TWI Industrial Member Report Summary 867/2007
By G Ng
Background
Additive manufacturing techniques rely on the build up of material to create a required shape. These processes frequently compete against techniques, such as machining, that rely on the removal of material. Direct Metal Deposition (DMD) using lasers, Shaped Metal Deposition (SMD) using TIG sources and wire deposition using electron beam systems, are examples of additive manufacturing techniques that have been used for a number of years. The nature of these techniques allows parts to be created in a more flexible manner than with more conventional processes. It is the flexible nature of additive manufacture that has been investigated in this Core Research Project.
To create the desired shape, it is common practice to design the part as a CAD drawing and then use a post-processing package to 'slice' the material into a number of layers. The resulting toolpath is then used to build the component one layer at a time. This can allow complex structures to be deposited. Manipulation of the powder delivered to the substrate can also be used to create parts of high value, tailoring mechanical properties to different parts of the component as required by variation of chemical compositions. This can have the advantage of minimising local stress concentrations resulting in a less defect susceptible deposit.
The potential of additive manufacturing techniques to create a part that is of functionally graded material (FGM), presents a significant opportunity. Material costs can be reduced, compared to conventional additive manufacturing techniques, by placing expensive materials where they are required and using inexpensive structural material elsewhere within the part. This report details the results of an initial study at TWI, into the manufacture of FGM, using laser DMD. Because of the innovative nature of this work, a fundamental approach was adopted. This has taken the form, in the first instance, of work on an obviously compatible low cost material combination, which was used for proof of concept, followed by a more ambitious material combination of more direct industrial relevance.
The work presents results of the development of a graded deposit between mild steel and H13 tool steel. This combination was chosen for the initial study because both of these powders are inexpensive and because H13 has a greater hardness than mild steel, which can be easily used for the verification of the process. The subsequent phase of the work used a combination of Ti-6Al-4V and Ti-6242, a combination which has potential use in aeroengine manufacture. The work entailed the procedure development of the deposition processes, as well as characterisation of the deposits, and an evaluation of resulting mechanical properties, including hardness, tensile strength, and ductility.
Objectives
The objectives of this Core Research Project were, by using laser DMD, to:
- Demonstrate the feasibility of depositing functionally graded material using mild steel and H13 tool steel.
- Develop procedures for the deposition of Ti-6Al-4V and Ti-6242.
- Deposit a functionally graded structure using a combination of Ti-6Al-4V and Ti-6242.