Defence Materials Technology Centre (Australia)

This paper provides a detailed historical review of the cavity flow phenomena in fluid mechanics, from recorded studies in the late 19th century to more recent work. Research has been reviewed, independently and in culmination with other studies, to summarise the major and minor governing parameters of the flow. Outlined are influences of technology, regarding numerical models, experimental methods, analysis, and control techniques. All Mach regimes are assessed; low incompressible, sub-, trans-, super- and hypersonic where substantial research was available. A large variety of cavity geometry was presented, mostly rectangular, with more complex features akin to industry application, and where available, assessment of the boundary layer structure is also included. Conclusions on present understanding, and requirements for future work are given, with an aligned set of available data.Cavity flow-field initialisation and development is dependent on; upstream (U/S) flow conditions of; airspeed M∞, boundary layer (BL) disturbance (δ), displacement (δ∗) and momentum (θ) thickness, either laminar or turbulent, and cavity geometry; length (L), depth (D) and width (W), with ratios L/D,L/W,δ/D and L/θ defining cavity response. I.e., a narrow cavity with a thin BL U/S tends toward a periodic 3D flow-field, with 3D effects and periodicity decreasing as W and δ increase. Control is achievable through SL stabilisation via spanwise disturbance from the leading edge (LE), or thickening the BL, thus shear layer (SL). Experiments are preferred over numerical models, due to the inefficiency and high cost of required models (Colonius, 2001; Rowley and Williams, 2006; Lawson and Barakos, 2011). We understand effects of L, D, L/D, and M∞, thus future work should focus on W, BL and how they impact mode switching and stream/spanwise flow propagation. Also introducing more complex geometry, realistic to application, to observe additional 3D effects and U/S momentum change, in contribution to a scaling parameter and determination of criteria for activation of material displacement.

Abstract Laser-directed energy deposition (L-DED) is a key enabling technology for the repair of high-value aerospace components, as damaged regions can be removed and replaced with additively deposited material. While L-DED repair improves strength and fatigue performance compared to conventional subtractive techniques, mechanical performance can be limited by process-related defects. To assess the role of oxygen on defect formation, local and chamber-based shielding methods were applied in the repair of 300M high strength steel. Oxidation between layers for locally shielded specimens is confirmed to cause large gas pores which have deleterious effects on fatigue life. Such pores are eliminated for chamber shielded specimens, resulting in an increased ductility of ∼15%, compared to ∼11% with chamber shielding. Despite this, unmelted powder defects are not affected by oxygen content and are found in both chamber- and locally shielded samples, which still have negative consequences for fatigue.

Polycrystalline diamond (PCD) tools are widely used in industry due to their outstanding physical properties. However, the ultra-high hardness of PCD significantly limits the machining efficiency of conventional abrasive grinding processes, which are utilized to manufacture PCD tools. In contrast, electrical discharge grinding (EDG) has significantly higher machining efficiency because of its unique material removal mechanism. In this study, the quality and performance of PCD tools machined by abrasive grinding and EDG were investigated. The performance of cutting tools consisted of different PCD materials was tested by high-speed turning of titanium alloy Ti6Al4V. Flank wear and crater wear were investigated by analyzing the worn profile, micro morphology, chemical decomposition, and cutting forces. The results showed that an adhesive-abrasive process dominated the processes of flank wear and crater wear. Tool material loss in the wear process was caused by the development of thermal cracks. The development of PCD tools’ wear made of small-sized diamond grains was a steady adhesion-abrasion process without any catastrophic damage. In contrast, a large-scale fracture happened in the wear process of PCD tools made of large-sized diamond grains. Adhesive wear was more severe on the PCD tools machined by EDG.

Laser metal deposition (LMD) presents a novel method for the repair of high value aerospace components as cracked or wear damaged areas may be locally removed and replaced with fresh materials. To better understand the influence of the extent of repair on mechanical performance, tensile specimens of 300M high strength steel were repaired using LMD to account for 10%, 20%, and 40% of the total cross section depth, with 420SS used as the feed material. LMD repair provided significant improvements to strength compared to the grind-out condition, with greater benefit for larger fill thicknesses. The repaired ultimate tensile strength (UTS) decreased from 1900 to 1750 MPa as the fill thickness increased from 10% to 40%. This was in line with the increasing proportion of 420SS, as this steel has a lower UTS (1360 Mpa) compared to 300M (2000 MPa), with the repaired samples showing typical composite behavior. Brittle martensite in the upper layers of the deposit caused ductility to drop from 13% to below 3% following repair. In situ tempering eliminates untempered martensite in the heat-affected zone and lower layers of the deposit, with hardness in the tempered region ranging from 425 to 550 HV, compared to 600 HV in the substrate and 650 HV in the untempered region. The softest regions of the deposit have the greatest influence over the yield strength, while the UTS is governed by the average strength over the entire region.

The colonisation of biomaterial surfaces by pathogenic bacteria is a significant issue of concern, particularly in light of the rapid rise of antibiotic resistance.