Laser powder bed fusion (L-PBF) is an additive manufacturing technology that uses laser beam energy to melt layers of material powders in order to create a fully dense part. It uses a computer aided approach to creates a 3D model that is later sliced into distinct layers for printing. The printing process (part production) involves addition of material powders in a layer after layer process which is contrary to the subtractive conventional metal production process. As a technology powder bed fusion encompasses several printing techniques namely Electron beam melting (EBM), Selective heat sintering (SHS), Direct metal laser sintering (DMLS), Selective laser melting (SLM) and Selective laser sintering (SLS).
As a novel technology L-PBF has gained hearts in both industrial (airspace, medical, oil and gas industries) and research field due to its ability to produce part having intricate shape which means flexibility and creativity, also due to little or no waste material produced, and reduction of production time which means short time to market (which important business wise) and also due to possibility to produce fully dense parts having density of up to 99.98%. as such, several materials like polymers, aluminum, copper, iron and titanium alloys like Ti6Al4V are produced using L-PBF.
Even though L-PBF production process offers some exciting technical advantages, Ti6Al4V parts produced by it proves otherwise. It has been reported by several studies and noted during this study that L-PBF produced Ti6Al4V (as-build Ti6Al4V) has higher strength but very low elongation percentage when compared to conventionally produced Ti6Al4V as a result of higher thermal gradient that exist in L-PBF process that leads to the formation of acicular martensite microstructure. This pre-existing fallibility is the Achilles’ heel of L-PBF produced Ti6Al4V and renders Ti6Al4V parts produced by L-PBF inadequate for industries in which conventionally produce Ti6Al4V is used. As industries such as biomedical, aviation, oil and gas, automotive, marines, and offshores relies on Ti6Al4V properties.
Studies have shown that it is possible to transform the acicular martensite microstructure observed in as-build Ti6Al4V through heat treatment and by doing so improve its mechanical properties as well. However, a bi-lamellar microstructure has not been achieved from L-PBF Ti6Al4V produced Ti6Al4V.
This thesis aim was to improve mechanical properties of L-PBF produced Ti6Al4V through heat treatment to transform the acicular martensite microstructure found in L-PBF to bi-lamellar microstructure and conducted comparative studies of observed mechanical properties from as-build, lamellar, and bi-lamellar microstructure. Two heat treatment approaches: one stage heat treatment and two-stage heat treatment were utilised in this thesis.
The one stage heat treatment approach in was consisted of heat-treating L-PBF produced Ti6Al4V parts above the β transus to a temperature of 1030°C then slowly furnace cooled while the two-stage heat treatment approach consisted of the first stage in which L-PBF produced Ti6Al4V samples were solution heat treated above the β transus at 1030°C followed by slow furnace cooling (like in one stage heat treatment), and a second stage heat treatment consisting of ageing L-PBF produced Ti6Al4V parts to different ageing temperature vis 800, 880 and 960°C then air cooled.
All the samples (as-build, heat treated) were analyzed using scanning electron microscopy (SEM) then mechanically tested (tensile test, fracture crack growth test).
The obtained results showed that the acicular martensite phase initially observed in as-build parts was transformed to lamellar microstructure following one stage heat treatment approach and to bi-lamellar microstructure after the two-stage heat treatment approach. Mechanical test of all samples showed that as-build samples had higher tensile strength, lower elongation and no resistance to crack propagation, while heat-treated samples had very good combination of strength, improved elongation and good resistance to crack compared to as-build samples.
Based on the observed results it was concluded there are scientific possibilities to transform the acicular martensite microstructure obtained in as-build Ti6Al4V to lamellar and bi-lamellar microstructure and improve mechanical properties of L-PBF produced Ti6Al4V and it was noted that bi-lamellar microstructure offered the best combination of mechanical strength when compared to as-build and lamellar samples.