting equipment.