GB2361710A

GB2361710A - Precipitation hardening of aluminium castings

Info

Publication number: GB2361710A
Application number: GB0102136A
Authority: GB
Inventors: John Edmond Allison; James Maurice Boileau; Michael Edward Hall; Venkat Nara; William Thomas Donlon
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2000-02-11
Filing date: 2001-01-29
Publication date: 2001-10-31
Also published as: GB0102136D0; DE10106640A1

Abstract

An aluminium casting is solution heat treated at a temperature in the range of about 480 {C to about 515 {C for a time in the range of about 1 to about 3 hours, cooled at a rate of at least about 0.6 {C/second and aged at a temperature in the range of about 220 {C to about 260 {C for a time in the range of about 1 to about 2.5 hours. Alternatively, the casting heat treated simultaneously with removing the sand casting core by solution heat treating at a temperature above the combustion temperature of binder and below the temperature at which incipient melting of the aluminium begins for no more then about 3 hours, cooling and aging for no more than about 2.5 hours at a temperature sufficient to obtain a hardness in the range of about 80 to about 120 on the 500 kg Brinnell hardness scale. The cooling may be performed using a water-based, an air-based or a synthetic quenchant. The aluminium casting may be made from an Al-Si-Cu alloy of the 319 or 320 type.

Description

2361710 ACCELERATED HEAT TREATMENT FOR CAST ALUMINUM ALLOYS The invention

relates generally to the field of foundry processing, and more particularly it relates to a heat treatment for cast aluminium having a reduced cycle time and aluminium castings produced by that process.

Reducing the weight of automotive components has become a key f ocus over the last twenty years because of the need to comply with regulations on fuel economy while meeting increased customer expectations for vehicle safety and performance. One method of weight reduction involves redesigning existing components using lightweight materials.

As a result, the use of cast aluminium has dramatically increased for components such as engine blocks and cylinder is heads.

For most powertrain applications, the aluminium component undergoes a heat treatment prior to use. Heat treatments are performed for a number of reasons including thermal removal of the sand cores used to make the casting, relief of residual stresses, dimensional stabilisation of the cast structure, and optimisation of the strength, ductility, and machinability required by a particular application. Heat treatment is a key parameter affecting the mechanical properties and functionality of a cast 319 type aluminium alloy component.

Most of the increase in tensile properties that accompany heat treatment is due to the formation of precipitates in the aluminium dendrites during ageing.

Precipitates interfere with the movements of dislocations through the matrix. The degree of strength is dependent upon precipitate size, spacing, and coherency. Since precipitate structure and morphology are determined by the time and temperature used in the ageing sequences, this allows for considerable control to optimise the strength, ductility, and toughness of these alloys.

Aluminium silicon copper casting alloys of the 319 and 320 types can undergo a number of different heat treatments 2 to obtain different combinations of properties. However, the designations of heat treatments specify only the generalities, not specific process parameters. For example, the Aluminium Association designation T7 states: "Solution heat treated and stabilised: Applies to products that are stabilised after solution heat treatment to carry them beyond the point of maximum strength to provide control of some special characteristic." The generalisation is present because heat treatment is affected by a number of parameters including solidification rate and alloy chemistry. In addition, process parameters such as heat-up time, actual time at heat treatment temperatures, and furnace temperature gradients in multiple-part heat treat ovens also affect the heat treatment.

is Heat treatment generally consists of a combination of three steps: a solution treatment, a rapid quench, and an treatment. Before the casting can be heat treated, ageing t the sand used to form the casting must be removed. Core removal can be accomplished by impact and vibration devices, and/or by breaking and prying the cores out manually, and/or- by heating to destroy the binders on the sand. Thermal sand removal generally takes place at temperatures greater than 4000C with long cycle times, about 4 to 10 hours. Sand removal can be followed by solution treatment at a temperature of about 500'C for from about 4 to 10 hours. In these cases, if the sand removal process is carried out at about 5000C, part of the solution treatment can take place during the sand removal process, reducing costs and improving efficiency. After quenching the casting, it is typically aged at a temperature in the range of 120' to 260'C for about 4 to 20 hours.

Thus, known heat treatment processes require more than 8 hours to perf orm. Accordingly there is a need for a heat treatment process which reduces cycle time, while still providing the needed physical properties in the casting.

According to the invention there is provided process for heat treating an aluminium casting, the process comprising: a) solution treating the casting by heating the casting and maintaining the casting at a temperature in the range of about 4800 to about 5150C for a time in the range of about 1 to about 3 hours; b) cooling the casting at a rate of at least about 0. 6cC/second; and c) ageing the casting by heating the casting and maintaining the casting at a temperature in the range of about 220' to about 260'C for a time in the range of about 1 to about 2. 5 hours.

A process embodying the present invention provides a heat treatment of an aluminium casting at a reduced cycle t ime. It includes: 1) thermally removing the sand core and at the same time solution treating the casting by heating the casting and maintaining the casting at a temperature above the combustion temperature of the binder and below the is temperature at which the incipient melting of the aluminium begins for no more than about 3 hours, 2) cooling the casting at a rate sufficient to retain the chemical elements needed to form the precipitates in solid solution, and 3) ageing the casting by heating the casting and maintaining the casting at a temperature sufficient to obtain a hardness in the range of about 80 to 120 on the 500 kg Brinnell Hardness Scale for no more than about 2.5 hours.

For a 319-type aluminium alloy, during thermal sand removal and solution treatment (step 1) the temperature is preferably maintained in the range of about 480' to about 5150C, and more preferably in the range about 4800 to about 50CC. During this step, the temperature is preferably maintained for about 1 to 3 hours, more preferably about 2 hours. 30 The cooling (step 2) preferably takes place at a cooling rate of at least about 0.6'C/second. The cooling can be performed with a water-based quenchant, an air-based quenchant or a synthetic (polymersolution) quenchant. During ageing (step 3), the temperature is preferably 35 maintained in the range of about 2200 to about 2600C, and the temperature is preferably maintained for about 1 to 2.5 hours, more preferably about 2 hours.

The present invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a graph showing the effect of solution treatment temperature on the hardness of a 319-type aluminium alloy; Fig. 2 is a graph showing the effect of solution treatment time on the hardness of a 319-type aluminium alloy; Fig. 3 is a graph showing the effect of cooling rate from the solution treatment temperature on the hardness of a 319-type aluminium alloy aged at 2600C for 1 hour; Fig. 4 is a graph showing the effect of ageing time on the hardness of a 319-type aluminium alloy; Fig. 5 is a graph showing the effect of ageing time on the dimensional growth of samples from 319-type aluminium alloy engine block bulkheads; and Fig. 6 is a graph showing the effect of thermal exposure time and. temperature on the dimensional growth of samples from 319-type aluminium alloy engine block bulkheads.

The reduced cycle time heat treatment consists of three steps. First, there is a solution treatment step. As part of the solution treatment step, the sand core can be removed if one is present. Alternatively, the sand core could be removed by some other process, such as a mechanical one. The purpose behind the combined step is to take advantage of one controlled temperature to perform both operations. In general, as the temperature increases, the time will be decreased within certain limits. 'The thermal sand removal operation is performed to pyrolise the resin binding the sand together in the mould. Since the resin will pyrolise at temperatures over about 400'C, time and temperature are not very sensitive parameters for this operation. However, time and temperature are critical in performing the solution treatment properly. As shown in Fig. 1, the solution treatment temperature for a 319type aluminium alloy needs to be above about 4800C to ensure the optimum properties upon quenching and ageing. Further, the temperature cannot exceed about 515'C for a 319 or 320-type alloy without risking incipient melting (i.e., permanent, microstructurally-altering damage). Therefore, for a 319 or 320-type aluminium alloy, the solution treatment should take place at a temperature in the range of about 4800 to 5150C, preferably about 4800 to 5000C.

Fig. 2 shows that a solution treatment time over 2 hours does not provide appreciable improvement in the hardness of a 319-type aluminium alloy, and that a minimum time of about 1 hour can provide the necessary hardness.

Therefore, the solution treatment should be maintained for between about 1 and 3 hours, preferably no more than about 2 is hours.

The second step is quenching the casting. The quench step must be rapid enough to retain the chemical elements needed to form the precipitates in solid solution. A cooling rate of greater than 0.60C/second provides approximately the same maximised hardness, as shown in Fig.

3. This cooling rate can be obtained using either rapid air-based, water-based, or synthetic (polymersolution) quenchants.

The third step, ageing, is used to control the precipitate structure and morphology, and hence, mechanical properties. Fig. 4 shows that ageing for 2 hours at 2400C yields equal or improved hardness as compared to ageing for 2 hours at 2600C.

Comparative Example The mechanical properties of cylinder heads produced according to the present invention and a typical heat treatment process were compared. As shown in Table 1, the mechanical properties for the cylinder produced according to the present invention (T7-M) are comparable to those of a typical process (T7). The typical heat treatment cycle for 319-type aluminium takes about 9 hours.It starts with a 5 hour thermal sand removal solution treatment step in which the casting is held at the solution treatment temperature of about 5000C for approximately 3.5 hours. The casting is then quenched, for example in boiling water, and held there for s 20 minutes. It is placed in a thermal ageing oven at about 255"C for 4.33 hours (3 hours at temperature) TABLE 1

Mechanical Property Comparisons Of T7M and T7 Heat Treatments Heat Treatment T7M T7 Ageing Time 2.5 Hours 4 Hours Ageing Temperature 240 C 255 C 500 kg. Brinell Hardness 110 95 Ultimate Tensile Strength (Mpa) 300 260 0.2% Yield Strength (MPa) 290 220 % Elongation 1.00% 1.25% % Dimensional Growth at 180 C <0.1-'. <0.1% The fatigue properties for casting made using the present invention are not expected to be significantly different from the existing process. Previous research on 319-type alloys has shown that heat treatment has little effect on the fatigue properties when porosity is present.

The heat treatment of the present invention maintains the dimensional stability of the casting. Figs. 5 and 6 show that a minimum of time of 2 hours at 2400C provides dimensional stability to samples manufactured from 319-type aluminium alloy.

The casting must have a 500 kg Brinnell Hardness of 100 + 20. One embodiment of a heat treatment cycle according to the present invention which produced the required hardness involved a solution treatment time of 1 hour at 4950C and an ageing time of 1 hour at 2550C. This provided a reduction in the heat treatment cycle of 50% as compared to the typical process described above (4. 5 hours v. 9 hours). Another embodiment of a heat treatment cycle with the necessary hardness (just under 100) had a 2 hour solution treatment at 4950C and a 1 hour ageing at 2550C, providing a reduction in cycle time of 3.5 hours or 39%. When the ageing temperature was reduced to 240'C, an ageing time of 2 hours was required to produce the necessary hardness. This provides a reduction of heat treatment cycle time of 28%.

Claims

1. A process for heat treating an aluminium casting, the process comprising:

a) solution treating the casting by heating the casting and maintaining the casting at a temperature in the range of about 4800 to about 5150C for a time in the range of about 1 to about 3 hours; b) cooling the casting at a rate of at least about 0. 60C/second; and c) ageing the casting by heating the casting and maintaining the casting at a temperature in the range of about 2200 to about 260'C for a time in the range of about 1 to about

2.5 hours.

is 2. A process as claimed in claim 1, wherein during a) solution treatment, the casting is maintained at a temperature in the range of about 4800 to about 5000C.

3. A process as claimed in claim 1, wherein during a) solution treatment,the casting is maintained at the temperature for no more than about 2.5 hours.

4. A process as claimed in claim 1, wherein during a) solution treatment, the casting is maintained at the temperature for no more than about 2 hours.

5. A process as claimed in claim 1, wherein during c) ageing, the casting is maintained at the temperature for no more than about 2 hours.

6. A process as claimed in claim 1, wherein during c) ageing, the casting is maintained at the temperature for no more than about 1.5 hours.

7. A process as claimed in claim 1, wherein the b) cooling is performed using a quenchant selected from the group consisting of water-based quenchants, air-based quenchants, and synthetic quenchants.

8. A process as claimed in claim 1, wherein the aluminium casting comprises an aluminium silicon copper casting alloy selected from 319 and 320-type aluminium alloys.

9. A process f or heat treating an aluminium casting, the process comprising:

a) solution treating the casting by heating the casting and maintaining the casting at a temperature above the combustion temperature of the binder and below the temperature at which the incipient melting of the aluminium begins for no more than about 3 hours, b) cooling the casting at a rate sufficient to retain in solid solution the chemical elements needed to form the precipitates; and c) ageing the casting by heating the casting and maintaining the casting at a temperature sufficient to obtain a hardness in the range of about 80 to about 120 on the 500 kg Brinnell Hardness Scale for no more than about 2.5 hours.

10. A process as claimed in claim 9, wherein during a) solution treatment, the casting is maintained at a temperature in the range of about 4800 to about 5150C.

11. A process as claimed in claim 9, wherein during a) solution treatment, the casting is maintained at a temperature in the range of about 4800 to about 500'C.

12. A process as claimed in claim 9, wherein during a) solution treatment, the casting is maintained at the temperature for no more than about 2.5 hours.

13. A process as claimed in claim 9, wherein during a) solution treatment, the casting is maintained at the temperature for no more than about 2 hours.

14. A process as claimed in claim 9, wherein during c) ageing, the casting is maintained at a temperature in the range of about 22C to about 2600C.

15. A process as claimed in claim 9, wherein during c) 10 ageing, the casting is maintained at the temperature for no more than about 2 hours.

16. A process as claimed in claim 9, wherein during c) ageing, the casting is maintained at the temperature for no is more than about 1.5 hours.

17. A process as claimed in claim 9, wherein the b) cooling takes place at a rate of at least about 0.60C/second.

18. A process as claimed in claim 9, wherein the b) cooling is performed using a quenchant selected from the group consisting of water-based quenchants, air-based quenchants, and synthetic quenchants.

19. A process for heat treating an aluminium casting substantially as hereinbefore described with reference to the accompanying drawings.