High velocity impact properties of as-built and heat treated ti6al4v (eli) specimens built through direct metal laser sintering

Abstract

Direct metal laser sintering (DMLS) is a metal additive manufacturing (AM) technology specific to the company Electro Optical Systems (EOS). Generally, this technology is known as selective laser melting (SLM) and it fits in the broad category of powder bed fusion (PBF) (ASTM F2792-12a). The process is a laser-based technique that uses the digital input of CAD models to create three-dimensional (3D) parts from metallic powders. In the Centre for Rapid Prototyping and Manufacturing (CRPM), an EOSINT M280 system was used for building the DMLS parts for this study. The term, high velocity, is used to describe speeds in the range of 50 to 1500 m/s in ballistics. In general, the mechanisms that are activated in ballistic impact depend on the thickness, ductility, strength, stiffness, hardness, toughness, strain rate, density of the target, and velocity of the projectile. In order to get insight on the high velocity impact behaviour of the Ti6Al4V alloy, preliminary testing was conducted on wrought Ti6Al4V as a prelude to considerations of the as-built and stress relieved DMLS Ti6Al4V (ELI) alloy as an alternative to Rolled Homogeneous Armour steel (RHA) and aluminium alloy AA 5083 for high velocity impact applications. Preliminary ballistic impact testing of wrought Ti6Al4V involved the manufacturing of test plates of different thicknesses, and thereafter ballistic impact testing using 7.62 x 39 mm (.30 calibre) bullets travelling at an average velocity of 702 m/s. The preliminary testing of the wrought Ti6Al4V revealed that its minimum thickness to prevent through penetration was above 14 mm. This value was found to lie between 14 mm and 18 mm. Based on the foregoing, the value falls between that of RHA and AA 5083 at 11.43 mm and 19.8 mm, respectively. Fractographic analysis of the wrought Ti6Al4V plates further revealed that, although Ti6Al4V is usually classified as a ductile metal under normal loading conditions, when exposed to high velocity impact, the alloy exhibits both brittle and ductile behaviour. Microstructural analysis of the wrought Ti6Al4V revealed that the high strain rate imposed on the plates, brought about alteration of the microstructure during high velocity ballistic impact. This was clear from the gradually increasing refinement of grains and decreasing measured average values of the widths of 𝛼-laths, radially away from the edges of the penetration holes of the projectiles. Both the refinement of grains and the decreasing average measured values of the widths of  grains were a function of the high rise in temperature followed by cooling. The effect of impact on the plates tested was to create a compressive stress wave in the direction of impact (longitudinal axis of the penetration hole) and tensile stress waves in the radial and circumferential directions with respect to the penetration hole. These induced stresses tended to lengthen and widen the grains along the radial and circumferential directions, respectively. It was noted, however, that the predominant change in grains towards the edge of the hole was a reduction of size (refinement of grains). This suggested that the effect of grain refinement due to exposure to high temperatures overrode that of the induced stresses. In addition, the high strain rate and attendant high rise in temperature arising from high velocity impact was enough to cause the formation of adiabatic shear bands (ASBs), whose incidence reduced with distance away from the edge of the projectile hole. The ASBs were seen in this work to be sites in which cracks propagated and therefore formed paths for failure of the material. After the preliminary ballistic impact testing of wrought Ti6Al4V, a justification was formulated for investigation of the high velocity impact properties of as-built and stress relieved DMLS Ti6Al4V (ELI). The justification yielded values of optimum thickness for the DMLS alloy of 13.64 mm and 10.40 mm, based on the V50 ballistic limit and shear strain energy theories, respectively, which were lower than the respective values for wrought Ti6Al4V of 15.38 mm and 14 mm. Ballistic impact testing of as-built and stress relieved DMLS Ti6Al4V (ELI) also revealed a minimum thickness to prevent through penetration greater than 14 mm. This value fell short of the estimated value of 10.40 mm in the justification for testing the alloy, which can be attributed to the assumption made in the calculations of normal deformation with the usual high plastic deformation encountered at low strain rates. Visual inspection of the as-built and stress relieved DMLS Ti6Al4V (ELI) ballistically impacted plates unveiled plastic deformation, induced bulging and spalling on the entry side of the penetration holes for all the plate thicknesses. Furthermore, apart from the 8 mm thick plate, all the plates exhibited ductile failure on the exit side of the penetration hole, evidenced by the existence of petals there. Fractographic analysis of the as-built and stress relieved DMLS Ti6Al4V (ELI) plates revealed brittle behaviour at the entry points of the penetration holes and ductile behaviour at the exit points of the penetration holes for all the plate thicknesses. Microstructural analysis of the as-built and stress relieved DMLS Ti6Al4V (ELI) plates further revealed that the high strain rates imposed on the plates, also brought about alteration of the microstructure during high velocity ballistic impact, as was observed for wrought Ti6Al4V. This was evidenced by a gradual increase of the 𝛽 phase fraction towards the edge of the projectile hole. There was an attendant decrease in hardness towards the edge of the projectile hole as evidenced by the Vickers hardness measurements. It was posed that due to the low thermal conductivity of Ti6Al4V (ELI) of 6.7 W/m-K, the heat generated by the high induced strain rates during high velocity impact, could not be conducted away effectively from the point where it was generated. This retention of heat is thought to have led to softening of the material and the formation of ASBs near the edge of the projectile hole. Moreover, the average velocity of the projectiles and the related primary and secondary penetrations in the present work was high enough to induce a temperature rise above the beta transus temperature over the period of impact. Though the time periods of impact (typically is about 2 – 5 ms) are much shorter than the normal heat treatment periods typified by heating and soaking, cooling in the present case occurred at ambient conditions and may thus be considered as fast cooling. Therefore, while there was not enough time to support whole scale transformation of to grains upon impact, there was adequate time for grains to transform back to grains after impact to form lamellae. This effect was observed to reduce with distance away from the edge of the penetration hole. The results obtained from the studies here revealed that as-built and stress relieved DMLS Ti6Al4V (ELI) manufactured through the DMLS process using an EOSINT M280 system with standard process parameters, can be used for high velocity impact applications, such as in the military and the aerospace industry.