Redesign and characterisation of the nose wheel fork of a light aircraft for production in Ti6al4v(ELI) through laser powder bed fusion

Abstract

In agreement with the AHRLAC company (currently known as Paramount Aerospace Industries), the nose wheel fork of the AHRLAC, which was conventionally machined in 7050 aluminium alloy, was selected for this study. A technique of topology optimisation, aligned with design for additive manufacturing (TO-DfAM), was applied to redesign the nose wheel fork. This was done to obtain a lightweight component without compromising the required strength and to allow the use of laser powder bed fusion (L-PBF) for the production of the nose wheel fork in Ti6Al4V(ELI). The redesigned Ti6Al4V(ELI) nose wheel fork was 20% lighter than the fork produced through conventional machining from 7050 aluminium alloy. For the current research, the dimensions of the component were adjusted to a size which allowed the production of the prototype scaled-down nose wheel fork in Ti6Al4V(ELI) in the available EOSINT M290 direct metal laser sintering (DMLS) system. The prototype nose wheel fork, together with standard test specimens built in the same machine process parameters, were submitted to a two-phase heat treatment. This heat treatment consisted of a stress-relieving heat treatment at 650 °C for 3 hours, which was followed by high-temperature annealing at 950 °C for 2 hours. Subsequently, the built and heat-treated test specimens were characterised through porosity assessment, dimensional accuracy determination and surface roughness testing. Standard tests were done according to the respective ASTM standards to determine the mechanical and fatigue properties of the DMLS Ti6Al4V(ELI) test specimens. To confirm the integrity of the building process, tensile test specimens, built along the deposition direction, were tested following ASTM E8. Other properties included tensile strength, impact toughness, fracture toughness (KIC), fatigue crack growth rate (FCGR) and high cycle fatigue (HCF). For impact toughness determination, two sets of specimens were tested, one set had a V-notch printed and the other V-notches were created using electrical discharge machining (EDM). Before testing these specimens based on the ASTM E23 standard, they were conditioned to a temperature of -50 °C. The KIC and FCGR were determined in accordance with the ASTM E399 and ASTM E647 standards, respectively. To determine the impact of surface roughness on the fatigue properties of the alloy, HCF specimens with as-built surface roughness were subjected to tension-tension fatigue testing in compliance with the ATSM E466 standard. Subsequently, the operational performance of the DMLS Ti6Al4V(ELI) scaled-down nose wheel fork, also with as-built surface roughness, was experimentally tested using a unique test jig designed to allow static and fatigue loading. Initially, the nose wheel fork was tested under maximum static loads along the X- and Y-axes that were set 24% higher than the design loads required by the General Aviation (GA) Code of Federal Regulations (CFR). The results of this test were used to validate the finite element analysis (FEA) executed to model the performance of the component. Secondly, the component was tested under cyclic loading along the X- and Y-axes to investigate its fatigue performance. These cyclic loads were also 24% higher than the accepted design load. All loads were applied at a frequency of 3 Hz with a fully reversed (stress ratio R = -1) fatigue loading as the baseline. The microstructure near the crack regions and the analysis of the fracture surfaces of the scaled-down nose wheel fork were analysed to confirm the failure mechanisms. The average impact toughness of 26 J which was measured for both the as-built and wire-cut V-notch Charpy impact test specimens, was 8% higher than the impact energy required by the aviation industry. When comparing the FCGR results obtained in the current study with the one reported in literature, higher crack growth resistance was observed. These were attributed to the heat-treatment post-process that was executed. The fatigue strengths of 190 MPa and 225 MPa recorded for the standard DMLS Ti6Al4V(ELI) HCF test specimens with as-built surface roughness that were built in, and perpendicular to, the plane of the build platform, respectively, were 50% lower than those of machined test specimens. This confirmed the negative impact of the surface roughness obtained with the DMLS process. The DMLS Ti6Al4V(ELI) scaled-down nose wheel fork withstood static loading without any plastic deformation. It resisted more than 100 000 cycles during the first X-loading of 6 000 N, however, failure was observed after 15 000 cycles in an area predicted to have high cycle fatigue under Z-loading. These results illustrated that the inherent surface roughness on the DMLS Ti6Al4V(ELI) scaled-down nose wheel fork, due to the staircase effect and partially melted powder particles on the surface, was detrimental to the fatigue life of the component. The current study was a unique first attempt to redesign and characterise a mission-critical structural component of an actual aircraft for production in Ti6Al4V(ELI) through DMLS technology and assess its compliance with the performance requirements of the component. It was confirmed that a digital design method that includes FEA can be used to design a unique intricately shaped DMLS Ti6Al4V(ELI) nose wheel fork, provided experimentally determined mechanical properties, fatigue properties and inherent surface roughness were integrated into the model. Based on static and fatigue performance testing, as well as standardised destructive and non-destructive testing, the potential to produce a dimensionally accurate full-scale nose wheel fork of the AHRLAC in Ti6Al4V(ELI) through DMLS was confirmed. Improved fatigue performance of a full-scale DMLS Ti6Al4V(ELI) nose wheel fork can be obtained through surface finishing.