Additive layer manufacturing characterized by melting of pre-alloyed metal powders is a processing technique that has developed from rapid prototyping with the capacity to produce functional net-shape parts with the strength attributes of wrought parts. This method is suitable for small volume production and is cost competitive as opposed to CNC machining methods. In nearly all powder-based processes including sintering, hot isostatic pressing, and powder compaction, and net shape manufacturing processes including casting, there exists an inherent porosity related to the process.
In literature, several studies report on particular combinations of alloys, additive layer manufacturing and applications. Titanium alloys, for instance Ti-6Al-4V, are increasingly being investigated for implementation in unique aerospace as well as biomedical applications such as orthopedics devices, and dental implants. For that reason, they have received a large proportion of the effort.
In general, the material development cycle begins by analyzing the relationships between porosity and tensile strength, as indicated by powder bed laser fusion processes, including optical lasers to melt the powder, wire-feed methods, and electron beam approaches. Anisotropic mechanical attributes have been found to occur at a varying degree, and it has been observed that there are a number of levels of porosity that pose negative effects on ductility followed by high levels of hardness as well as yield strength.
Swansea University researchers led by Dr Nicholas Lavery investigated 316L with a detailed description of the alloy material. The aspect being detailed is based on a thorough description of the measurement methodologies for as-built as well as hot isostatically pressed samples. The study also entailed the use of ultrasound testing to determine elasticity properties, a method which has not be extensively used on additively manufactured materials. Their research work is published in Materials Science & Engineering A.
The authors correlated against laser energy density, porosity and relative density established using several experimental methods. They then performed high-throughput ultrasound measurements in a bid to compute elasticity attributes and indicate that lower porosities from builds with higher energy densities had higher elasticity moduli consistent with empirical relationships, and that hot isostatic pressing improved the elasticity attributes to levels similar to wrought 316L steel.
The authors then performed finite element analysis based on the porosity microstructures. This was in a bid to understand the effect of pore size distributions and morphology on the Young’s modulus.
The authors observed that two types of porosities were formed as the laser energy input was increased. However, the overall as-built porosity could be below 2% at the optimal laser energy input. Hot isostatic pressing was further able to minimize porosity to below 1%. In addition, it marginally increased the peak ultimate tensile strength values and lowered the standard deviation. One of the strong points of this publication is the thorough comparison with other published data for 316L steel.
Hot isostatic pressing considerably reduced the yield strength, but also increased elongation and in both cases reducing standard deviation. Upper tensile strength was observed to be higher for horizontally built samples as opposed to vertically built samples. After hot isostatic pressing, the authors recorded a 100MPa difference in ultimate tensile strength between vertically built samples and horizontally built samples.
The researchers observed improvement in ductility as it was reflected in the fracture surfaces of the hot isostatically pressed samples. Ultrasound measurements enabled the authors to examine the relationships between elasticity properties and porosity, for instance, Young’s modulus. Their study found that properties were directly related to the laser energy input. Finite element micromodels exhibited a similarity with empirical equations employed for sintered parts.
N.P. Lavery, J. Cherry, S. Mehmood, H. Davies, B. Girling, E. Sackett, S.G.R. Brown, J. Sienz. Effects of hot isostatic pressing on the elastic modulus and tensile properties of 316L parts made by powder bed laser fusion. Materials Science & Engineering A, volume 693 (2017), pages 186–213.Go To Materials Science & Engineering A