The present invention relates to a method of additive manufacturing a superalloy component.
Additive Manufacturing is a group of processes characterised by manufacturing three-dimensional components by building up substantially two-dimensional layers (or slices) on a layer by layer basis. Each layer is generally very thin (for example between 20 to 100 microns) and many layers are formed in a sequence with the two dimensional shape varying on each layer to provide the desired final three-dimensional profile. In contrast to traditional “subtractive” manufacturing processes where material is removed to form a desired component profile, additive manufacturing processes progressively add material to form a net shape or near net shape final component.
There is a desire to use Additive Manufacturing for the manufacture of superalloy components, for example for the manufacture of aero engine components. Superalloys are alloys which are designed for high performance at elevated temperatures. In particular, superalloys are generally defined as an alloy with excellent mechanical strength and creep resistance at high temperatures. However, the nature of superalloy materials results in several difficulties for additive manufacturing. For example, the high temperature performance of a superalloy is the result of a microstructure that makes them brittle and, therefore, prone to cracking. A number of superalloys are generally considered to be “difficult to weld” (and therefore difficult to form in an Additive Manufacturing process) due their tendency to cracking, in particular nickel superalloys with a high proportion of gamma prime forming elements typically aluminium and titanium are known to be problematic. While the degree of welding difficulty is subjective it is associated in the superalloys principally with their gamma prime hardening elements—aluminium and titanium. As shown in FIG. 1, different alloys may be graphically plotted based upon their compositions of aluminium and titanium. An approximate boundary may be defined between about aluminium 3 wt %+titanium 0 wt % to about aluminium 0 wt. %+titanium 6 wt %; to the right of this boundary alloys may be generally classified as “difficult to weld”.
One solution to reduce or avoid cracking during additive manufacture is to maintain the bulk part close to its melting temperature during formation. However, in the case of high temperature materials, such as superalloys, the temperature required is extremely high. The consequence of this is that the equipment is costly and complex, particularly for laser based systems, and the process slowed by the need for heat up and cool down times rendering any such manufacturing process costly and difficult to practice.
An alternative solution has recently been proposed in “Presentation of EC Project FANTASIA; Session 4C: Advanced Manufacturing Technics for Engine Components” pages 31-35, dated 31 Mar. 2011, and presented by Konrad Wissenbach, Fraunhofer Institute for Laser Technology ILT, Aachen, Germany (available at: www.cdti.es/recursos/doc/eventosCDTI/Aerodays2011/4C2.pdf). In this proposal a part is laser powder bed additive manufactured conventionally in Mar-M-247 (a widely used superalloy) resulting in cracks. The cracks formed during the additive manufacturing of the Mar-M-247 component are then treated by pre-heating the whole component to a temperature of 1150° C. (in excess of its operating temperature of 1040° C.) before laser remelting the entire surface of the component. This provides a component having a sealed surface which is then treated by Hot Isostatic Pressing (HIP) and is reported to remove internal cracks and provide a substantially crack free final component. The surface sealing step is required in this proposal, as HIP is only able to collapse fully enclosed cracks.
During HIP processing any gas within crack voids must be placed into solution in the alloy if they are to close. The applicants have identified two potential disadvantages of this process. First, commercially available HIP process have, experimentally, been found to be unable to collapse voids unless the crack is substantially gas free. However, laser based Additive Manufacture typically takes place in a chamber containing a tightly controlled atmosphere of inert gas at close to ambient pressure and therefore purge gas will be present in cracks formed during layer formation. Such cracks will not be closed by a commercial HIP process. As such the proposed method, which already requires additional and potentially costly processing steps, may require layer melting in a vacuum chamber—which is particularly difficult for laser processing as they emit a vapour which covers any window in the vessel to allow the laser to enter. Laser melting processes are therefore generally carried out in a purged environment with gas flow to keep the laser window clear.
Secondly, even if a HIP process is selected capable of putting gas contained in cracks into solution in the alloy, this is not a reliable manufacturing process for a nickel superalloy. It may be expected that this gas will subsequently come back out of solution thereby recreating voids during high temperature operation of that component.
Only cracks formed after solidification (e.g. by a relaxation process) will be reliably closed by HIP processing.