Method of forming parts from sheet metal alloy
A method of forming a part from sheet metal alloy is provided, for example, forming a part from aluminium alloy. The method comprises heating (A) the sheet metal alloy to a temperature at which solution heat treatment of the alloy occurs and so as to achieve solution heat treatment. The sheet is cooled (B) at at least the critical cooling rate for the alloy and then placed between dies to form (C) it into or towards the part.
1. A method of forming a part of complex shape from sheet metal alloy, the method comprising the sequential steps of:
(a) heating the sheet to a temperature at which solution heat treatment of the alloy occurs so as to achieve solution heat treatment;
(b) controlling a cooling rate of the sheet to at or above a critical cooling rate of the alloy until a target temperature is reached based on a measured temperature at one or more positions, where the cooling rate is controlled by:
measuring the temperature of the sheet at the one or more positions on the sheet,
comparing the measured temperature with a reference temperature, where the reference temperature defines a rate of cooling at which microstructural precipitation in the alloy is avoided, and
cooling the sheet by applying a cooling medium to at least a portion of the sheet,
wherein the target temperature is 250° C. to less than 450° C.,
wherein the cooling rate of the sheet is controlled by adjusting a flow rate of the cooling medium applied to at least a portion of the sheet by increasing the flow rate of the cooling medium when the measured temperature is decreasing at a rate lower than a rate of the reference temperature, and decreasing the flow rate of the cooling medium when the measured temperature is decreasing at a rate higher than the rate of the reference temperature;
(c) placing the sheet between dies to form it into or towards the complex shape; and then
(d) quenching the sheet between the dies, whilst the dies are in contact with the sheet.
2. The method of claim 1 , wherein step (b) comprises cooling the sheet at at least the rate at which microstructural precipitation in the alloy is avoided.
3. The method of claim 1 , wherein the sheet is cooled to the lowest temperature that still allows forming of the part.
4. The method of claim 1 , wherein the cooling medium is a forced air flow.
5. The method of claim 1 , wherein the cooling medium is a fluid.
6. The method of claim 1 , wherein step (b) comprises selectively cooling at least a first area of the sheet to a first temperature which is lower than a second temperature, to which at least a second area of the sheet is cooled.
7. The method of claim 6 , wherein step (b) comprises selectively cooling at least a first area of the sheet to a first temperature which is lower than a second temperature to which at least a second area of the sheet is cooled by directing a fluid at the first area of the sheet with a longer duration, lower temperature and/or greater mass flow than at the second area.
8. The method of claim 1 , wherein step (a) comprises heating the sheet to above the solution heat treatment temperature and maintaining the sheet at the solution heat treatment temperature for at least 15 seconds.
9. The method of claim 1 , wherein the dies are cooled.
10. The method of claim 1 , wherein the sheet is of an aluminium alloy.
11. The method of claim 1 , wherein the sheet is of an AA6XXX aluminium alloy, and step (a) comprises heating the sheet to between 520° C. and 575° C.
12. The method of claim 1 , wherein step (b) comprises cooling the sheet at a cooling station forming part of an apparatus arranged to transfer the sheet to the dies.
13. The method of claim 1 , wherein quenching the sheet between the dies includes quenching a portion of the sheet in contact with the dies.