Net shape manufacturing
Net shape manufacturing of components is becoming more important due to increasing metals costs and the need for reduced energy use and carbon dioxide emissions. Professor Xinhua Wu from The University of Birmingham, UK, describes the processes involved.
The buy-to-fly ratio of titanium (Ti) and nickel (Ni) alloys in aerospace applications is often more than 10:1 when using conventional manufacturing methods. The waste of ~90% of the material as machining chips and swarf, together with the carbon dioxide emissions generated, make net shape manufacturing an attractive process.
Direct laser deposition (DLD) and net shape hot isotopic pressing (HIPping) can use off-the-shelf powders specified by customers and could become serious contenders in manufacturing since the mechanical properties obtained meet the requirements for aerospace applications.
Under the spotlight
Direct laser deposition technologies use a laser as a heat source to melt the metal powder either blown into the laser focal point or layered on a powder bed. The laser follows the path defined by the CAD file of a component, which is built by repeating the laser scanning layer by layer.
When blown powder is used, the component is either built upon a suitable substrate or by adding features to a pre-form. It is a faster process than the powder bed technique because it uses a 10 times larger Z-increment and a higher melting rate. This unfortunately downgrades the surface roughness. However, the latest development may overcome this where a variable spot size can be used to allow rapid building in the middle and refined deposition for the surface of a component.
Precision repair has been applied to aeroengine compressor and turbine blades, seal segments and to tooling as shown below, left. The capability for adding features on a large substrate has been demonstrated by AeroMet Corporation in the USA, where Ti6Al4V components were built using a ‘travelling’ glove box.
Laser powder bed processing often referred to as SLM (selective laser melting) is a development from rapid prototyping where the difference is in the materials used.
The key issues for DLD are control of microstructure, mechanical properties, surface finish, reproducibility and efficiency/cost. With powder being fed continuously into the moving laser beam, there are concerns about density, unmelted powder, the effect of the environment (argon atmosphere is essential for DLD Ti alloys), and microstructural variation with the height, all of which influence the mechanical properties of the finished component. These issues are strongly influenced by process parameters, laser scan speed and spacing, laser power, Z-increment (or layer height), and powder feed rate, which must be optimised.
Hot isotopic pressing is carried in a vessel where high temperature and pressure are exerted on a component to densify the powder and remove any porosity. It is used routinely for closing the porosity in castings.
Net shape HIPping uses a capsule with pre-defined geometry through computer modelling to shape a component. This capsule is filled with metal powder outgassed to a good vacuum and sealed, followed by HIPping. The capsule’s design needs to take into account the densification of the powder from a typical tap density of 65% and plastic deformation of the capsule at the HIPping temperature. Mild steel is generally used to form the capsule or any internal tooling required. If the modelling is adequate, the component will, after HIPping, be within the specification of the dimensions defined by the customer.
The metal powders used are in the size-range of 50-200µm. The required quantity is normally less than 1.2 times the net weight of the component – an enormous improvement in the buy-to-fly ratio from 10 to 1.2.
It is possible to manufacture sizable components from commercially supplied PREP Ti6Al4V powder using HIPping. The maximum component dimension is limited only by the size of the available HIP facility.
The microstructure of as-HIPped Ti6Al4V is different from that found in the alpha-beta forged alloy, as can be seen in the image above, right. The as-HIPped sample consists of regions of approximately equiaxed grains of alpha and beta surrounding thin laths of alpha, together with some equiaxed alpha grains within a beta matrix. The Ti6Al4V powder is martensitic and HIPping results in alpha plates replacing the martensite plates. The sample forged in the two-phase region contains fine primary alpha within beta grains, whereas samples forged above the beta transus show coarse grain boundary alpha and coarse alpha laths within the original beta grains.
Typical tensile, fatigue, fracture toughness properties of HIPped and (α+β)-forged samples are listed in the table below and it is clear that the balance of properties are comparable, with the powder HIPped samples showing a slightly better fracture toughness.
The influence that the as-HIPped surface has on properties is important since a typical net shape HIPped component has a large amount of as-HIPped surface. This surface is formed as a result of acid leaching the mild steel capsule after HIPping, and is influenced by any diffusion occurring from the steel into the capsule during HIPping, as well as by the acid.
The tensile strength of bulk samples (machined to remove the HIPped surface) is similar to that of samples which contain the as-HIPped surface. The fatigue properties of the latter are significantly better, as shown above.
Conventionally, tooling is made from mild steel, which has to be machined to an accuracy that will result in a component within specification after allowing for dimensional changes occurring during HIPping. The cost of this disposable tooling is at least 50% of the cost of the final component. Lower cost tooling to produce near net shape components is being developed. A critical assessment of the costs of various options available when producing components must be conducted. The holes in the engine casing and the end flanges were machined for the casing shown on the previous page and this was the most cost-effective process route for this component.
The dimensional control and the properties resulting from HIPping powder are reproducible so that successive components will have similar dimensions and properties – much more so than with forged components, where segregation that occurrs during solidification is not eliminated in large components.
Near net shape manufacture using DLD or HIPping provides two very different manufacturing processes that are of growing importance, in particular in aerospace where material costs are high.