Keywords

15.1 Introduction

Aluminium, being one of the most prevalent metals found in the earth's crust, is commonly employed for the purpose of reducing the weight of industrial products, particularly in the domains of construction, aerospace, and automotive engineering. Consequently, this lightweight metal possesses a distinctive function within the aerospace and aviation sectors, which are associated with the domain of transportation. The materials in question demonstrate noteworthy attributes including but not limited to fatigue and corrosion resistance, elevated fracture toughness, and a high degree of strength, often surpassing that of low carbon/low alloy steels [1]. The utilisation of aluminium in autobody applications has become increasingly prevalent due to the necessity for weight reduction in the construction of automobiles [2]. The automotive industry is currently encountering a predicament due to the escalating demand for vehicles that are more fuel-efficient, with the aim of reducing energy consumption and mitigating air pollution. Aluminium is considered to be a superior material for replacing heavier alternatives due to its exceptional strength, favourable stiffness to weight ratio, remarkable formability, commendable resistance to corrosion, and potential for recyclability [3]. Due to its exceptional specific strength and ductility, aluminium is a noteworthy material for energy absorption. Consequently, high-strength aluminium alloys are well-suited for utilisation in automotive bumper systems designed for this purpose. Although possessing numerous advantageous characteristics, pure aluminium is inadequate for the majority of industrial uses due to its softness and lack of strength. Aluminium alloys possess a strength that can surpass steel in terms of strength-to-weight ratios, often exceeding it by a factor of thirty, despite being derived from pure aluminium [4]. The utilisation of lightweight materials in the production of automotive engine components has been observed to enhance engine performance by reducing fuel consumption, noise, and vibration. Aluminium alloys are highly sought-after lightweight materials owing to their exceptional material properties, including formability and fatigue strength. Industries commonly utilise heat treatable aluminium alloys belonging to the 2000 and 7000 series. One of the significant characteristics of these aluminium alloys is their consistent ageing throughout the operational lifespan of the aircraft. These materials demonstrate outstanding characteristics such as high resistance to fatigue and corrosion, exceptional fracture toughness, and elevated levels of strength [5]. Aluminium alloys are commonly utilised in the aerospace industry owing to their low density, high fracture toughness, high fatigue durability, and ease of manufacturing. Alloys containing Al, Zn, Mg, and Cu are commonly utilised in the production of high-performance aircraft components such as wing skins and stringers. The aerospace industry's demands necessitate the incorporation of superior attributes such as elevated strength, acceptable corrosion resistance, and robust fracture toughness. In recent times, significant efforts have been directed towards enhancing heat treatment methodologies to satisfy the prerequisites for desirable synthetic attributes of Al–Zn–Mg–Cu alloys [6]. Aluminium alloys have been widely utilised as structural materials. These materials possess many of advantageous characteristics, including commendable resistance to corrosion, favourable weldability, exceptional surface properties, and a comparatively modest expense [7]. The widespread utilisation of aluminium alloys can be attributed to their exceptional specific strength and machinability. The manipulation of the thermomechanical cycle enables the attainment of a diverse spectrum of mechanical characteristics for these alloys, as they can undergo heat treatment to generate varying degrees of precipitation [8]. Industries commonly utilise heat treatable aluminium alloys belonging to the 2000 and 7000 series. One of the significant characteristics of these aluminium alloys is their continuous ageing throughout the operational lifespan of the aircraft. The specimens demonstrate outstanding characteristics such as durability against fatigue and corrosion, elevated fracture toughness, and a high degree of strength. The Al-Cu-Mg aluminium alloy 2618, belonging to the 2XXX series of aluminium alloys, is commonly employed in the transportation and aerospace sectors due to its favourable material properties. This alloy is capable of withstanding high temperatures over extended periods of time. The composition contains sufficient amounts of iron (Fe) and nickel (Ni) to form intermetallic complexes of micrometre size, which enhance dispersion and uphold microstructural stability under elevated temperatures. The aluminium alloy 2618 is a highly advantageous material due to its unique composition of aluminium, copper, magnesium, and silicon. This combination results in exceptional mechanical and physical properties, as well as superior resistance to corrosion and heat. As a result, this alloy is highly suitable for a diverse range of applications [9]. The process of age hardening, also referred to as precipitation hardening, is a heat treatment technique utilised to enhance the strength of metals and their corresponding alloys. The strengthening process commonly referred to as precipitation hardening involves the utilisation of solid impurities or precipitates. The process of ageing the metal involves inducing precipitation through either thermal treatment or cold storage. Alfred Wilm is credited with the discovery of the age hardening process. Metals and alloys composed of nickel, magnesium, and titanium possess desirable malleability and are deemed appropriate for undergoing the Age hardening process. The process of Age hardening has been observed to result in an increase in both the tensile and yield strength. The formation of precipitates impedes the mobility of dislocations or imperfections within the crystal lattice of metals. Age hardening is a process that involves subjecting metals and alloys to high temperatures for extended periods to facilitate precipitation. The objective of this investigation is to ascertain the optimal temperature for Age hardening through the utilisation of wear and hardness testing methodologies [10].

15.2 Literature Review

The impact of cold deformation on the mechanical properties and microstructure of artificially aged AA2024 aluminium alloy has been observed to be substantial. Specifically, the mechanical properties at 180 °C for 2 h, which is considered the peak ageing condition, have been found to be enhanced. The AA2024 alloy exhibits the potential to attain elevated levels of strength and ductility via precipitation strengthening, rendering it a viable option for deployment in key industrial sectors [11]. This investigation examined the effects of both natural ageing and pre-straining on the mechanical characteristics of AA 6016 alloy. The combination of extended natural ageing and pre-straining resulted in notable fluctuations in the characteristics of the material [2]. The aim of this research was to examine the effects of distinct thermal processes, specifically retrogression and re-aging (RRA) and duplex ageing, on the electrical conductivity and corrosion resistance of AA 7075-T6 alloy. The experiment involved retrogression at a temperature of 200 °C for a duration of 2 h, succeeded by re-aging for 24 h at a temperature of 120 °C. Additionally, duplex ageing was carried out at a temperature of 163 °C for different time intervals. Experiments were carried out to investigate exfoliation corrosion, and the findings indicated that duplex ageing at a temperature of 163 °C for a duration of 16 h exhibited the most favourable amalgamation of electrical conductivity and resistance to corrosion [3]. This study is centred on the examination of the work-hardening and ageing characteristics of a commercially available AA7108 alloy belonging to the 7xxx aluminium alloy family. The investigation encompasses an analysis of the alloy in its as-cast state as well as in its homogenised state. The hardening behaviour of an alloy is comprehensively investigated by analysing several factors, including but not limited to particle size, distribution, dislocation density, and alloying elements present in solid solution. The aim of this empirical investigation is to enhance comprehension of the correlation among microstructural attributes, mechanical features, and thermal treatments in the AA7108 alloy, which is part of the 7xxx aluminium alloying system [12]. The current study provides a thorough analysis of the fatigue properties of AA2618 aluminium alloy, a material that has been extensively utilised in the aviation industry, particularly in the Concorde aircraft. Although AA2618 has been largely substituted by alloys such as 7075 in modern aircraft owing to their superior fatigue resistance, it is currently being reintroduced for structural components that are subjected to high-temperature fatigue loading [5]. The present research endeavours to examine the stress-aging behaviour of an Al–Zn–Mg–Cu alloy in two stages and its influence on precipitate evolution. The phenomenon of ageing is characterised by the continuous formation of precipitates along the boundaries of grains, without the presence of clearly defined zones that are free of precipitates. During the second stage of stress-aging, the coarsening of ageing precipitates occurs, and their density within grains is contingent upon the temperature of the first stage of stress-aging. Discontinuous h phases arise along the grain boundaries, resulting in a wider region devoid of precipitates. The stress-aging process, which occurs in two stages, is designed to enhance the distribution of ageing precipitates both within grains and at grain boundaries [6]. This research investigates the susceptibility to quench sensitivity and the effects of natural ageing on a low-copper Al–Mg-Si alloy (AA6060). The phenomenon of quench sensitivity is predominantly ascribed to the depletion of solutes at the nucleation sites of dispersoids during the gradual cooling process subsequent to extrusion. The quench sensitivity is significantly influenced by the number density of dispersoids, the type of solute, and its concentration. The findings suggest that the sensitivity of quenching is impacted by the degree of vacancy supersaturation subsequent to the cooling process. The quench sensitivity of short natural ageing times, which last for 30 min, is higher as a result of vacancy annihilation and clustering. Conversely, longer natural ageing times, which last for 24 h, alleviate quench sensitivity. The extended process of natural ageing not only results in an increase in hardness but also enhances the quench sensitivity of lean Al–Mg–Si alloys [7]. The objective of this investigation is to examine the rate sensitivity of different obstacles present in age hardenable aluminium alloys. This will be achieved by determining the activation volume in relation to the flow stress. The findings suggest that solutes and precipitates demonstrate greater sensitivity to rate in comparison to dislocations. Shearable precipitates exhibit the most significant rate sensitivity among all the hindrances [8]. The present investigation aimed to analyse the influence of creep loading on the evolution of precipitate radii in aluminium alloy 2618A. The alloy was subjected to over-aging while under load at a temperature of 190 °C, in comparison to undergoing isothermal ageing without any stress. The findings indicate that the process of creep resulted in a statistically significant increase in the average size of precipitate radii when compared to samples that had not undergone deformation. The obtained distributions of radii proved to be valuable in the comparative analysis of the matured specimens, while the coarsening mechanism, which entails the diffusion of pipes along dislocations, accounted for the observed findings in the creep specimens [13]. This study offers valuable insights into the correlation between the chemical composition of alloys, their processing techniques, and the resulting microstructure of nanoscale precipitates. Furthermore, the constraints of current models and approaches are analysed along with potential solutions to address them. In general, this review provides a thorough examination of the present status of investigations pertaining to age hardening in aluminium alloys [10]. The present investigation scrutinised the impact of Sn concentration on the age-hardening kinetics and the formation of peak ageing precipitates in Al–Mg-Si alloys at a temperature of 250 °C. The findings indicate that the peak hardness exhibited an initial rise followed by a decline upon increasing the Sn content, with the highest level of hardness detected in the alloy comprising 0.1 wt.% Sn. The alloys containing Sn displayed a greater peak hardness in comparison to the Sn-free alloy. The augmentation in hardness observed in the alloys containing Sn was attributed to the refinement and amplified number density of precipitates, as evidenced by microstructural analysis [14]. The present study employed Cow horn particles (CHp) as a reinforcing agent to fabricate Metal Matrix Composites (MMCs) with Aluminium Alloy A356, aiming to achieve a composite material that is both lightweight and economical. The composite's age hardening process was precisely simulated through the utilisation of an artificial neural network (ANN), which yielded a notably high correlation coefficient of 0.9921. The process parameters for age hardening were optimised through the utilisation of a simulated annealing optimisation algorithm (SA-NN system). This resulted in a high degree of concordance with experimental values and negligible discrepancies [15]. The fabrication process of the alloy and composite involved a liquid metallurgy approach, which was subsequently followed by solutionizing and ageing. The results of the analyses indicate the presence of Mg2Si phase within the alloy and MgAl2O4 spinel at the interface of the composite. The results of the hardness testing indicated a noteworthy augmentation in the hardness of the specimens that underwent ageing at a temperature of 155 °C. The composite of LM25 and 10 wt% Al2O3, when subjected to ageing at 155 °C, demonstrated the least amount of wear, indicating its viability for use in the automotive sector [16]. Aluminium bronzes, specifically C95200 and C95300, were subjected to casting and ageing processes. A study was conducted to investigate the impact of ageing on the wear behaviour, friction coefficient, and microstructure. The process of ageing has been observed to cause a reduction in the size of alpha grains, leading to an improvement in both hardness and wear resistance. Nevertheless, at elevated ageing temperatures, the rate of wear exhibited an initial reduction followed by subsequent escalation [17]. The present research endeavours to examine the abrasive wear characteristics of 2014 Al alloys that have undergone solution treatment, in the context of both reciprocating and continuous sliding conditions. The wear patterns exhibited by both modes are comparable, however, reciprocating wear induces a greater mean surface roughness. The lower wear loss and abrasive wear coefficient in reciprocating wear as compared to continuous wear can be attributed to this phenomenon [18]. The present study investigates the abrasive wear rate of Al (6061) alloy that has been reinforced with SiC particulate and E-glass fibre, while being subjected to varying conditions. This study aims to examine the correlation between the duration of ageing and the rate of wear. The production of castings was carried out utilising the liquid melt methodology, followed by a subsequent heat treatment process. The study employed a pin-on-disc apparatus to conduct wear tests, which demonstrated that the wear rate decreases with increasing ageing durations [19]. The present investigation aims to examine the dry sliding wear characteristics of Al5083 alloy via a Pin-on-disc wear test apparatus. The aim is to improve the durability of the aluminium alloy through increased wear resistance. The experimental conditions involved manipulating load and sliding speed, while maintaining a constant sliding time. The experimental pins were fabricated using Al5083 material that had undergone forging. Surface deformation mechanisms were analysed via scanning electron microscopy (SEM) at ambient temperature [20]. The present investigation is centred on the alteration of the characteristics of low and high silicon stainless steel by means of diverse heat treatment circumstances, particularly age hardening with different durations of ageing. The findings suggest that stainless steel containing elevated levels of silicon exhibits enhanced mechanical characteristics, wear resistance, and resistance to corrosion. The optimal ultimate tensile strength and hardness values are achieved at intermediate ageing times, whereas shorter ageing times are found to minimise wear and corrosion rates. The examination of microstructure reveals the impact of varying heat treatment durations on the characteristics of materials, underscoring the possibility of optimising the mechanical and corrosion properties of stainless steel by judiciously selecting the appropriate heat treatment [21]. The present investigation was conducted to assess the hot and warm formability of 2618 aluminium alloy in its as-solutioned state by means of torsion testing. The deformation-induced precipitation of particles of a secondary phase has been observed to have an impact on both the shape and stress levels of the flow curves. The flow stress exhibited a continuous rise as a result of decreased temperatures, whereas elevated temperatures caused a peak followed by a softening effect due to precipitation and coarsening of precipitates. The study involved the derivation of constitutive equations and subsequent evaluation of the ductility of the alloy using strain-to-fracture values [22]. The present investigation involved the development of an aluminium matrix composite (AMC) by utilising aluminium alloy 2618 and integrating Si3N4, AlN, and ZrB2 particles. The tribological characteristics of the composite material were analysed under varying temperature conditions, with emphasis on parameters such as wear rate, wear resistance, specific wear rate, and coefficient of friction. The mechanical characteristics were assessed, and the examination of worn surfaces was conducted using scanning electron microscopy (SEM). The study employed Taguchi, ANOVA, and GA techniques to examine process parameters and enhance wear rates in the composite material [23].

15.3 Casting Techniques for Aluminium Alloys

The process of casting is a prevalent method employed in the manufacturing industry to fabricate intricate shapes of metallic constituents, such as those composed of aluminium alloys. The process of aluminium casting entails the introduction of liquid aluminium into a designated mould, which subsequently undergoes solidification to attain the intended configuration. Various casting techniques are at available for aluminium alloys, each presenting distinctive benefits and appropriate for particular applications [24].

15.3.1 Stir Casting Method

The stir casting process employs a mechanical stirrer to generate a vortex that facilitates the homogenization of the reinforcement and matrix material (refer to Fig. 15.1). The cost-effectiveness, mass production capability, simplicity, near-net shaping, and manageable composite structure control make this process a suitable option to produce metal matrix composites [25].

Fig. 15.1
An experimental setup of stir casting. It is equipped with a motor, stirrer, furnace, crucible, molten metal, and bottom pouring system.

Stir casting method

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In the process of stir casting, a furnace is employed to elevate the temperature and liquefy the constituent components. Meanwhile, a mechanical stirrer is utilized to generate a vortex, thereby facilitating the homogenization of the reinforcement materials that are introduced to the molten mixture. The crucible experiences an axial flow pattern generated by the impeller blade, which is affixed to a motor with adjustable speed. The reinforcement particles are introduced into the vortex through a feeder mechanism, and the agitation process is sustained for a predetermined duration. Subsequently, the resultant liquid amalgamation is introduced into a cast and permitted to undergo the process of cooling and solidification. Subsequent to casting, a series of post-casting procedures, including heat treatment, machining, testing, and inspection, are carried out. The utilization of bottom pouring furnaces is favored in the process of stir casting due to their ability to facilitate immediate pouring and mitigate the occurrence of solid particle settling. The utilization of single-stage impeller stirrers is prevalent in the manufacturing of metal matrix composites owing to their economic benefits, versatility, and uncomplicated nature [26, 27].

15.3.2 Squeeze Casting

The hybrid process of squeeze casting amalgamates the techniques of casting and forging, thereby resulting in cast products that exhibit exceptional mechanical properties (see Fig. 15.2). The utilization of this technique offers the possibility of fundamentally transforming the manufacturing process of aluminium alloy constituents and presenting itself as a feasible substitute for the replacement of crucial components. The procedure entails the introduction of liquefied metal into a warmed mould, succeeded by the exertion of force as the metal undergoes solidification. The squeeze casting process enables the production of intricate details and facilitates the integration of coring techniques for the creation of holes and recesses. The application of high pressure and the proximity of the material to the die surface lead to reduced porosity and enhanced mechanical characteristics. This manufacturing technique is applicable for the processing of both ferrous and non-ferrous metals and can be employed in conjunction with fiber cake preforms to yield castings that are reinforced with fibers.

Fig. 15.2
2 schematic representations of the squeeze casting method. The model is composed of an upper movable mold half, molten metal, a lower fixed mold half, a preform, a metal matrix composite, and an ejector pin. In the right model, the upper movable mold half is fixed to the lower fixed mold half.

Squeeze casting

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The Direct Liquid Metal Forging (DLMF) technique corresponds to the conventional forging process, wherein the liquid metal is introduced into a lower die segment and exposed to substantial pressure (minimum of 100 MPa) until the component solidifies. This approach facilitates the fabrication of components exhibiting exceptional mechanical characteristics. The Indirect Squeeze Casting (ISC) process bears resemblance to die casting and is executed using a die casting apparatus. The purified and granulated molten material is introduced into the mould via wide openings at a comparatively low speed. The solidification of the melt occurs within a pressure range of 55 to 300 MPa. The components manufactured via the ISC process demonstrate commendable tensile strength. The utilization of squeeze casting techniques offers notable benefits such as the capability to manufacture components exhibiting enhanced mechanical characteristics in contrast to the traditional casting methods. These materials offer improved mechanical properties and structural robustness, facilitating the fabrication of intricate component configurations. Notwithstanding, the limitations of this approach encompass diminished adaptability in the geometry of the parts, decreased efficiency, more stringent machining prerequisites, and elevated expenses in comparison to conventional casting techniques [28,29,30].

15.3.3 Diffusion Bonding

The process of diffusion bonding (Fig. 15.3) is a method of solid-state joining that facilitates the bonding of diverse materials by means of atomic diffusion at elevated temperatures. Autogenous bonding can be employed for materials that are identical, while interlayers can be utilized to bond families of materials that are different. The procedure necessitates the application of force to bring the surfaces into intimate contact and promote the phenomenon of diffusion. The quality of the joint is significantly influenced by the surface roughness and flatness of the parts. Diffusion bonding offers several benefits such as the capacity to generate robust joints without the requirement of filler materials, the aptitude to bond intricate internal structures, and the creation of joints with commendable bond strength and helium leak-tightness. The procedure facilitates the production of heat exchangers featuring diminutive diameter and depth conduits, thereby yielding advantageous ratios of heat transfer to weight. Diffusion bonding has been effectively employed for the joining of various materials, including metals, ceramics, and composites. Nevertheless, there exist certain drawbacks such as the intricacy involved in preparing workpieces and the substantial initial setup expenditure. Notwithstanding these constraints, diffusion bonding presents a dependable and efficient technique for amalgamating materials with elevated strength and integrity [31, 32].

Fig. 15.3
An experimental design of the diffusion bonding method. It is equipped with a load cell, a furnace with a high vacuum facility, a movable rod, a chiller to supply cold water, bonding dies, and a fixed rod.

Diffusion bonding

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15.3.4 Powder Metallurgy

The process of powder metallurgy (depicted in Fig. 15.4) involves the utilization of powdered metals and alloys to fabricate parts with high precision and accuracy. The components, namely bushings, bearings, gears, and structural components, are extensively utilized in manufacturing processes. The sintering procedure, which involves the application of heat to powdered particles below their melting point to create solid bonds, is an essential aspect of the powder metallurgy process. Throughout time, powder metallurgy has progressed into a proficient and productive technology that exhibits elevated tolerances and reduced waste. The procedure comprises of four primary stages, namely powder preparation, mixing and blending, compacting, and sintering. The aforementioned procedures have undergone modifications and adjustments in order to fulfil particular component specifications. Powder metallurgy has a lengthy historical background that can be traced back to ancient times. However, it gained significant recognition during the industrial revolution due to its ability to facilitate mass production. The field has witnessed notable progressions, encompassing the emergence of diverse methodologies such as traditional, injection moulding, isostatic pressing, and metal additive manufacturing. In general, powder metallurgy presents a flexible and dependable approach for producing components with elevated accuracy and customized characteristics, rendering it a valuable methodology in diverse sectors [33,34,35,36,37,38].

Fig. 15.4
A schematic diagram demonstrates the process of power metallurgy. The process includes mixing, sintering, and compaction.

Powder metallurgy

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15.4 Performance Analysis of Age Hardened Aluminium Alloy

The fatigue behaviour of 2618 aluminium alloy is notably influenced by its microstructure, which is distinguished by a dense population of Al9FeNi coarse particles located within the grains. In contrast to the Al2214 and Al7050 alloys, the microstructural composition of the 2618 alloy effectively mitigates the influence of surface roughness on its fatigue resistance properties. The observed phenomenon could potentially elucidate the significant impact of average shear stress on the endurance limit of materials subjected to cyclic loading [5]. The age hardening characteristics of components made of 2219 aluminium alloy are subject to variation depending on the specific spinning reductions employed. The observed variations are ascribed to alterations in the microstructural characteristics, such as the density of dislocations and the state of precipitation. The utilisation of DSC, TEM, and EBSD methodologies was implemented to examine the microstructures and comprehend the underlying mechanisms that account for the diverse age hardening behaviours. The aforementioned techniques facilitated an understanding of the thermal phenomena, enabled the examination of microstructural characteristics at the atomic level, and yielded crystallographic data [39]. This investigation analysed the impact of cold deformation and ageing on the mechanical characteristics of AA2024 aluminium alloy. The study revealed that a precise degree of deformation, coupled with a subsequent ageing process, resulted in a notable enhancement of the alloy's hardness and strength. Increased levels of deformation facilitate the attainment of optimal ageing outcomes at reduced temperatures and abbreviated timeframes. The decline in mechanical properties can be attributed to the enlarged precipitate size and prolonged recovery resulting from over-aging conditions. The results underscore the significance of meticulous regulation of the deformation and ageing procedures in order to augment the mechanical characteristics of aluminium alloy [11]. The creep performance of the 2A14 aluminium alloy is subject to the impact of temperature, whereby elevated temperatures result in heightened creep strain and steady-state creep rate. The alloy demonstrates the least primary creep stage and the minimum stress sensitivity at a temperature of 433 K. Accurate predictions of creep strain for the alloy under different temperature and stress conditions can be achieved by integrating the optimal ageing precipitation temperature into the creep model. The results underscore the significance of taking into account the impact of temperature and precipitation-induced ageing when comprehending and forecasting the creep performance of 2A14 aluminium alloy [40]. The investigation of the ageing phenomenon of alloy 2618A was conducted via isothermal ageing at a temperature of 190 °C, with and without the application of a creep load. The DFTEM analysis has indicated that the S-phase, which is accountable for the desired material properties, experiences coarsening while undergoing ageing. The impact of supplementary stress on the ageing process has been observed to exert a substantial effect on the coarsening of Al2CuMg precipitates, resulting in a marked increase in particle radii in stressed specimens. The significance of stress in lifetime prediction models for alloy 2618A is underscored by the accelerated degradation observed under external load. This research offers significant contributions to the understanding of the coarsening mechanism of the alloy, emphasising the necessity for additional exploration of the dislocation's function in this phenomenon [13]. The production of Al-Cu alloys was carried out through the utilisation of vertical continuous casting. The process parameters, specifically the temperature and time for solution treatment and artificial ageing, were modified accordingly. Through the implementation of a solution treatment, the residual phases of CuAl2 present in the alloy were effectively dissolved, leading to a notable enhancement in the mechanical properties. Elevating the solution treatment temperature to 525 °C resulted in augmented recrystallized grain sizes and an increased proportion of precipitation within the alloy, thereby amplifying its tensile strength and hardness. The present study emphasises the significance of meticulous regulation of solution treatment parameters for the purpose of enhancing the characteristics of Al-Cu alloy materials [41]. The precipitation behaviour and mechanical properties of AA2195, an alloy composed of aluminium and lithium, were examined via various heat treatments. The outcomes of the DSC analysis indicated that the precipitation patterns of the T0, T6, and T8 heat treatments were comparable. The alloy exhibited the existence of diverse precipitates at temperatures lower than 300 °C, with the T1 precipitates having an acicular shape and being the primary reinforcement phase. Consequently, the alloy's strength and toughness were enhanced. The results underscore the feasibility of employing distinct thermal processing techniques to augment the characteristics of Al-Li alloy AA2195 [42]. The process of stress-aging involves the decomposition of a supersaturated solid solution, resulting in the formation of finely distributed ageing precipitates within the grains. The morphology and dimensions of said precipitates exhibit variability contingent upon the initial stage stress-aging temperature. The interaction between dislocations and intermediate precipitates is affected by external stress. The microstructural formations that arise in the initial phase act as antecedents for the precipitation phases that occur in the subsequent stage. During the second stage of the stress-aging process, it has been observed that the ageing precipitates tend to experience more facile growth. The density of ageing precipitates exhibits an initial increase followed by a subsequent decrease at elevated first stage stress-aging temperatures, owing to fluctuations in nucleation mechanism and growth kinetics [6]. Through the manipulation of input conditions during the process of wear testing, the wear rate, specific wear rate, and coefficient of friction were successfully reduced. Elevating the weight percentage of the composite material resulted in enhanced wear resistance. The analysis of the abraded surfaces pre- and post-testing revealed significant findings such as the displacement of particles, the existence of reinforcements, minor fissures, and porosity. The aforementioned discoveries provide significant perspectives on the wear characteristics and efficacy of the composites in diverse circumstances, facilitating the advancement of sturdier and more effective materials for a range of uses [23]. The stress drops are notably elevated upon ageing the alloys to their corresponding peak strength, namely T6 for AA 6061 and T8 for AA 2195, in contrast to the solutionized and overaged state. The determination of the activation volume often involves utilising a logarithmic correlation between stress drop and time [8]. The implementation of deformation ageing treatment (DAT) on alloy 2618 leads to the formation of a more refined S’ precipitate phase and the elimination of dislocation-precipitate tangles, as compared to the T6 state. The process of Differential Ageing Treatment (DAT) has been observed to result in a reduction in the width of the Precipitate Free Zone (PFZ) within the alloy. Furthermore, even after extended exposure to a temperature of 200 °C, the S’ phase exhibits a finer morphology compared to that observed in the T6 state. The state of DAT demonstrates elevated strength and hardness at both ambient and elevated temperatures, owing to the reduced size and spacing of precipitates, as well as the diminished width of the precipitate-free zone [43]. The Al7075/B4C metal matrix composites (MMCs) in both as-cast and age-hardened states display a uniform microstructure, albeit the age-hardened specimens exhibit reduced porosity. The wear resistance of Al7075/B4C metal matrix composites (MMCs) is subject to the influence of various factors, including age hardening, sliding velocity, applied load, weight percentage of reinforcement, and microstructural characteristics. The composites that have undergone age-hardening exhibit greater hardness and wear-resistance in comparison to the as-cast Metal Matrix Composites (MMCs), thereby indicating the favourable influence of age-hardening on the wear performance [44]. The impact of silicon (Si) on the ageing hardening behaviours of Al-Sc and Al-Sc-Zr alloys has been examined in this study. The present study investigated the content range of Si, spanning from 0.05 wt% to 0.30 wt%, through the utilisation of microhardness measurements and transmission electron microscopy (TEM). The findings of the study indicate that the most favourable outcomes were observed at an optimal silicon concentration of approximately 0.15%. Comprehending the impact of silicon content on the process of ageing can facilitate the advancement of alloys that possess customised mechanical properties for applications [45]. In order to ascertain the optimal combination of alloying elements for the purpose of age hardening, such as Iron and Nickel, a methodical approach to experimentation is imperative. The ideal configuration for age hardening is contingent upon the targeted temperature range. By means of methodical experimentation and analysis of diverse alloying elements and their amalgamations, it is feasible to ascertain the most advantageous composition for attaining intended age hardening characteristics at temperatures. The present study has the potential to make a significant contribution towards the advancement of alloys possessing customised mechanical characteristics for applications, through the judicious selection of alloying constituents and their respective ratios [46]. The sensitivity of an alloy to quench rate is influenced by the Zn/Mg ratio. An alloy exhibiting a high Zn/Mg ratio displays greater sensitivity to quench rate in comparison to an alloy with a low Zn/Mg ratio. The present study reveals that the high Zn/Mg ratio alloy exhibits a substantially elevated critical cooling rate of approximately 100 K/s, in contrast to the low Zn/Mg ratio alloy which displays a critical cooling rate of approximately 1 K/s, indicating the absence of precipitation during cooling. The observation suggests that the high Zn/Mg ratio alloy is more susceptible to precipitation suppression through rapid cooling compared to the low Zn/Mg ratio alloy. The results indicate that the regulation of the cooling rate plays a pivotal role in enhancing the precipitation kinetics in alloys featuring varying Zn/Mg ratios [47]. The present study aimed to investigate the precipitation hardening behaviour of the Al–Zn–Mg–Cu (7A85) alloy through artificial ageing at a temperature of 434 K.. The primary reason for precipitation hardening in the 7A85 alloy has been attributed to the identification of intermediate precipitates situated between the GP-II (Guinier Preston) and η′ (intermediate structures) phases. The results offer valuable perspectives on the underlying mechanisms that contribute to the enhancement of the 7A85 alloy's strength during the process of ageing [48]. An inquiry was conducted to explore the enhancement of fatigue strength in the 7075-aluminium alloy. Various ageing conditions were investigated, encompassing assessments of hardness, tensile strength, fatigue strength, and microstructural analysis. The findings indicated that the spacing of precipitation exhibited a decreasing trend followed by an increasing trend as the ageing time progressed. This phenomenon exhibited a correlation with alterations in both hardness and tensile strength, whereby an initial increase was followed by a subsequent decrease. The specimen that underwent an ageing process of 48 h at a temperature of 120 °C exhibited the maximum fatigue strength of 165 MPa [49].

15.5 Challenges in Age Hardening of Aluminium Alloys

It is absolutely necessary to select the appropriate alloy composition in order to achieve good age hardening. This procedure does not work well with all aluminium alloys, and the ageing of some alloys may not result in a considerable increase in the material's strength. As a result, it is essential to take the composition of the alloy into serious account. It is essential to have a solid understanding of the precipitation kinetics of the alloy. Ageing is a time-dependent process that involves the development and expansion of fine precipitates. Failure to create the appropriate strengthening precipitates or their growth to excessive sizes due to improper ageing conditions can lead to weakened materials. Optimal strength and ductility can only be achieved by careful manipulation of the ageing process. Over aging is a phenomenon that arises from subjecting an alloy to prolonged ageing or elevated temperatures beyond the necessary range. This leads to the enlargement of precipitates and a consequent reduction in the alloy's strength. On the other side, under aging occurs when neither enough time nor heat is applied during the ageing process, resulting in incomplete precipitation and diminished strength. Solution heat treatment is commonly used to remove any existing precipitates from aluminium alloys before they are aged. In order for the material to age properly, it must first be transformed into a uniform, single-phase solid solution. When solute atoms are not uniformly distributed during solution heat treatment, the subsequent precipitation hardening process might be negatively impacted. Dimensional alterations and distortion are possible results of thermal expansion and contraction of the material during the age hardening process. When working with huge or intricate structures it can be especially tricky to keep their dimensions consistent. During the heat treatment process, temperature variances, duration variations, and quenching rate differences can all have a significant impact on the age hardening effect. Even little adjustments to these factors can have a considerable influence on the microstructure and mechanical qualities of the final product. There are a variety of issues specific to age hardening that are presented by various aluminium alloys. For instance, the ageing temperature range for certain alloys may be rather limited, which highlights the need of maintaining exact control over the process. For others, achieving the appropriate characteristics may necessitate the use of specialised quenching procedures or ageing processes that include many steps. It is vital to have a comprehensive understanding of the behaviour of the particular alloy in addition to meticulous process optimisation in order to be successful in overcoming these problems. Precise control of temperature, time, and cooling rates is required. In addition, characterization strategies and modelling strategies can be of assistance in forecasting and optimising the age-hardening process for aluminium alloys.

15.6 Applications of Age Hardening of Aluminium Alloys

The utilisation of age hardenable aluminium alloys is prevalent in the aerospace industry owing to their amalgamation of superior strength, low mass, and commendable resistance to corrosion. The application of these alloys is observed in the construction of aircraft structures, including wings, fuselage components, and landing gear, where the crucial factors of strength and weight reduction are considered. Aluminium alloys that are capable of age hardening have been identified as having significant utility in the automotive sector, specifically in the areas of weight reduction and enhancement of fuel economy. The alloys are utilised in various vehicular applications such as engine components, suspension systems, and body panels, thereby imparting enhanced mechanical robustness while concurrently mitigating the overall mass of the automobile. Aluminium alloys possessing the ability to undergo age hardening are employed in marine contexts owing to their corrosion-resistant properties and high ratio of strength to weight. They are utilised in the construction of boat hulls, superstructures, and offshore structures [50,51,52,53,54]. The process of age hardening can be employed in the context of aluminium alloys utilised in the field of structural engineering with the aim of augmenting their ability to bear loads and bolstering their overall strength. The application of these alloys is observed in the fabrication of bridges, edifices, and various other infrastructure undertakings. Aluminium alloys possessing age hardening characteristics are employed in the domain of electrical transmission lines and conductors. Due to their exceptional strength and conductivity, they are well-suited for utilisation in scenarios where there is a need for efficient power transmission while maintaining a lightweight structure. The process of age hardening aluminium alloys holds significance in defence-related contexts. The alloys find their application in the production of defence-related structures such as armoured vehicles and military aircraft, where their ability to offer a blend of robustness, reduced weight, and immunity to corrosion is of utmost importance [55, 56]. In the end, the authors have a considerable experience in the field of polymer and its based composites [57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77].

15.7 Conclusions

The effective performance of age-hardened aluminium alloys and composites has been revealed, demonstrating their capacity to offer efficacious reinforcement solutions for diverse engineering predicaments. The materials have exhibited significant enhancements in mechanical characteristics, including strength, hardness, and wear resistance, as a result of undergoing age hardening. The research has underscored the adaptability of age-hardened aluminium alloys and composites, enabling their utilisation in a diverse array of applications spanning various sectors, including aerospace, automotive, construction, and manufacturing. Due to their heightened strength and performance, they are well-suited for use in crucial components and structures that necessitate exceptional mechanical properties. In addition, age-hardened aluminium alloys and composites possess favourable attributes such as reduced weight, resistance to corrosion, and superior machinability, thereby augmenting their potential utility and merits. It is imperative to acknowledge that the efficacious execution of age-hardened materials necessitates meticulous contemplation of factors such as alloy composition, heat treatment parameters, and processing techniques. The appropriate selection and management of these variables are of utmost importance in attaining the intended mechanical characteristics and maintaining uniformity in quality. With the on-going progress in material science, it is anticipated that additional research and development endeavours will result in more inventive solutions that employ age-hardened aluminium alloys and composites. The investigation of innovative alloy compositions, the refinement of heat treatment procedures, and the incorporation of innovative manufacturing methods are expected to facilitate the discovery of fresh opportunities for enhancing strengthening solutions in the times ahead. In general, the efficacy of age-hardened aluminium alloys and composites as viable strengthening remedies for engineering predicaments is incontrovertible. Through the utilisation of their exceptional mechanical characteristics and comprehensive comprehension of their processing, engineers and researchers have the ability to further advance the limits of performance, thereby facilitating the creation of products that are more secure, efficient, and technologically sophisticated.