WO2019090398A1

WO2019090398A1 - Procedure for post-heat treatment of aluminium-silicon-magnesium components made by selective laser melting (3d metal printing)

Info

Publication number: WO2019090398A1
Application number: PCT/AU2018/051214
Authority: WO
Inventors: Xinhua Wu; Kun Yang; Elena Labelle; Paul Rometsch; Heng RAO
Original assignee: Monash University
Current assignee: Monash University
Priority date: 2017-11-13
Filing date: 2018-11-13
Publication date: 2019-05-16
Anticipated expiration: 2020-05-13

Abstract

The heat treatment of a component made of an Al-Si-Mg alloy by an SLM process comprises subjecting the component to a stress relieving heat treatment, without sintering loose powder entrapped with or within the component, prior to cutting the component from a build platform on which the component was made by the SLM process. The stress relieved component then is separated from the build platform, and then subjected to solution heat treatment to take solute elements and phases into solid solution and quenched and artificially aged.

Description

PROCEDURE FOR POST-HEAT TREATMENT OF ALUMINIUM-SILICON-MAGNESIUM COMPONENTS MADE BY SELECTIVE LASER MELTING (3D METAL PRINTING)

FIELD OF THE INVENTION

[001 ] This invention relates to a method for heat treatment of Al-Si-Mg components made by a selective laser melting (SLM) process.

BACKGROUND OF THE INVENTION

[002] SLM processes are poised to transform the metal manufacturing industry, particularly in those areas with which conventional manufacturing reaches its limitations in terms of both design freedom and manufacturing capabilities. However, due to the layer by layer scan manner by which components are built up in SLM processes, material of which the components are made experiences repeated thermal cycling. In addition, high thermal gradients are normally obtained due to a combination of high melt temperature, small melt pool sizes and the short liquid life-time that results from extremely rapid solidification. These thermal effects generate important plastic strains that result in residual stresses in SLM-manufactured components. Consequently the components are prone to cracking or distortion, causing difficulty in cutting the components off the build platform (particularly with large components), while a reduction in fatigue life is caused by crack initiation and propagation. Stress relief therefore is necessary to remove the internal stress.

[003] For Al-Si-Mg alloys, the occurrence of both cracking and distortion in components made by an SLM process is such that unacceptably high reject rates can be encountered, adding considerably to the cost of the SLM process. The present invention seeks to provide a method that facilitates the more cost effective use of an SLM process for making components of an Al- Si-Mg alloy.

SUMMARY OF THE INVENTION

[004] For Al-Si-Mg alloys, enhanced yield strength mainly stems from precipitation hardening due to precipitation of Mg2Si phases, which largely depend on the level of Mg and Si in solution before artificial ageing starts. This is normally achieved by quenching the alloy after solution heat treatment for cast alloys. Solution heat treatment temperatures and times depend on the alloy system under consideration and, for cast Al-Si-Mg alloys, solution heat treatment is conducted between 500^°C to 550^°C for several hours so that Si particles become somewhat spheroidal. For this, the time of solution heat treatment for cast Al-Si-Mg alloys is specified as between 4 to 12 hours in Metals Handbook, 9^th edition, vol. 15 pp. 758-759. Our research has found that with a component made of an Al-Si-Mg alloy by an SLM process, a super- saturation of solutes, including Si and Mg, is present in the alloy of the component on completion of the SLM process. This is due to the fast solidification explained above. However, as stress relief requires heating to a temperature comparable to or higher than a temperature appropriate for artificial ageing, stress relief will cause the alloy to be over-aged. While over- ageing is undesirable, the present invention utilizes this understanding to provide a process for the heat treatment that obviates the problem of over ageing and a resultant diminution of physical properties.

[005] The present invention provides a process for the heat treatment of a component made of an Al-Si-Mg alloy by an SLM process, wherein the process comprises the steps of:

(1 ) prior to cutting the component from a build platform on which the

component was made by the SLM process, subjecting the component to a heat treatment within a temperature range and for a period of time, both sufficient for substantially stress relieving the component;

(2) separating the stress relieved component produced in step (1 ) from the build platform smoothly and without sintering loose powder with or within the component;

(3) subjecting the separated component from step (2) to a heat treatment at a temperature range and for a period of time enabling solution treatment in which solute elements and phases are taken into solid solution;

(4) quenching the solution heat treated component produced in step (3); and

(5) subjecting the quenched component produced in step (4) to artificial ageing, optionally after a period of natural ageing. [006] Thus, the present invention provides five steps and corresponding conditions for the heat treatment of Al-Si-Mg alloys produced by 3D printing (i.e., SLM). In the alloy with which the invention is employed, the Si concentration may be between 6wt% and 12wt%, while the Mg content may be up to 0.8wt%. The alloy may conform to the specification for The Aluminium Association (AA) designated alloy A357 with around 7wt% Si and 0.5wt% Mg, or to the specification for AA designated alloy A360 with around 10wt% Si and 0.4wt% Mg. More specifically, the alloys preferably comprise:

(i) A357: in wt.% - 6.5 to 7.5 silicon; 0.4 to 0.7 magnesium; 0.040 to 0.2 titanium; 0 to 0.2 iron; 0 to 0.2 copper; 0.040 to 0.070 beryllium; 0 to 0.1 manganese; 0. to 0.1 tin; 0 to 0.15 residuals; and a balance of 90.8 to 93 Al; or

(ii) A360: in wt.% - 9.0 to 10.0 silicon; 0.4 to 0.6 magnesium; 0 to 1 .3 iron; 0 to 0.6 copper; 0 to 0.5 nickel; 0 to 0.5 zinc; 0 to 0.35 manganese; 0 to 0.15 tin; 0 to 0.25 residuals and a balance of 85.8 to 90.6 aluminium.

The thickness of the component can vary widely, such as typically from 1 mm to 50mm.

[007] There is no specific requirement for how long the components need to be kept on the build platform before being stress relieved. However, the stress relief preferably is conducted as quickly as possible or convenient once the build is finished and, as indicated, before the component is cut off the build platform. The temperature range for the stress relief of step (1 ) of the process of the invention preferably is between 150^°C to 350^°C, with holding time preferably between 1 h to 12h. The temperature at the lower end of the range is selected so as not to be too low for efficient removal of the internal stress, while at the upper end of the range the temperature needs to be not so high as to sinter the loose powder on the components. The holding time should be longer for larger components or at lower temperature.

[008] The stress relief of step (1 ) of the process of the invention will lead to over ageing of Mg2Si phases in Al-Si-Mg alloy compositions, as indicated above. As a consequence, a resultant microstructure will be dominated by a broken up Si network, in contrast with the mostly connected Si network in the component before step (1 ), in the as produced condition resulting from the SLM process.

[009] In step (2) of the process of the invention, the component preferably is cut off from the build platform by electrical discharge machining (EDM). However, other conventional machine tool hardware or process involving sawing or machining can be used.

[010] The solution heat treatment of step (3) of the invention is conducted to cause solutes, including Mg2Si phases, to be taken back into solid solution. The following quench of step (4) then results in super-saturation of the alloy by the solutes being retained in solid solution at lower temperatures, down to ambient temperatures and below. The solution heat treatment temperature preferably is from 500^°C to 550^°C. Al-Si-Mg alloys containing about 0.6 wt% Mg most preferentially are solutionised at about 543^°C, while alloys containing about 0.4 wt% Mg most preferentially are solutionised at about 530^°C. The time of solution heat treatment of step (3) is normally considered to commence once the metal temperature approaches an onset solution heat treatment temperature within a certain margin. In the current case, the soak commences when the metal/load temperature is within 7^°C of the onset point. By taking into consideration the thickness and the mass of the components, the optimum holding time is kept between 0.25h to 12h. This will enable sufficient super-saturation for the desired mechanical properties to be achieved after artificial ageing. The holding time should be sufficient to ensure all the solutes go back in solution, and it should not be so long as to enable enlargement of gas porosities and grain size, leading to reduced elongation to fracture and also fatigue life.

[01 1 ] During solution heat treatment of step (3), the Si particles grow (preferentially at grain boundaries) at the expense of adjacent smaller Si particles. The most significant growth occurs within the first hour of solution treatment. The Si particle sizes should be no larger than 5pm in equivalent diameter after 6 h of solution heat treatment. Iron-containing intermetallic particles (e.g. β-phase needles) start to precipitate out and grow during solution heat treatment as well. Since such particles can also impair mechanical properties, shorter solution treatments are preferred. The preferred solution treatment soak time is therefore less than 3 hours and more preferably in the range of 0.5 to 1.5 hours.

[012] Step (4) preferably is conducted by quenching into a non-aggressive quench medium such as an aqueous polyethylene glycol (PEG) solution or warm water to achieve super-saturation. For such quenching, the PEG or warm water are selected to reduce the quench severity and avoid undue residual thermal stresses that could lead to quench cracking or distortion, such as could happen when quenching into cold or ambient water. The quenching preferably is conducted with a minimal delay time after removal of the component, from opening a furnace in which the solution heat treatment is conducted, and the immersion of the component in the quenching solution. The delay time most preferably does not exceed 15 seconds in order to minimize any loss of super-saturation, in compliance with Aerospace Material Specification AMS2771 E 3.2.9.4.1 . Also in accordance with that specification, the quenching preferably is conducted so that the quenching solution temperature does not rise by more than 14^°C, with this necessitating the need for a sufficiently large volume of solution for large components. Again, as in AMS2771 E 3.2.9.4.1 , the component preferably has a contact time with the quenching solution of not less than 2 min per inch of thickness of the component.

[013] Any storage time interval between quenching in step (4) and artificial ageing in step (5), at a temperature such as ambient temperature enabling natural ageing, preferably is controlled to minimise any negative effects of natural ageing. Any natural ageing time interval preferably is as short as practical and, in any event, it is desirable that the time interval is less than 24 hours and preferably less than 2 hours. However, to the extent that there is natural ageing time interval before step (5), it preferably is for a fixed time interval such as 1 hour to enable more uniform mechanical properties to be achieved after ageing in similar components produced in respective manufacturing cycles. [014] The artificial ageing of step (5) may be conducted within a temperature range from 150^°C to 190^°C, such as for a holding time of from 2 to 16 hours. Preferably the temperature for artificial ageing is from 160 to 170^°C for from 6 to 10 hours. The microstructure produced by the artificial ageing is such as to require the high level of resolution provided by transmission electron microscopy (TEM) to enable the changes produced by artificial ageing to be distinguished from the as quenched microstructure. With lower resolutions, the microstructure can appear to remain the same as in the as-quenched material. This is because the main change is due to precipitation hardening that occurs during artificial ageing involving precipitation of Mg2Si that is observed to form and grow as nano-scale particles.

GENERAL DESCRIPTION OF THE DRAWINGS

[015] In order that the invention may more readily be understood, description now is directed to the accompanying drawings, in which:

[016] Figure 1 provides a schematic representation of a process according to the present invention for the heat treatment of a component of an Al-Si-Mg alloy made by an SLM process;

[017] Figure 2 is a series of micrographs showing the microstructure of A357 alloy (AI-7Si-Mg) after respective treatment conditions;

[018] Figure 3 is a series of micrographs showing the microstructure of A357 alloy (AI-7Si-Mg) showing gas porosity evolution after respective heat treatment conditions;

[019] Figure 4 is a series of micrographs showing the microstructure of A360 alloy (AI-10Si-Mg) after respective treatment conditions;

[020] Figure 5 proves a graphical representation of the equivalent diameter of the Si particles calculated by image analysis of selected micrographs of Figure 4; [021 ] Figure 6 illustrates the development of yield strength with the various heat treatments of Figure 2 for A357 alloy;

[022] Figure 7 illustrates the development of ultimate tensile strength with the various heat treatments of Figure 2 for A357 alloy;

[023] Figure 8 illustrates the development of elongation to fracture with the various heat treatments of Figure 2 for A357 alloy;

[024] Figure 9 illustrates the development of yield strength with the various heat treatments of Figure 4 for A360 alloy;

[025] Figure 10 illustrates the development of ultimate tensile strength with the various heat treatments of Figure 4 for A360 alloy; and

[026] Figure 1 1 illustrates the development of elongation to fracture with the various heat treatments of Figure 4 for A360 alloy.

DETAILED DESCRIPTIONOF THE DRAWINGS

[027] Figure 1 schematically illustrates the process of the invention, for the heat treatment of a component made of an Al-Si-Mg alloy by an SLM process. The workflow of the process is depicted on a step-by-step basis, illustrating the sequential steps of:

Step (1 ), stress relief;

Step (2), cutting the component from the build platform on which the component has been built up by the SLM process;

Step (3), solution treatment;

Step (4), quenching; and

Step (5), artificial ageing.

[028] In step (1 ) the component and the build board on which the component was built up layer by layer by the SLM process, are together heated in a furnace, with the component still attached to the platform. The furnace is maintained in a temperature range suitable for stress relieving the Al-Si-Mg alloy of which the component has been made, such as from 150 to 350 °C. After the component has been ramped up to that temperature range, the component is maintained in the range for a period of time sufficient to achieve stress relief of the alloy of the component, such as from 1 to 12 hours. The time at temperature needs to observe the usual inverse relationship between time and temperature, but with the time increasing with the thickness and mass of the component. On completion of stress relief step, the component, and the build board to which the component is still attached, are allowed air cool sufficiently to enable step (2) to be commenced, such as to ambient temperature.

[029] In step (2) of the illustration provided by Figure 1 , the stress relieved component is separated from the build platform. This separation preferably is achieved by electrical discharge machining (EDM). However, as previously indicated, other conventional machine tool hardware or process involving sawing or machining can be used. With the component stress relieved it is able to be separated from the build platform with minimal risk of the component being cracked or broken in the course of separation from the build board. In contrast, without the stress relief of step (1 ), directly cutting the component from the build platform would result in the residual stresses in the component due to the SLM manufacturing process making the removal from the build platform extremely difficult. The constant relaxation of stress in the course of cutting would make cutting unstable or difficult to control, producing an irregular, somewhat zig-zag cut surface; while distortion of the component would be likely to be caused through stress relaxation away from the cutting line. If, as an alternative to the stress relief of step (1 ), the component was subjected to a solution treatment above 500 °C, loose powder entrapped with the component would be sintered into a solid mass that would not be readily removed and that effectively would destroy or invalidate the component.

[030] After its removal from the build platform, the component is subjected in step (3) to a heat treatment at a temperature range and for a period of time enabling solution treatment in which solute elements and phases are taken into solution. In the arrangement illustrated in Figure 1 , the component is loaded into a furnace maintained at a solution treatment temperature in the range of from about 500 °C to 550 °C and, after the component has ramped up to the furnace temperature, the component is maintained at the furnace temperature for a dwell time of from 0.25 hour to 12 hours, depending on the thickness and mass of the component. While it is desirable that the component is held close to the require solution treatment temperature throughout the dwell time, the component and the furnace can cycle about that temperature, but preferably by not more than plus or minus about 7 °C.

[031 ] After completion of the dwell time for step (3), the solution treated component is cooled in step (4) by quenching to retain the solutes in solid solution, such that the alloy of the component is supersaturated at lower temperatures down to ambient temperatures, and below. The quench medium preferably is an aqueous polyethylene glycol (PEG) solution or warm water, although other non-aggressive quench media can be used, in each case to reduce the quench severity and avoid undue residual thermal stresses that could lead to quench cracking or distortion. There preferably is minimal delay time between removal of the component from a furnace in which the solution heat treatment is conducted and immersing the component into the quench medium, such as not more than about 15 seconds. The quenching solution temperature preferably does not rise by more than 14^°C, while the component preferably has a contact time with the quenching solution of not less than 2 minutes per inch of thickness of the component.

[032] In step (5), the quenched component is subjected to a heat treatment at a temperature range and for a period of time providing artificial ageing. In the arrangement illustrated in Figure 1 , the component is loaded into a furnace maintained at an artificial ageing temperature in the range of from about 150 °C to 190 °C, or heated with heating of the furnace to a temperature in that range. After the component has a temperature in that range, the component is maintained at the furnace temperature for a dwell time of from 2 to 16 hours, depending on the thickness and mass of the component. Preferably the temperature for artificial ageing is from 160 to 170^°C for from 6 to 10 hours. As previously indicated, it typically requires the level of resolution provided by transmission electron microscopy (TEM) for discerning changes in the microstructure achieved by artificial ageing, as the artificial ageing involves precipitation of MgSi that forms and grows as nano-scale particles.

[033] The series of six micrographs (a) to (f) of Figure 2 show the microstructure of A357 alloy (AI-7Si-Mg) after respective treatment conditions of:

(a) the alloy of a component as produced by an SLM process;

(b) the alloy as for (a), after stress relieving heat treatment at 300 °C for 2 hours, followed by air cooling;

(c) the alloy as for (b), followed by solution heat treatment at 543 °C for 1 hour, quenching into an aqueous PEG solution and then artificial ageing at 160 °C for 8 hours and air cooling from the ageing temperature;

(d) the alloy as for (c), except for a dwell time of 2 hours for solution heat treatment;

(e) the alloy as for (c), except for a dwell time of 3 hours for solution heat treatment; and

(f) the alloy as for (c), except for a dwell time of 4 hours for solution heat treatment.

In Figure 2, the Z-axis of the 3-axis co-ordinate system depicted to the right of micrographs (c) and (f) indicates the build direction, such that the X- and Y- axes are parallel to successive layers built up in producing the component. Each of micrographs (a) to (f) was taken from a respective sample that had been etched by Keller's reagent. The dark contrast is the a-AI matrix, while the bright contrast is the eutectic Si phases. As can be seen, the Si particles are interconnected at cell boundaries in the as-SLM-produced condition shown by micrograph (a). The interconnected Si particles break down after stress relief stage shown by micrograph (b), although the original network still can be traced. The Si particles grow significantly during solution heat treatment and iron-containing intermetallic (needle like β-phase) starts precipitating out. [0034] The series of six micrographs (a) to (f) of Figure 3 show the microstructure of A357 alloy (AI-7Si-Mg) after respective treatment conditions (a) to (f) that were the same as for the respective conditions (a) to (f) described in relation to Figure 2. The 3-axis system is as for Figure 2. However, the micrographs of Figure 3 are at a considerably lower resolution than those of Figure 2, such that porosity due to gas evolution during heat treatment is apparent. It is clear that the gas porosities enlarge and increase in volume fraction once solution heat treatment is conducted, and maximizes at the longest solution heat treatment holding time.

[035] The series of micrographs (a) to (f) of Figure 4 show the microstructure of A360 alloy (AI-10Si-Mg) after respective treatment conditions (a) to (e) of:

(a) the alloy of a component as produced by an SLM process, after solution heat treatment at 530 °C for 15 minutes, quenching into an aqueous PEG solution and then artificial ageing at 160 °C for 8 hours and air cooling from the ageing temperature;

(b) the alloy as for (a), except for a dwell time of 30 minutes for solution heat treatment;

(c) the alloy as for (a), except for a dwell time of 1 hour for solution heat treatment; and

(d) the alloy as for (a), except for a dwell time of 2 hours for solution heat treatment; and

(e) the alloy as for (a), except for a dwell time of 6 hours for solution heat treatment.

Again the 3-axis system is as for Figure 2, while each of micrographs (a) to (e) was taken from a respective sample that had been etched by Keller's reagent. As with Fig. 2, the dark and bright contrast represent the a-AI matrix and Si particles, respectively, and the brightest contrast shows the needle like iron-containing intermetallic β-phase. It can be seen that the growth of Si particles tends to saturate at 1 -hour solution heat treatment.

[036] The graphical representation of Figure 5 shows the equivalent diameter of the Si particles calculated by image analysis of micrographs (a), (c), (d) and (e) of Figure 4. It can be seen that significant Si particle growth occurs in the first 2 hours of solution heat treatment, with the maximum Si particle size increasing from around 2.8 pm at 1 hour solution heat treatment to around 5 pm at 2 hours solution heat treatment.

[037] Sets of mechanical properties including yield strength, ultimate tensile strength, and elongation to fracture were obtained for A357 alloy (AI-7wt%Si- Mg alloy) under different heat treatment conditions. The data on yield strength is illustrated in Figure 6, the data on UTS is shown in Figure 7, while the data on elongation is provided by Figure 8. Samples heat treated in accordance with the present invention achieved a combination of yield strength of 240MPa, ultimate strength of 320MPa, and elongation to fracture of 4%.

[038] Sets of mechanical properties including yield strength, ultimate tensile strength, and elongation to fracture were obtained for A360 alloy (AI-10wt%Si- Mg alloy) under different heat treatment conditions. The data on yield strength is illustrated in Figure 9; the data on UTS is shown in Figure 10; while the data on elongation is provided by Figure 1 1 . Samples heat treated in accordance with the present invention achieved a combination of yield strength of 240MPa, ultimate strength of 320MPa, and elongation to fracture of 4%.

[039]. For a component made of an Al-Si-Mg alloy by an SLM process, the present invention, such as illustrated by Figure 1 , takes into consideration the need for, and benefit of, stress relief step prior to the component being separated from the build platform on which it was formed. The invention is found to minimize the risk of the component cracking when being cut from a build platform. As illustrated by Figures 6 to 1 1 , the invention enables the manufacture of components with comparable, or even better, mechanical properties than components that are similarly made and heat treated, but without stress relief prior to being separated from the build platform. The invention enables variation to allow for differences in the geometry and mass of components, such as for thicknesses ranging from 1 mm to 50mm, achieving similar microstructure as in Figures 2 and 4, and similar mechanical properties.

Claims

1 . A process for the heat treatment of a component made of an Al-Si-Mg alloy by an SLM process, wherein the process comprises the steps of:

(1 ) prior to cutting the component from a build platform on which the component was made by the SLM process, subjecting the component to a heat treatment within a temperature range and for a period of time, both sufficient for substantially stress relieving the component;

(2) separating the stress relieved component produced in step (1 ) from the build platform smoothly and without sintering loose powder entrapped with or within the component;

(4) quenching the solution heat treated component produced in step (3); and

(5) subjecting the quenched component produced in step (4) to artificial ageing, optionally after a period of natural ageing.

2. The process of claim 1 , wherein the alloy has a silicon concentration between 6wt% and 12wt% and a magnesium content of up to 0.8 wt%.

3. The process of claim 1 or claim 2, wherein the alloy conforms to the specification for The Aluminium Association (AA) designated alloy A357 with, in wt.%, 6.5 to 7.5 silicon; 0.4 to 0.7 magnesium; 0.040 to 0.2 titanium; 0 to 0.2 iron; 0 to 0.2 copper; 0.040 to 0.070 beryllium; 0 to 0.1 manganese; 0. to 0.1 tin; 0 to 0.15 residuals; and a balance of 90.8 to 93 Al.

4. The process of claim 1 or claim 2, wherein the alloy conforms to the specification for The Aluminium Association (AA) designated alloy A357 with, in wt.%, 9.0 to 10.0 silicon; 0.4 to 0.6 magnesium; 0 to 1 .3 iron; 0 to 0.6 copper; 0 to 0.5 nickel; 0 to 0.5 zinc; 0 to 0.35 manganese; 0 to 0.15 tin; 0 to 0.25 residuals and a balance of 85.8 to 90.6 aluminium. 5. The process of any one of claims 1 to 4, wherein the stress relief preferably is conducted as quickly as possible or convenient once the build is finished.

6. The process of any one of claims 1 to 5, wherein the stress relief of step (1 ) is conducted at a temperature in the range between 150^°C to 350^°C, with a holding time in that range of between 1 hour and 12 hours.

7. The process of any one of claims 1 to 6, wherein the stress relief of step (1 ) leads to over ageing of Mg₂Si phases in Al-Si-Mg alloy compositions and a resultant microstructure with a broken up Si network.

8. The process of any one of claims 1 to 7, wherein the component is cut off from the build platform by electrical discharge machining (EDM).

9. The process of any one of claims 1 to 8, wherein the solution heat treatment of step (3) is conducted to cause solutes, including Mg₂Si phases, to be taken back into solid solution.

10. The process of any one of claims 1 to 9, wherein the solution heat treatment temperature is from 500^°C to 550^°C.

1 1 . The process of claim 10, wherein the Al-Si-Mg alloys contains about 0.6 wt% Mg and the solution heat treatment is at about 543^°C.

12. The process of claim 10, wherein the Al-Si-Mg alloys contains about 0.4 wt% Mg and the solution heat treatment is at about 530^°C.

13. The process of any one of claims 10 to 12, wherein the time of solution heat treatment of step (3) is from 0.25 hour to 12 hours.

14. The process of claim 13, wherein the solution heat treatment time is less than 3 hours, such as in the range of 0.5 to 1 .5 hours.

15. The process of any one of claims 1 to 14, wherein the quenching of step (4) is conducted by quenching into a non-aggressive quench medium such as an aqueous polyethylene glycol (PEG) solution or warm water to achieve super-saturation.

16. The process of claim 15, wherein the quenching is conducted with a minimal delay time after removal of the component, from opening a furnace in which the solution heat treatment is conducted, and the immersion of the component in the quenching solution, such as a delay not exceeding 15 seconds, and so that the quenching solution temperature does not rise by more than 14^°C, and with the component having a contact time with the quenching solution of not be less than 2 minutes per inch of thickness of the component.

17. The process of any one of claims 1 to 16, wherein any storage time interval between quenching in step (4) and artificial ageing in step (5), at a temperature such as ambient temperature enabling natural ageing, is controlled to minimise any negative effects of natural ageing.

18. The process of claim 16 or claim 17, wherein any natural ageing time interval preferably is less than 24 hours, such as less than 2 hours.

19. The process of any one of claims 1 to 18, wherein the artificial ageing of step (5) is conducted within a temperature range from 150^°C to 190^°C, such as for a holding time of from 2 to 16 hours.

20. The process of claim 19, wherein the temperature for artificial ageing is from 160 to 170^°C for from 6 to 10 hours.