The growth of additive layer manufacturing

Materials World magazine
,
1 Oct 2017

Additive layer manufacturing is a growing technology, finding use in a range of sectors, as Robert Lancaster* explains.

Additive layer manufacturing (ALM) is a rapidly growing technology receiving widespread attention from a number of industrial sectors. The layer-by-layer process has evolved significantly over the past couple of decades and is now capable of fabricating near-net shape, fully dense structural parts with a high potential for geometrical optimisation (read more about the importance of ALM on page 35). 

The emergence of ALM is linked to the benefits it provides compared with more conventional manufacturing processes, such as forging or casting. These include considerable cost savings due to less waste, short lead times and improved buy-to-fly ratios. ALM can also form highly intricate components that would not be possible with traditional methods. 

ALM processes are used in the aerospace industry for two distinct purposes – repairs and component manufacture. Blown-powder (BP), where the metal powder is blown coaxially to the laser beam, which melts the particles on a base metal to form a metallurgical bond when cooled, is used to repair advanced, aero-gas turbine bladed disk components, known as blisks. 

A complete overhaul of the part could be costly and time consuming, whereas additive repair can be performed in situ. Powder-bed (PB) technologies, however, are more prominent in the manufacture of whole components. This process consists of a high-energy laser or beam that selectively melts or fuses pre-alloyed metal powder on a retractable bed to produce a three-dimensional part direct from a CAD model. 

In 2015, this method was realised as a flying test bed with the A380 aircraft from Airbus took flight and provided a significant milestone in the world of ALM. The aircraft was equipped with a Rolls-Royce Trent XWB-97 engine that contained 48 aerofoil shaped vane components – all produced by ALM. 

However, the influence of either of these manufacturing methods on the resultant mechanical properties of aerospace alloys must be fully understood before being applied in service. 

Handling the transient microstructures that are typically produced in ALM built material, which can lead to a significant variance in the mechanical properties across the component, is a fundamental research requirement. 

Furthermore, considering the multiple interactions of intra- and inter-built process variables on the integrity and consistency of the final structure, traditional laboratory-scale test approaches are deemed unsuitable for mechanical characterisation as it is difficult to extract representative test specimens that comply to international test standards.

Research is fundamental

In recognition of this, academics and researchers from the Rolls-Royce University Technology Centre (RRUTC) in Materials at Swansea University, UK – supported by EPSRC funding from the Strategic Partnership in Structural Metals in collaboration with the universities of Birmingham and Cambridge – have worked to develop methodologies that can accurately define the mechanical characteristics of such complex materials. The RRUTC is based within the Institute of Structural Materials (ISM) at Swansea University’s new Bay Campus, and currently consists of a team of six academics supported by 15 research officers and a rolling cohort of over 30 postgraduate PhD or EngD students. 

The ISM is supported by Swansea Materials Research & Testing Ltd. (SMaRT), an ISO 17025 accredited commercial test facility that supplies design-quality mechanical data to a range of companies within the structural material sectors. From this group, a team of six scientists has formed, working on related projects in unison with senior technologists and engineers from the Rolls-Royce technology centre.

One approach used by the team to characterise 3D-build geometries is small-scale testing, which can provide a discrete means of attaining mechanical property information from materials of limited dimensions. Take, for example, the small punch (SP) test, in which miniature disc specimens can be extracted from a variety of orientations from the build to evaluate the anisotropic nature of the part, providing important localised data. 

SP is a miniaturised mechanical test method that has previously been employed to evaluate neutron irradiation damage in nuclear reactor materials and for remnant life assessment of power plant components extracted from operation. The primary advantage of this approach is the small volume of test material required. 

The small punch test has been employed across laboratories worldwide to obtain creep rupture lives and tensile fracture data on a wide variety of different material systems.
SP also provides the ability to correlate test data that is produced by conventional approaches, therefore making the test methodology a practical and attractive solution for many geometry-related issues where material characterisation is required. Research has been published by the Swansea-led collaborative team on materials, ranging from single crystal superalloys for high-pressure turbine blade applications, titanium alloys for fan blades and intermetallic titanium aluminides for low-pressure turbine blade components. The SP test is now being used to assess the properties of additive layer structures.

Creating a European standard

The test typically comprises a round miniature disc specimen, 9.5mm in diameter and 0.5mm in thickness. This is clamped in location between a die set, from which a load is exerted onto the top surface through a hemi-spherical ended punch. From here, depending on whether the load is applied under constant displacement or constant load, a representative tensile or creep curve is produced.

The results are similar to more conventional test approaches, particularly the SP creep test, which typically exhibits the classical three stages of deformation that are widely recognised from uniaxial experiments. 

A European Code of Practice for small punch testing was formulated in 2009 to ensure consistency across international test houses and institutions. From this, a creep correlation factor, or kSP factor, was proposed, where the SP load may be correlated to a uniaxial creep stress in order to compare SP and conventional creep data. This method has found success in a number of material systems and is now widely accepted.

Swansea University is one of the leading partners towards a European standard for SP testing and is also breaking new ground in developing a novel SP fatigue testing capability in its research laboratories. Initial results have shown potential, with promising correlations to data generated from more traditional uniaxial approaches. 

Given the transient and anisotropic nature of the microstructure in ALM structures, SP specimens can sample isolated regions of interest and reveal any variance in the properties across the build. 

Naturally, this is important in the design of ALM components, helping to identify regions susceptible to a variation in properties – information that is impossible to gain from conventional, mechanical test approaches.

*Dr Robert Lancaster is an Associate Professor at Swansea University’s Institute of Structural Materials (ISM), specialising in the development of novel small-scale testing technologies. He is internationally recognised in the field of small scale testing, having taken an active role in standardising the test methodology and is chairing the 5th International Small Sample Test Techniques conference in Swansea in July 2018.

For more information, visit: www.sstt2018.com