An FE model of the solution heat treatment, forming and in-die quenching (HFQ) process was developed. Good correlation with a deviation of less than 5% was achieved between the thickness distribution of the simulated and experimentally formed parts, verifying the model. Subsequently, the model was able to provide a more detailed understanding of the HFQ process, and was used to study the effects of forming temperature and speed on the thickness distribution of the HFQ formed part. It was found that a higher forming speed is beneficial for HFQ forming, as it led to less thinning and improved thickness homogeneity

Aluminium alloy 2024 (AA2024) is extensively used as a structural material in the aircraft industry because of its good combination of strength and fatigue resistance. However, complex shaped components, particularly those made from sheet, are extremely difficult to form by traditional cold forming due to its low ductility at room temperature. A possible solution of this problem is to form sheet workpieces at elevated temperature. The aim of the work described in this paper is to determine the relationship between formability and temperature for AA2024 by conducting a series of tensile tests at elevated temperatures ranging from 350 to 493 °C. Ductility of AA2024 was found to increase gradually with increasing temperature up to 450 °C, followed by a sharp decrease with further increase in temperature. So-called cup tests confirmed that the formability of AA2024 is very high at a temperature of about 450 °C. Fracture surfaces and longitudinal sections of formed samples were examined by scanning electron microscope. It was found that fracture occurred in three different modes depending upon the temperature, and the sharp decrease in ductility when the temperature exceeds 450 °C was caused by softening of grain boundaries by solute enrichment (at higher heating rates liquation may be involved) and softening of the matrix around inclusion particles. Keywords AA2024 Fracture mechanisms Hot forming Formability 1 Introduction Aluminium alloy 2024 (AA2024) is widely used for structural applications in the aerospace industry, due to its good combination of strength and fatigue resistance. Age hardening markedly affects the microstructure and mechanical behaviour of AA2024. After appropriate heat treatment, finely dispersed precipitates are obtained and a high strength to weight ratio is achieved. Low ductility, however, is typically a limitation of AA2024, making complex-shaped parts difficult to form using traditional cold forming processes. Hence some thin-walled and complex-shaped parts are machined from solid blocks of metal, which can incur up to 90% material wastage for some applications, with corresponding cost and energy wastages. In recent years, efforts have been made to improve the strength and ductility of AA2024 [1–9] by developing advanced processing routes. For example, following severe plastic deformation (i.e. ECAP [1,4,5] , cryo-rolling [3] or high-pressure torsion processes [9] ) and suitable ageing, highly dispersed nanometre-sized precipitates can be obtained. Due to the low density of dislocations present after ageing, and the strong pinning effect and aggregation of precipitates to dislocations, an elongation of up to 18% can be achieved at room temperature [3] . However, the use of extreme strain levels is limited to very small samples, and is currently not a suitable alternative for bulk materials processing. Pre-homogenisation treatment [7,8] might be an effective way of improving the ductility of AA2024, but this would reduce the material strength. Material strength could be enhanced by adding SiC p to AA2024, but the ductility of the resulting composite has been found to be low [6] . As for most metallic alloys, the ductility of AA2024 increases with increasing deformation temperature [6] . However, hot stamping is seldom used for forming complex-shaped parts because the desired microstructure (which has been fixed in the sheet prior to forming) tends to be destroyed at high temperatures, thereby reducing the mechanical performance. Furthermore, if heat treatment to restore strength is carried out after stamping, the parts tend to lose their shape due to thermal distortion. Recently, a new technique, Hot Forming and cold-die Quenching (HFQ), has been developed [10] . The basic idea of this novel process is to (i) heat the sheet metal to its solution heat treatment (SHT) temperature, at which its ductility is expected to be maximal, and (ii) simultaneously form and quench the sheet by using cold dies. After forming, the workpiece material is held within the dies for roughly 5–6 s in order to reduce its temperature rapidly to approximately 100 °C and freeze the microstructure as a supersaturated solid solution (SSSS). If a heat-treatable, aluminium-based alloy is used, the part can then be aged to obtain full strength. The feasibility of this novel process has been demonstrated for AA6XXX in [11] . This technique has gained favour over traditional forming methods because it produces parts with high formability, negligible springback, rapid processing and efficacious mechanical properties. In AA2024, copper is the most effective strengthening constituent. The age hardening effect can be increased by increasing copper content up to 6 wt% [12] . Magnesium is used in combination with copper to accelerate the age hardening effect at room temperature. The equilibrium precipitate phases for this system are mainly CuMgAl 2 (S phase) and CuAl 2 (θ phase) [13] , although CuAl 2 is less often observed in the alloy compared to CuMgAl 2 . Both of these phases are largely soluble during SHT. In addition, manganese has a marked strengthening effect, because it can influence material properties through the formation of intermetallics that provide Zener drag and limit grain size. The intermetallic compounds (IMCs) in AA2024 are very complicated [14–16] , and the shape, size and chemical composition of the IMCs vary remarkably, which are dictated by the processing route [16] . A range of IMC compositions have been reported for AA2024, such as Al 20 (Cu,Fe,Mn) 5 Si(Al 8 Fe 2 Si), periphery, Al 10 (Cu,Mg), (Al,Cu) 93 (Fe,Mn) 5 (Mg,Si) 2 , Al 3 (Cu,Fe,Mn), Al 2 Cu (θ phase), CuMgAl 2 (S phase) and Al 7 Cu 2 Fe [15] . These are normally formed during the casting or homogenization stages of processing, i.e. high temperature stages; these are normally not soluble during SHT. A number of studies have been carried out on the fracture...

This paper discussed the friction coefficient evolution of a TiN coated contact during sliding wear. A hard TiN coating was prepared on a bearing steel (GCr15) substrate by applying the composite method of cathode arc and magnetron sputtering. The microstructure, microhardness, microscratch and tribological behaviour of this coating were studied to obtain the relationship between friction coefficient and other coating properties, meanwhile the full friction coefficient evolution curve with different stages during the wear process of TiN coating was shown. It was found that, coating friction and wear, as two interactive responses from a tribo-system, mutually affected each other and should be studied as a single physical phenomenon. Therefore, a novel friction-wear interactive friction model was developed to represent the evolution of friction coefficient and to predict coating breakdown. The results show that the evolution of the friction coefficient curve can reflect different stages of the wear process and the wear life can be estimated using the new friction model

Sustainability is a key factor in an automotive OEMs’ business strategy. Vehicle electrification in particular has received increased attention, and major manufacturers have already undertaken significant investments in this area. However, in order to fully confront the sustainability challenge in the automotive industry, lightweight design in additional to alternative propulsion technologies is also required. Vehicle weight is closely correlated with fuel consumption and range for internal combustion and electrified vehicles, respectively, and therefore, weight reduction is a primary objective. Over the past decades, advanced steel and aluminium-forming technologies have seen considerable development, resulting in significant weight reduction of vehicle components. Hot stamping is one of the most established processes for advanced steel and aluminium alloys. The process offers low-forming loads and high formability as well as parts with high strength and minimal springback. However, the high temperatures of the formed materials over numerous cycles and the significant cooling required to ensure desirable component properties necessitate advanced tooling designs. Traditionally, casting and machining are used to manufacture tools; although in recent years, additive manufacturing has gained significant interest due to the design freedom offered. In this paper, a comprehensive review is performed for the state-of-the-art hot-forming tooling designs in addition to identifying the future direction of Additive Manufactured (AM) tools. Specifically, material properties of widely used tooling materials are first reviewed and selection criteria are proposed which can be used for the transition to AM tools. Moreover, key variables affecting the success of hot stamping, for example cooling rate of the component, are reviewed with the various approaches analysed by analytical and numerical techniques. Finally, a number of future directions for adopting additive manufacturing in the production of hot stamping tools are proposed, based on a thorough analysis of the literature.