A Critical Review of Diffusion—Thermomechanical and Composite Reinforcement Approaches for Surface Hardening of Aluminum Alloys and Matrix Composites

Abstract

Aluminum alloys require improved surface performance to satisfy the demands of today’s aerospace, automotive, marine, and structural applications. This paper compares three key surface hardening methods: diffusion-assisted microalloying, thermomechanical deformation-based treatments, and composite/hybrid reinforcing procedures. Diffusion-assisted Zn/Mg enrichment allows for localized precipitation hardening but is limited by the native Al₂O₃ barrier, slow solute mobility, alloy-dependent solubility, and shallow hardened depths. In contrast, thermomechanical techniques such as shot peening, surface mechanical attrition treatment (SMAT), and laser shock peening produce ultrafine/nanocrystalline layers, high dislocation densities, and deep compressive residual stresses, allowing for predictable increases in hardness, fatigue resistance, and corrosion performance. Composite and hybrid reinforcement systems, such as SiC, B₄C, graphene, and graphite-based aluminum matrix composites (AMCs), use load transfer, Orowan looping, interfacial strengthening, and solid lubrication effects to enhance wear resistance and through-thickness strengthening. Comparative evaluations show that, while diffusion-assisted procedures are still labor-intensive and solute-sensitive, thermomechanical treatments are more industrially established and scalable. Composite and hybrid systems provide the best tribological and load-bearing performance but necessitate more sophisticated processing approaches. Recent corrosion studies show that interfacial chemistry, precipitate distribution, and galvanic coupling all have a significant impact on pitting and stress corrosion cracking (SCC). These findings highlight the importance of treating corrosion as a fundamental design variable in all surface hardening techniques. This work uses unified tables and drawings to provide a thorough examination of strengthening mechanisms, corrosion and fatigue behavior, hardening depth, alloy suitability, and industrial feasibility. Future research focuses on overcoming diffusion barriers, establishing next-generation gradient topologies and hybrid processing approaches, improving strength ductility corrosion trade-offs, and utilizing machine-learning-guided alloy design. This research presents the first comprehensive framework for selecting multifunctional aluminum surfaces in demanding aerospace, automotive, and marine applications by seeing composite reinforcements as supplements rather than strict alternatives to diffusion-assisted and thermomechanical approaches.

1. Introduction

Aluminum alloys and aluminum matrix composites (AMCs) find extensive applications in the aerospace, automotive, marine, and defense sectors, where there is a need for lightweight construction combined with high wear resistance, fatigue durability, and corrosion stability. Their advantageous strength-to-weight ratio, ease of fabrication, thermal conductivity, and inherent corrosion resistance render them suitable for various components, including fuselage skins, airframe linkages, suspension arms, heat exchangers, and marine structures [1,2,3]. However, the inherently low surface hardness of aluminum results in poor tribological performance under high contact or sliding conditions, which constrains its use in applications requiring superior surface integrity. This limitation has spurred significant research into surface hardening strategies aimed at enhancing near-surface properties without compromising the ductile core that is essential for load-bearing performance. Recent reviews highlight the increasing adoption of aluminum matrix composites in automotive and aerospace components due to their improved stiffness-to weight-ratio and enhanced tribological performance compared to monolithic alloys [4,5,6,7,8].

Three major surface hardening pathways have emerged. (i) Diffusion-assisted and thermomechanical approaches rely on controlled heat treatment, microalloying, surface activation, and precipitation engineering to develop hardened gradients. Representative techniques include Zn/Mg diffusion, conventional T6 and multi-step aging treatments, retrogression–re-aging (RRA), laser and shot peening-assisted precipitation, and severe surface plastic deformation (SSPD). These processes operate through established metallurgical mechanisms—precipitation strengthening, dislocation multiplication, grain refinement, and solid solution strengthening—and, when optimized, can significantly improve hardness, wear behavior, and corrosion performance [9,10,11,12,13,14,15,16]. Diffusion-based routes are largely ineffective for commercial purity alloys (e.g., AA1100) due to minimal solute solubility; consequently, thermomechanical and composite approaches remain the primary options for these grades [13,14,17,18,19,20,21,22,23].

(ii) Composite and hybrid reinforcement strategies enhance both surface and bulk properties by incorporating ceramic (SiC, B₄C, Al₂O₃, TiC), carbonaceous (graphite, graphene), or bio-derived reinforcement particles within the aluminum matrix. Systems such as Al/SiC/Gr, Al/B₄C/h BN, and multi-component nanocomposites demonstrate that synergistic reinforcement effects can increase hardness, improve load transfer efficiency, reduce wear rate, and modify corrosion pathways relative to monolithic alloys [24,25,26,27]. Recent studies highlight the sensitivity of composite performance to reinforcement chemistry, morphology, and interface quality; subtle differences in secondary phases (e.g., TiO₂ vs. Al₂O₃) can shift the balance between hardness and corrosion resistance [26,27,28]. Similar observations in Al–Si alloy matrix composites show that reinforcement mineralogy and particle morphology exert strong influence on hardness, wear rate, and frictional response during sliding contact [7,8,24,25,26,27].

While diffusion-controlled precipitation and composite reinforcement are typically examined separately, they reveal microstructural synergies that yield similar gradient effects—specifically, the development of hard, wear-resistant surfaces paired with ductile cores. These attributes are increasingly desirable in advanced aluminum systems.

However, existing reviews fail to provide a thorough comparison of diffusion-assisted, thermomechanical, and composite reinforcement strategies, often concentrating on individual techniques instead. Key questions regarding industrial feasibility, scalability, cost, hardening depth, corrosion performance, and the applicability to various precipitation hardenable alloys remain inadequately addressed. Additionally, discussions surrounding diffusion-based treatments frequently neglect practical limitations, such as the presence of the native Al₂O₃ barrier, solubility constraints, and the limited responsiveness of alloys like AA1100 due to their low Zn/Mg solubility.

For instance, studies on laser or shot peening-based surface treatments examine deformation and residual stress mechanisms without benchmarking against diffusion-based or composite alternatives [10,11,12]. Similarly, surveys of aluminum matrix composites concentrate on reinforcement characteristics and tribological behavior with limited comparison to thermomechanical treatments [5,6,7,8]. Unlike these focused studies, the present work provides the first integrated comparison of all three pathways within a single evaluation framework, incorporating hardening depth, alloy-specific applicability, and cost–feasibility trade-offs.

To address these gaps, this review provides a comprehensive, mechanistic, and application-oriented evaluation of three major surface hardening pathways: (i) diffusion-assisted microalloying, (ii) thermomechanical surface severe plastic deformation (SSPD), and (iii) composite or hybrid reinforcement strategies. While diffusion-assisted and thermomechanical treatments are both discussed in Section 2.1 due to their shared emphasis on gradient microstructure development, they represent distinct process families with fundamentally different depth scales, industrial maturity, and alloy applicability. Composite reinforcement (Section 2.2) constitutes a third, bulk-level approach that differs qualitatively from surface-only treatments. This unified framework enables direct comparison of mechanisms, property trade-offs, and industrial feasibility across all major hardening routes for aluminum systems.

The specific objectives are to perform the following:

• Compare diffusion-assisted, thermomechanical, and composite-based treatments in terms of mechanisms, processability, and performance outcomes.
• Synthesize advantages, limitations, and trade-offs among methods with respect to hardness–ductility balance, corrosion resistance, hardening depth, and cost–feasibility relationships.
• Present comparative evaluation tables, strengthening mechanism maps, and method selection flowcharts for precipitation hardenable alloy families (2xxx, 6xxx, 7xxx) and commercially pure grades.
• Highlight emerging directions, including hybrid diffusion–composite approaches, rare earth microalloying, gradient microstructures, and sustainability-driven processing strategies.

This comparative framework is designed for material engineers and process designers selecting surface hardening strategies for lightweight structural components in aerospace, automotive, and marine applications. The evaluation tables and method selection flowcharts (Section 2.3) provide immediate, actionable guidance for matching treatment routes to specific alloy families and performance requirements (wear resistance, fatigue life, corrosion durability). Building on these motivations, Section 2.1 critically evaluates diffusion-assisted and thermomechanical treatments, Section 2.2 examines composite reinforcement strategies, and Section 2.3 consolidates comparative analyses through property-based tables and selection flowcharts. Future hybrid approaches are discussed in Section 3, while Section 4 provides design guidelines.

Unlike previous reviews that treat diffusion-assisted aging, thermomechanical treatments, or aluminum matrix composites in isolation [10,11,12,21,22,23,29], this work synthesizes all major surface hardening routes through integrated mechanistic analysis, industrial feasibility assessment, cost–performance relationships, and alloy-specific applicability across both high solute (2xxx, 6xxx, 7xxx) and commercial purity grades. The review also introduces consolidated comparison frameworks—including comparative evaluation tables, strengthening mechanism maps, and method selection flowcharts—currently absent from the literature. These frameworks highlight where each approach excels, where it fails, and how hybrid or complementary treatment routes can be strategically deployed in modern aluminum engineering.

Scope and Methodology

This review adopts a structured, mechanism-centered framework to evaluate the major surface hardening pathways for aluminum alloys, with emphasis on diffusion-assisted microalloying, thermomechanical processes, and composite or hybrid reinforcement strategies. The methodology was formulated to ensure comprehensive coverage, clarity, and strong alignment with industrial and scientific relevance.

Literature Search Strategy:

A systematic literature survey was conducted using Scopus, Web of Science, ScienceDirect, and MDPI databases. Key search terms included the following: Aluminum alloy surface hardening, diffusion-assisted microalloying, precipitation strengthening, thermomechanical treatments, shot peening, laser shock peening, surface severe plastic deformation, Aluminum matrix composites, hybrid composites, and tribological behavior. Additional publications were identified through backward and forward citation tracking of influential works on precipitation mechanisms, surface mechanical treatments, composite reinforcement, and broader surveys on aluminum alloy behavior in structural and welded applications [30].

Inclusion Criteria:

Studies were included if they

• Reported experimental, numerical, or mechanistic findings on surface hardening of aluminum alloys or AMCs;
• Provided quantifiable performance metrics such as hardness, wear rate, fatigue life, or corrosion/SCC/IGC behavior;
• Offered mechanistic insights on diffusion kinetics, precipitation pathways, grain refinement, dislocation behavior, or reinforcement–matrix interaction;
• Addressed processing routes of practical interest, including aging treatments, peening, laser processing, stir casting, powder metallurgy, ultrasonic-assisted casting, surface alloying, or hybrid methods.

Exclusion Criteria:

Studies were excluded when they

• Focused exclusively on non-aluminum systems without transferable mechanisms;
• Presented only qualitative outcomes without measurable data;
• Described highly specialized laboratory techniques lacking foreseeable industrial applicability;
• Reproduced existing results without offering new mechanistic understanding.

Evaluation Framework:

All selected studies were assessed using a consistent set of criteria:

• Strengthening mechanisms: diffusion-induced precipitation, Orowan looping, load transfer, thermal mismatch strengthening, grain refinement, and interface-mediated effects;
• Hardening depth and gradient development: distinguishing diffusion layers (10–100 µm), thermomechanical deformation layers (200–1000 + µm), and through-thickness reinforcement in AMCs;
• Mechanical and tribological performance: hardness enhancement, ductility retention, wear behavior, frictional response, fatigue and vibration performance;
• Corrosion and SCC behavior: influence of precipitate distribution, solute uniformity, grain structure, and residual stresses on pitting, IGC, and SCC susceptibility;
• Processability and industrial feasibility: cost, energy requirements, reinforcement distribution, interfacial quality, scalability, and compatibility with industrial workflows.

Rather than listing processes, this review provides a critical synthesis of mechanisms, property trade-offs, and industrial feasibility, highlighting where each route fails as well as where it excels.

Scope of the Review:

Using this evaluation framework, the review integrates mechanistic, performance-based, and feasibility-oriented insights to develop a unified comparison of diffusion-assisted microalloying, thermomechanical surface treatments (including SSPD and peening-assisted aging), and composite or hybrid reinforcement architectures. Specific emphasis is placed on challenges highlighted in recent studies—limited Zn/Mg diffusivity, the persistent Al₂O₃ barrier, alloy family specific responses, hardening depth limitations, long-term corrosion durability, and the cost–feasibility relationship of different processing routes. This structured approach ensures that the comparative analysis presented in Section 2 is rigorous, coherent, and relevant to both research and industrial applications. Recent AI-driven alloy design frameworks similarly emphasize the need for integrated, data-rich evaluation strategies to address such multi-factor interactions in aluminum alloys, reinforcing the importance of unified comparison methodologies adopted in this review.

2. Literature Review

A wide range of surface hardening techniques have been developed for aluminum alloys and aluminum matrix composites, each exploiting different microstructural mechanisms and depth responses. These approaches can be broadly grouped into three categories:

(i) Diffusion-assisted microalloying;

(ii) Thermomechanical Surface Severe Plastic Deformation (SSPD);

(iii) Composite or hybrid reinforcement strategies.

A schematic overview of these pathways and their selection logic is presented in Figure 1. Each pathway interacts with precipitation behavior, dislocation activity, grain refinement, solute mobility, and reinforcement–matrix interfaces in distinct ways, influencing hardness, wear resistance, fatigue life, and corrosion performance.

The following subsections critically evaluate these methods, beginning with diffusion-assisted and thermomechanical treatments (Section 2.1), followed by composite-based approaches (Section 2.2), and concluding with a comparative assessment (Section 2.3).

2.1. Diffusion-Assisted and Thermomechanical Surface Treatments

Surface hardening in aluminum alloys is governed by precipitation kinetics, dislocation activity, grain refinement, solute mobility, and combined thermomechanical effects. Over the years, multiple strategies have been developed to enhance hardness, wear resistance, corrosion behavior, and fatigue performance. These approaches can be broadly grouped into diffusion-assisted microalloying, thermomechanical surface severe plastic deformation, laser-based surface modification, and hybrid chemical–mechanical routes. Each pathway is discussed with emphasis on governing mechanisms, achievable improvements, and alloy-dependent limitations.

2.1.1. Precipitation and Aging Mechanisms Relevant to Surface Hardening

Precipitation hardenable aluminum alloys (2xxx, 6xxx, 7xxx) strengthen through the formation of Guinier Preston (GP) zones, metastable intermediates, and equilibrium phases [9,10,11,31,32,33]. Conventional T6 and multi-step aging promote fine precipitate dispersion that impedes dislocation motion; however, prolonged exposure to elevated temperatures accelerates precipitate coarsening, reducing hardness, fatigue strength, and corrosion resistance.

Advanced aging routes such as retrogression and re-aging (RRA), interrupted aging, and thermomechanical-assisted aging help counteract coarsening. RRA dissolves coarse grain boundary precipitates during retrogression and re-precipitates refined phases during re-aging, simultaneously improving strength and stress corrosion cracking (SCC) resistance [13,14,15,16,23,34]. These routes can produce desirable “hard skin/ductile core” gradients that enhance resistance to bending, fretting, and cyclic fatigue loading. Cu segregation and Q phase formation at grain boundaries in Al Mg Si (Cu) alloys can significantly change IGC and SCC susceptibility, as highlighted in recent reviews [35,36]. However, precipitation alone cannot generate sufficiently deep hardened layers, making thermomechanical routes the primary industrial benchmark.

2.1.2. Thermomechanical Treatments: Shot Peening, LSP, and SMAT

Thermomechanical surface severe plastic deformation (SSPD) methods—including shot peening, laser shock peening (LSP), and surface mechanical attrition treatment (SMAT)—are among the most industrially mature surface hardening techniques for aluminum alloys. Shot peening generates high dislocation densities and compressive residual stresses extending 200–600 µm, delaying crack initiation and improving fatigue life [12,15]. SMAT produces ultrafine/nanocrystalline surface layers with high defect density, accelerating precipitation during subsequent aging and increasing hardness [9,12,34]. LSP introduces deep (>1 mm) compressive stress fields with minimal surface roughening, while simultaneously promoting metastable precipitate formation in precipitation hardenable alloys [10,11,12,37]. Recent investigations on marine-grade alloys such as AA6061 T6 and AA6082 T6 demonstrate that optimized LSP parameters can induce hardened layers extending up to 1.5 mm, increasing surface hardness by over 40% and extending fatigue life by a factor of two [9,16,33,37]. These methods consistently improve hardness, wear resistance, fatigue performance, and corrosion behavior, offering scalability superior to diffusion-based treatments.

2.1.3. Laser-Assisted Surface Modification and Accelerated Precipitation/Diffusion

Laser alloying, laser melting, and laser texturing modify surface chemistry, refine grains, and enhance local diffusion by rapid heating and cooling [10,11,13]. Laser-induced textures also improve tribological behavior and wettability [11]. Combined laser + thermomechanical routes (e.g., LSP + aging) further intensify solute–dislocation interactions and microstructural refinement. This synergy enables deeper hardening and improves thermal stability [9,12,37]. The next subsection examines diffusion-assisted Zn/Mg microalloying, an academically important but industrially limited approach.

2.1.4. Diffusion-Assisted Zn/Mg Microalloying: Potential and Limitations

Diffusion-assisted microalloying aims to enrich the surface with Zn/Mg to stimulate precipitation during aging. However, several fundamental limitations restrict applicability:

• Al₂O₃ barrier: Native oxide severely restricts diffusion unless aggressively removed [20,21,23,34].
• Meaningful hardening is achievable mainly in high Zn/Mg alloys (7xxx), and Zn-modified Al–Mg sheet systems also demonstrate strong work hardening and ductility improvements due to enhanced solute–dislocation interactions [34].
• Unsuitability for AA1100: Low solute affinity and thermal sensitivity promote cracking and hinder diffusion [20,21,22,23].
• Shallow depth: Diffusion layers rarely exceed 50–100 µm, much lower than thermomechanical or composite routes [21,22,23].
• Poor industrial feasibility: Slow kinetics, high temperature demands, and non-uniform gradients limit use [23,33,34].

Although localized hardness improvements are reported, diffusion-assisted treatments remain laboratory-scale and supplementary rather than primary surface hardening routes. Given these different microstructural effects, corrosion behavior requires separate evaluation. Even in non-precipitation hardening systems such as Al–Si–Cu alloys, Zn additions have been shown to enhance strength and reduce wear rate, demonstrating the broader role of solute-assisted strengthening across aluminum families [38].

2.1.5. Corrosion, SCC, and IGC Behavior Under Thermomechanical and Diffusion Treatments

Corrosion behavior of aluminum alloys is influenced by grain size, precipitate distribution, solute uniformity, and residual stresses [31]. Investigations on Zn-modified Al-Mg systems show that Zn additions significantly alter grain boundary precipitation and intergranular corrosion susceptibility [39,40] thereby linking microstructural evolution directly to corrosion response.

Broad SCC assessments across aluminum alloy families show that the combined influence of anodic dissolution, hydrogen-induced cracking, grain boundary microchemistry, and PFZ evolution governs susceptibility, underscoring the need to balance microstructural refinement with corrosion resistance during thermomechanical and diffusion-based treatments. Environmentally assisted cracking in Al Mg Si (Cu) alloys is strongly governed by grain boundary segregation, precipitate chemistry, and hydrogen–microstructure interactions, which collectively determine SCC and corrosion fatigue susceptibility. Similar effects of Zn additions on grain boundary precipitation and intergranular corrosion have also been reported in Mg rich 5xxx alloys, especially Al 5083, where Zn modifies boundary chemistry and influences corrosion susceptibility [41]. Similar effects of artificial aging on corrosion pathways have also been documented in Al Cu Mg systems such as AA2024 T3, where aging schedule strongly alters intergranular attack morphology and corrosion kinetics [42].

Near-atomic-scale investigations of 7xxx alloys have shown that hydrogen segregation to dislocations and grain boundaries, along with dissolution of strengthening η phase precipitates ahead of advancing cracks, significantly accelerates SCC through combined HELP and HEDE mechanisms. RRA-treated 7xxx alloys demonstrate enhanced SCC resistance due to controlled dissolution and re-precipitation of grain boundary phases [34,43,44,45,46].

Controlled multistage aging treatments in low Cu, Al, Zn, Mg, and Cu alloys have been shown to widen PFZs, increase grain boundary precipitate spacing, and enrich Cu within GBPs—microstructural adjustments that collectively reduce SCC susceptibility while maintaining mechanical performance. Diffusion-assisted treatments, in contrast, often create unstable galvanic gradients when solute distribution is non-uniform, leading to variable corrosion performance [22,23,47,48].

As summarized in

Table 1. Consolidated comparison of diffusion-assisted and thermomechanical pathways.

Parameter	Diffusion-Assisted	Thermomechanical (SSPD + Aging)	Laser-Assisted	Hybrid (SSPD + Diffusion/Aging)
Hardness increase	Moderate	High (20–200%)	Moderate–High	High
Hardening depth	10–100 µm	200–1000 + µm	100–500 µm	200–600 µm
Ductility retention	Good	Good–Excellent	Good	Good–Excellent
Corrosion behavior	Variable; galvanic gradients possible	Predictably improved	Improved–Neutral	Improved
Fatigue performance	Low–Moderate	Strong	Strong	Strong
Wear resistance	Slight improvement	Improved	Improved	Strong
Industrial feasibility	Low	High (mature)	Medium–High	Medium
Process complexity	High	Low–Medium	Medium–High	Medium
Cost	High	Low	Medium–High	Medium
Alloy compatibility	Requires high Zn/Mg (7xxx)	2xxx, 6xxx, 7xxx	Broad	2xxx, 6xxx, 7xxx

, thermomechanical routes offer superior depth and industrial feasibility, whereas diffusion-assisted methods remain limited by shallow penetration and alloy dependence.

2.1.6. Hybrid Thermomechanical–Diffusion Routes and ML-Based Optimization

Hybrid routes integrate SSPD, LSP, or SMAT with controlled aging or limited diffusion to enhance solute–dislocation interactions and precipitation kinetics [22,23,34]. These systems achieve deeper hardened regions, improved thermal stability, and stronger crack-resistant interfaces. Comprehensive evaluations of abrasive and sliding wear mechanisms in aluminum matrix composites further highlight how reinforcement type, size, and interfacial bonding govern hardness and wear resistance, complementing the mechanistic trends observed in hybrid and particle-reinforced AMC systems [49].

Emerging machine learning models further accelerate optimization of aging schedules, diffusion temperatures, and reinforcement configurations, enabling multi objective balancing of hardness, wear resistance, corrosion behavior, and manufacturability [49,50,51,52,53]. Broader AI-driven alloy design frameworks similarly emphasize the value of data-rich, multi-parameter optimization strategies for aluminum alloys, while active learning approaches have shown additional potential for guiding sustainable composition pathways. Recent ML-assisted corrosion prediction studies in high-strength Al systems also support the integration of corrosion performance metrics into optimization pipelines and large-scale statistical reviews highlight the emerging role of ML across alloy development workflows. Hybrid frameworks thus offer promising pathways where both surface integrity and bulk stability are required.

2.2. Composite and Hybrid Reinforcement Strategies

Composite and hybrid reinforcement techniques incorporate ceramic, carbonaceous, or multi-component particulates into aluminum matrices to enhance hardness, wear behavior, thermal stability, and load transfer efficiency [24,25,26,27,28,54]. These systems operate through mechanisms including Orowan looping, load transfer, interfacial strengthening, thermal mismatch dislocation generation, and solid lubrication, which also underpin the abrasive and sliding wear responses of aluminum matrix composites as highlighted in recent tribological reviews [49].

2.2.1. Reinforcement Types and Strengthening Mechanisms

Metal–matrix composites (MMCs) commonly employ ceramic reinforcements such as SiC, B₄C, Al₂O₃, TiC, TiB₂, and WC, or carbonaceous reinforcements such as graphite, graphene, and carbon nanotubes (CNTs) [24,25,26,27,28]. A detailed comparison of these reinforcement types and their dominant strengthening mechanisms is provided in

Table 2. Representative reinforcement materials for aluminum matrix composites.

Reinforcement	Key Properties	Dominant Strengthening Mechanisms	Advantages	Typical Applications
SiC	High hardness, stiffness	Load transfer, Orowan strengthening	Excellent wear resistance, thermal stability [24,25,26,27]	Pistons, brake rotors, aerospace components
Al2O3	High hardness, chemical stability	Grain refinement, dispersion strengthening	Improved wear + corrosion behavior [24,25,26,27,28]	Marine and structural components
B4C	Very hard, low density	Orowan strengthening, matrix constraint	High specific strength [39,40]	Armor, aerospace sliding surfaces
TiC	High hardness	Dispersion strengthening	Improved wear resistance [24,25,26,27]	High-temperature aerospace components
Graphite (Gr)	Low friction	Solid lubrication, crack deflection	Reduced wear, low friction [24,25]	Bearings, bushings, sliding components
Graphene/CNTs	Exceptional tensile strength, conductivity	Load transfer, interface strengthening	High stiffness, vibration damping [26,27]	Vibration resistant aerospace/automotive parts
Hybrid (Ceramic + Gr/GO)	Tunable hardness + lubricity	Combined Orowan + lubrication	Balanced stiffness–lubrication [24,25,26,27,28]	Multifunctional aerospace/automotive components

.

Their strengthening arises from several complementary mechanisms [24,25,26,27,28,49,51]:

• Load transfer: High modulus particles bear part of the applied stress.
• Orowan strengthening: Non-shearable particle pin dislocations, increasing flow stress.
• Thermal expansion mismatch strengthening: Differential contraction generates geometrically necessary dislocations.
• Interfacial strengthening: Strong particle–matrix bonding promotes crack deflection.
• Grain refinement: Particles act as heterogeneous nucleation sites, refining grains.
• Solid lubrication: Graphite, graphene, and CNTs reduce friction and improve wear behavior.

These mechanisms collectively improve hardness, wear resistance, and thermal stability, as illustrated in Figure 2.

2.2.2. Performance Characteristics and Reinforcement-Dependent Behavior

Ceramic reinforcements impart high hardness, stiffness, and thermal stability, offering superior wear performance under abrasive or sliding conditions [24,25,26,27]. Recent powder metallurgy-based evaluations further confirm that reinforcements such as SiC, B₄C, Al₂O₃, and MgO significantly enhance hardness and wear resistance in AMCs, with performance strongly governed by particle dispersion quality and matrix–reinforcement interfacial bonding. Carbonaceous reinforcements—graphite, graphene, CNTs—act as solid lubricants, reducing friction and improving tribological stability. Hybrid systems (e.g., SiC–Gr, SiC–GO, B₄C–Gr) combine stiffness with lubrication to achieve a balanced response including improved hardness, reduced friction, and stable wear characteristics [24,25,26,27,28].

These features make MMCs ideal for components such as pistons, brake rotors, aerospace brackets, and structural connectors that operate under dry sliding, vibrational, or high-temperature environments.

2.2.3. Comparative Role of Composite Approaches

Composite and hybrid systems differ fundamentally from diffusion and thermomechanical approaches in several aspects [24,25,26,27,28,49,51]:

• Depth of strengthening: MMCs provide through-thickness reinforcement.
• Wear resistance: Ceramic and carbonaceous reinforcements offer superior resistance compared to diffusion or SSPD [24,25,26,27,28,49,51].
• Thermal stability: High melting reinforcements stabilize the matrix under temperature fluctuation.
• Fatigue behavior: Stable interfaces and refined microstructures improve crack initiation resistance.
• Corrosion performance: Reinforcement-dependent; SiC/GO hybrids often outperform monolithic alloys.

These advantages justify a dedicated focus on MMC-based hardening routes. For components subjected to high-vibration loading, hybrid composites reinforced with damping phases (e.g., graphene or graphite) offer superior performance compared to monolithic surface-hardened alloys.

2.2.4. Hybrid Composite–Thermomechanical Approaches

Hybrid processing routes combine MMC reinforcements with thermomechanical techniques such as shot peening, SMAT, or LSP [22,24,25,26,27,28,34,55]. The synergy between particle induced and deformation-induced strengthening mechanisms results in the following:

• Increased dislocation density around reinforcements,
• Deeper compressive residual stress fields,
• Improved particle–matrix interfacial bonding,
• Enhanced precipitation response during aging.

Such hybrid systems have shown improved hardness, wear resistance, fatigue life, and thermal stability.

2.2.5. Alloy Applicability

Unlike diffusion-assisted treatments—which rely on solute chemistry and are only effective for high Zn/Mg alloys (e.g., 7xxx)—MMC and hybrid reinforcement strategies are applicable across all aluminum series (1xxx–7xxx) [24,25,26,27,28]. Recent systematic evaluations of stir cast Al7075-based composites demonstrate substantial improvements in hardness, wear resistance, and tribo corrosion stability through ceramic reinforcement additions, while powder metallurgy-based studies further confirm similar enhancements across a wide range of AMC systems. This makes them particularly useful for alloys like AA1100 that lack precipitation hardening capability.

2.2.6. Synergistic Hybrid Composite–Thermomechanical Approaches and Optimization

Hybrid MMC + thermomechanical routes provide multi-scale strengthening by combining load transfer, Orowan looping, interfacial strengthening, and deep compressive residual stresses [22,34,55]. Their combined effect leads to improved wear stability, thermal fatigue resistance, and overall structural integrity. Recent computational and machine-learning-based design approaches allow optimization of reinforcement fraction, particle size, aging schedules, and processing temperatures, enabling tailored performance for specific structural and tribological applications [50,51,52]. These hybrid systems are increasingly adopted in high-performance sectors. Representative applications of aluminum matrix composites in aerospace and automotive components are shown in Figure 3.

2.3. Comparative Evaluation of Surface Hardening Approaches

Surface hardening of aluminum alloys can be broadly achieved through diffusion-assisted methods, thermomechanical treatments, and composite or hybrid reinforcement [13,14,15,16,24,25,26,27]. Although each route enhances surface integrity through different microstructural pathways, their suitability varies depending on alloy system, target properties, component geometry, and service environment [9,10,11,12,24,25,26,27]. This section consolidates the distinct benefits and constraints of each approach, providing an integrated perspective on their relative performance and application relevance. Recent corrosion-focused evaluations of hybrid aluminum matrix composites indicate that interfacial chemistry, galvanic coupling, and inhibitor–matrix interactions strongly govern pitting and polarization behavior, highlighting the need to treat corrosion response as a decisive design variable when comparing composite-based hardening routes [5].

2.3.1. Mechanistic Comparison

Diffusion-assisted treatments rely on near surface solute enrichment and precipitation but are fundamentally constrained by the native Al₂O₃ barrier, slow diffusion kinetics, and solute-dependent hardening response. They produce localized strengthening within tens of micrometers and show variable corrosion behavior due to possible galvanic gradients [47,48,56,57].

Thermomechanical processes such as shot peening, SMAT, and LSP strengthen surfaces through severe plastic deformation, dislocation accumulation, grain refinement, and the introduction of deep compressive residual stresses. In precipitation hardenable alloys, these effects are further amplified by subsequent aging, enabling high hardness and superior fatigue resistance within hundreds of micrometers [9,10,11,22,31,34,55].

Composite and hybrid reinforcement routes differ fundamentally from both, as they generate strengthening throughout the entire matrix rather than at the surface alone [24,25,26,27,28]. Mechanisms such as load transfer, Orowan looping, interface strengthening, thermal expansion mismatch dislocation generation, and solid lubrication allow MMCs to achieve very high wear resistance, excellent thermal stability, and good fatigue behavior [24,25,26,27,28]. Hybrid systems combining SSPD or LSP with MMCs further deepen the strengthened region and stabilize reinforcement–matrix interfaces [22,24,25,26,27,28,34,55].

2.3.2. Property-Based Comparison

A consolidated comparison of the three strategies reveals the following trends:

Hardness and wear resistance: MMCs and hybrid reinforcements exhibit the highest improvements due to ceramic and carbonaceous load transfer mechanisms [24,25,26], with additional confirmation from hybrid composite studies [27,28]. Thermomechanical routes provide strong but surface-limited enhancements through deformation-induced grain refinement and precipitation-assisted strengthening [12,15]. Diffusion-assisted routes generally offer only moderate hardness improvements due to shallow solute penetration and limited precipitation efficiency [21,22,23]. While thermomechanical treatments are primarily recognized for fatigue enhancement, recent reviews confirm that laser shock peening also significantly reduces friction coefficient and wear rate due to severe plastic deformation and surface nanocrystallization, demonstrating its dual structural and tribological benefits [58].

Fatigue performance: Thermomechanical surface treatments exhibit the strongest fatigue improvements because of deep and stable compressive residual stresses [12,15,16]. Additional strengthening is achieved through accelerated precipitation and microstructural refinement during post-peening aging [34,59,60]. MMCs also show good fatigue resistance when reinforcement dispersion is uniform and interfaces remain stable [24,25,26,27,28].

Corrosion behavior: Thermomechanical treatments generally improve corrosion resistance through refined grains, homogenized precipitate structures, and reduction in anodic discontinuities [31,34,39], with further evidence from aging-optimized 2xxx/6xxx/7xxx systems [42,43,44,45] and aqueous/environmental durability studies [59,61,62,63,64]. Composite systems show reinforcement-dependent corrosion response, with SiC/GO and SiC–Gr hybrid composites often demonstrating superior pitting and polarization performance [24,25,26,27,28]. Diffusion-based treatments display the highest variability due to non-uniform solute gradients, oxide-barrier-limited penetration, and local galvanic effects [21,22,29], as also reported in recent corrosion and SCC assessments [47,48].

Depth of hardening:

• Diffusion: 10–100 µm [13,14,17,21];
• Thermomechanical: 200–1000+ µm [12,15,16,59,61];
• MMCs: full thickness (bulk response) [24,25,26,27,28].

Thermomechanical treatments typically achieve depths of 200–1000 + µm, whereas diffusion layers are restricted to <100 µm. A quantitative comparison of hardening depths across the three strategies is plotted in Figure 4.

• Alloy applicability:

• MMCs are universally applicable (1xxx–7xxx) [24,25,26,27,28];
• Thermomechanical treatments are suitable for 2xxx/6xxx/7xxx [9,10,11,15,16];
• While diffusion-assisted strategies require high Zn/Mg solubility (7xxx) [13,14,22,55].

Cost and feasibility:

• Thermomechanical processes offer the best balance between cost and industrial maturity [15,16,59,61].
• MMCs are more expensive due to reinforcement and processing requirements [24,25,26,27,28].
• Diffusion-based hardening is least feasible due to oxide barrier constraints and slow kinetics [13,14,17,18,19,21].

2.3.3. Selection Considerations for Practical Applications

To assist designers in selecting appropriate surface hardening strategies based on alloy family and functional requirements, a decision-making flowchart is provided in Figure 5. The choice among the three surface hardening routes depends on the dominant service demands:

• Wear-dominated environments (sliding, abrasive, dry contact): MMCs and hybrid reinforcement routes provide the best performance [24,25,26,27,28].
• High-cycle fatigue or vibrational loading: Thermomechanical processes such as shot peening and LSP are preferred [12,15,16,34,59,61].
• Low-solubility alloys (e.g., AA1100) requiring overall stiffness or thermal stability: MMCs or hybrid routes are ideal due to alloy-independent applicability [24,25,26,27,28].
• Localized strengthening with minimal dimensional change: Diffusion-assisted routes may be used, provided alloy chemistry allows effective solute uptake [13,14,22,23].

For components under vibration/fatigue-dominated loading: thermomechanical SSPD routes (shot peening, LSP, SMAT + aging) offer the best hardness–ductility balance due to deep compressive residual stresses and retention of a ductile core. Diffusion-only treatments lack sufficient depth, and AMCs can provide high stiffness but may reduce global ductility if reinforcement fraction is excessive.

Among all routes, the thermomechanical peening + aging is the most industrially mature and feasible, owing to existing deployment in aerospace/automotive industries, lower cost per component, and compatibility with standard heat treatment lines. Cost wise, the thermomechanical peening + aging is generally cheaper per unit surface area because equipment is already widespread in aerospace and automotive sectors, while AMCs require costly ceramic or carbonaceous reinforcements, powder metallurgy or advanced casting, and sometimes post processing, leading to overall higher initial cost despite superior lifetime performance. For 7xxx alloys: Thermomechanical (SP/LSP + aging) and limited diffusion-assisted Zn/Mg enrichment are both relevant, with the former being industrially preferred. For 2xxx and 6xxx: Thermomechanical + aging routes dominate, and diffusion-assisted enrichment is seldom practical. For 1xxx (e.g., AA1100): Diffusion-assisted precipitation is largely ineffective; AMCs or surface composites are required.

2.3.4. Summary of Comparative Insights

To quantitatively compare the industrial feasibility and performance metrics of these strategies, a detailed side-by-side assessment is presented in

Table 3. Comparative evaluation of diffusion-based, thermomechanical, and composite/hybrid surface hardening strategies.

Criteria	Diffusion-Assisted Zn/Mg Enrichment	Thermomechanical Routes (SP, LSP, SMAT, Aging)	Composite/Hybrid Reinforcements (AMCs)
Industrial Feasibility	Low; limited by oxide barrier, slow diffusion; only feasible for select 7xxx alloys	Very high; widely used in aerospace/automotive	Moderate; depends on fabrication route (PM, stir casting, ultrasonic processing)
Processing Cost	High (long time, high temperatures)	Low–moderate (SP cheapest; LSP moderate)	Moderate–high (reinforcement cost + processing)
Hardening Depth	10–80 µm (shallow)	200–1000+ µm (deep residual stresses)	Through thickness (bulk strengthening)
Hardness Improvement	Low–moderate	High (20–200% increase)	Very high (strong Orowan, load transfer contributions)
Fatigue Performance	Limited improvement	Excellent (deep compressive residual stresses)	Good–excellent; graphene and CNT improve damping
Wear Resistance	Slight improvement	High improvement	Very high (SiC, B4C, Gr provide strong wear resistance)
Corrosion Behavior	Variable; galvanic gradients may form	Often improved (refined grains, uniform precipitates)	Reinforcement-dependent; SiC–GO hybrids best
Ductility Retention	Good (surface gradient)	Good–excellent	Moderate (reduces with higher wt.% reinforcement)
Dimensional Change	Minimal	Minimal	Moderate (depends on composite processing)
Alloy Compatibility	Only for Zn/Mg rich alloys (7xxx)	2xxx, 6xxx, 7xxx	Works for all series (1xxx–7xxx)
Best Fit Applications	Localized surface chemical modification	Fatigue critical aerospace/automotive components	High wear, high load, structural components
Sustainability/Energy Demand	High energy requirement	Low energy (SP), moderate (LSP)	Moderate; depends on reinforcement fabrication

.

• The comparative analysis indicates the following:

• Diffusion-assisted treatments are limited by shallow penetration and alloy dependence [13,14,17,18,19,21,22,23].
• Thermomechanical treatments offer strong, deep, and reliable hardening with minimal variability and well-established industrial adoption [12,15,16,22,23,34,59,61].
• Composite and hybrid reinforcements provide the most comprehensive improvement—covering hardness, wear resistance, fatigue stability, and thermal performance—making them well-suited for demanding structural and tribological applications [22,23,24,25,26,27,28,34].

These insights provide the foundation for selecting or designing surface hardening strategies tailored to component requirements, alloy composition, and service conditions, and they motivate the development of multifunctional processing frameworks discussed in Section 3 [22,23,34,50,51,52].

To quantitatively compare the advantages, limitations, industrial feasibility, and performance metrics of the major hardening strategies, a comparative evaluation is presented in Table 3. This table provides a side-by-side assessment of diffusion-assisted microalloying, thermomechanical treatments, and composite/hybrid reinforcement routes across multiple practical criteria.

To assist researchers and designers in selecting appropriate surface hardening strategies for different aluminum alloy families, a method selection flowchart is presented in Figure 5. The diagram integrates alloy chemistry, desired functional properties, cost/feasibility considerations, and the comparative performance of diffusion, thermomechanical, and composite-based treatments.

Figure 5 shows the flowchart for the selection based on functional requirements such as wear resistance, fatigue strength, corrosion/SCC resistance, dimensional tolerance, and industrial feasibility. Composite/hybrid reinforcements are recommended for high-wear environments; thermomechanical routes (shot peening, laser shock peening, SMAT, and artificial aging) are optimal for fatigue critical applications; and diffusion-assisted Zn/Mg microalloying is restricted to select 7xxx alloys where minimal dimensional changes are required.

This comparative synthesis demonstrates that while diffusion-assisted microalloying remains scientifically relevant, its industrial practicality is limited. Thermomechanical routes offer the most established and scalable surface strengthening. Composite and hybrid reinforcement methods deliver the highest tribological and load-bearing performance, motivating increased interest in multifunctional and process-integrated hardening frameworks. These aspects are further explored in Section 3.

3. Future Work

Future research should integrate alloy design with advanced processing routes to address the specific limitations identified across diffusion-assisted, thermomechanical, and composite reinforcement strategies. The gradient design approach demonstrated in Al–Mn alloys, where concurrent microstructural and compositional gradients were introduced through surface mechanical treatment and aging, provides a promising model for achieving localized strengthening without compromising corrosion resistance. Similar gradient architectures should be explored within diffusion-assisted and thermomechanical treatments to overcome the conventional hardness–corrosion trade-off in aluminum alloys.

Recent advances in precipitation hardenable alloys further support this direction. In AA2024, constrained deformation followed by peak aging generated a multimodal gradient structure with controlled variations in grain size and precipitate distribution, enabling simultaneous improvements in strength, ductility, and corrosion resistance. Extending such concepts to alloys with limited Zn/Mg solubility—particularly AA1100—and integrating them with diffusion-assisted or additive-manufacturing-based processing may unlock new pathways for spatially optimized surface performance.

Durability challenges such as intergranular corrosion (IGC) and stress corrosion cracking (SCC) remain critical constraints on high-strength aluminum systems. While retrogression–re-aging (RRA) provides an effective framework for balancing strength with SCC/IGC resistance in 7xxx alloys [63,65], similar optimization is urgently needed for emerging systems undergoing Zn/Mg diffusion or thermomechanical-assisted diffusion routes. Surface modification techniques such as laser alloying and shot peening demonstrate the ability to enhance solute transport and strengthen near-surface regions while retaining ductility; however, long-term corrosion behavior and hydrogen-assisted cracking mechanisms under realistic service environments remain insufficiently characterized and represent a critical barrier to industrial adoption.

Hybrid processing strategies offer significant potential for next-generation components. Synergistic combination of diffusion-assisted enrichment, severe surface plastic deformation (SSPD), and controlled multistage aging may enable dual performance architectures—hard, wear-resistant surfaces mechanically and tribologically optimized, supported by tough, corrosion resistant cores engineered for fatigue durability. Emerging additive manufacturing routes—including laser-directed energy deposition (LDED) and selective laser melting (SLM)—provide inherent capabilities to couple compositional grading, localized thermal management, and in situ aging during solidification, enabling precise tailoring of diffusion pathways and precipitate structures beyond the scope of conventional processing [13,52,66].

Identified Research Priorities:

(i) Overcoming the diffusion barrier

The native Al₂O₃ oxide film remains the primary kinetic and thermodynamic limitation for all diffusion-assisted surface enrichment routes. Future work should systematically explore the following:

• Advanced surface pretreatments: nano scale pore engineering, electrochemical etching, and controlled oxide reduction to remove the diffusion barrier while preserving substrate integrity [31,61,67].
• Plasma-assisted and ion beam-assisted diffusion methods to enhance Zn/Mg and rare earth element mobility, targeting penetration depths of 100–500 µm (vs. current 10–100 µm) [31,61,67].
• In situ laser or ultrasonic-assisted surface activation and solute transport coupling to accelerate diffusion kinetics during processing [68,69,70].

(ii) Strength–ductility trade-offs

Achieving simultaneous strength and ductility improvements remains a persistent mechanical design challenge, particularly in high Zn/Cu alloys prone to coarse precipitate formation and grain boundary embrittlement. Strategic alloy microalloying combined with optimized heat treatment offers promise:

• Zn-enriched systems (7xxx class): Minor Sc (0.1–0.3 wt%) or Ag (0.4–0.8 wt%) additions, combined with retrogression–re-aging (RRA), refine precipitate morphology and suppress coarse boundary phases [57,60,71].
• Rare earth microalloying: Lanthanide (La) and dual rare earth approaches (Er + Zr) promote formation of fine, thermally stable precipitates while reducing precipitation-free zone (PFZ) width, simultaneously improving tensile strength, strain capacity, and SCC resistance [43,45].
• Emerging strategies: Transition metal additions (Ti, V, Mo) and grain boundary segregation control via thermomechanical processing further expand the design space for tailored mechanical responses [4,5].

(iii) Long-term corrosion durability

Long-term stress corrosion cracking (SCC) and intergranular corrosion (IGC) behavior, particularly following combined diffusion-assisted and thermomechanical routes, remain poorly characterized and severely limits industrial deployment. Critical research needs to include the following:

• Systematic environmental durability evaluation: Accelerated corrosion testing (ASTM G47, ISO 7539) combined with long-term in-service monitoring under realistic marine salt spray, aerospace thermal cycling, and automotive moisture/acid conditions [57,72,73,74,75,76,77,78].
• Electrochemical characterization: Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and galvanic coupling analysis of diffusion-enriched surfaces to quantify susceptibility to localized pitting and filiform corrosion [50,51,52].
• Mechanistic investigation: Near-atomic-scale studies combining hydrogen microprint technique (HMT), in situ transmission electron microscopy (TEM), and atom probe tomography (APT) to elucidate hydrogen-assisted cracking mechanisms at precipitate interfaces and grain boundaries following hybrid processing [4,5].
• Predictive modeling: Machine learning surrogate models trained on multi-parameter corrosion datasets to forecast SCC/IGC susceptibility as a function of alloying elements, processing parameters, and environmental conditions [6,7].

(iv) Industrial scalability

Industrial scalability of diffusion-assisted and hybrid thermomechanical routes is presently limited by energy intensity, processing time, and capital equipment requirements. To enable commercial deployment, future work must prioritize the following:

Energy efficient activation methods: Plasma-assisted surface pretreatment, radio frequency (RF) heating, and hybrid laser–ultrasonic surface conditioning to reduce processing temperatures and cycle times by 30–50% relative to conventional diffusion [31,61,67,70].

Scalable reinforcement integration: Surface composite fabrication via electrical resistance heating (ERH) and pressure-assisted embedding demonstrates substantial performance gains (>200% hardness increase, >50% wear reduction) in non-hardenable alloys such as AA1100 [48,57,60,71,76,79,80]. Scaling from laboratory trials to pilot manufacturing and eventually full-scale production requires optimization of equipment geometry, thermal management, and defect mitigation.

Inline quality control: Real-time, non-destructive monitoring of diffusion depth, hardness gradients, and residual stresses using eddy current, acoustic, or X-ray diffraction techniques to ensure process robustness and component reliability [8,35].

(v) Alloy and process design optimization

High Zn/Cu (7xxx) and high Cu (2xxx) alloys deliver exceptional strength but suffer from persistent stress corrosion cracking (SCC) and intergranular corrosion (IGC) susceptibility. Future research must balance competing objectives:

• Refining heat treatment schedules: Develop multi-parameter optimization of solution treatment, artificial aging, and potential retrogression–re-aging (RRA) protocols to maximize yield strength and fatigue performance while minimizing SCC/IGC susceptibility through controlled precipitation-free zone (PFZ) engineering and grain boundary chemistry management [36,81].
• Alloy composition redesign: Explore systematic reduction in Cu and Cr content in favor of sustainable, lower-solubility elements (Mg, Zn, Sc, rare earth) to mitigate intergranular attack and galvanic coupling effects while maintaining strength targets [82,83].
• Circular economy principles: Prioritize recycling compatible alloy compositions and processing routes that minimize segregation of alloying elements during remelting, enabling high recovery rates and closed-loop material cycles [53,84,85,86].
• Computational design: Leverage machine learning alloy design frameworks integrating thermodynamic predictions, mechanical property surrogate models, and corrosion susceptibility indices to rapidly explore vast compositional spaces and identify Pareto optimal alloys balancing strength, ductility, corrosion resistance, and recyclability [66,81].

(vi) Consolidation and joining technologies

Robust metallurgical integration of gradient hardened or composite reinforced surface layers with ductile bulk alloys is essential for structural integrity and performance reliability. Critical consolidation and joining technologies include the following:

• Hot isostatic pressing (HIP): Enables diffusion bonding and pore elimination under elevated temperature and isotropic pressure, ideal for powder metallurgy-based composites; however, processing times (10–50 h) and cost (USD 10,000–50,000 + per cycle) limit scalability.
• Spark plasma sintering (SPS/FAST): Rapid consolidation via pulsed direct current and applied pressure, reducing cycle time to 1–10 min and minimizing precipitate coarsening; residual stress and interface defect control remain challenging [20,21].
• Diffusion bonding: Solid-state joining at controlled temperature and pressure, preserving near surface microstructures and enabling selective layer-to-layer integration; requires precise surface preparation and extended bonding times (0.5–2 h) [18,19,87].
• Thermal spraying (cold spray, warm spray): High-velocity particle deposition enabling rapid surface layer build up; applicable to composite reinforcements and diffusion-alloyed surfaces, though residual stress and coating adhesion require optimization.
• Emerging hybrid approaches: Ultrasonic-assisted and transient liquid phase (TLP) bonding offer potential for reduced processing times and improved interface chemistry control, though further development is needed [47,88].
• Trade-offs among processing time, cost, process complexity, and residual stress state must be carefully managed during selection and scale up to balance mechanical performance with economic feasibility.
• Despite significant progress, critical challenges remain regarding oxide barriers and strength ductility trade-offs. Furthermore, the application of LSP to additively manufactured aluminum components (e.g., AlSi10Mg) is emerging as a critical post processing step to effectively neutralize tensile residual stresses and close near surface porosity inherent to laser powder bed fusion (LPBF), thereby significantly enhancing fatigue performance [89].

Key research gaps and corresponding future opportunities are summarized in

Table 4. Summary of critical research gaps, challenges, and future technological opportunities in surface hardening of aluminum alloys.

Research Gap/Challenge	Future Opportunity/Direction	Priority	Ref.
Native Al2O3 oxide film limiting deep alloying element diffusion	Development of nano coatings, surface pretreatments, plasma-assisted diffusion, and nano texturing to enhance solute penetration	High	[69,90,91,92]
Strength–ductility trade-off, especially in high Zn/Cu alloys	Alloy design integrating Zn, Mg with minor Sc and Ag additions; application of retrogression and re-aging (RRA) heat treatments for optimized balance	High	[22,23,29,34]
Limited understanding of long-term corrosion resistance after diffusion-based aging	Systematic evaluations of IGC and SCC under realistic service conditions; exploration of hybrid thermomechanical and laser-assisted diffusion treatments for durability	Critical	[50,51,52,57,68,72]
Industrial scalability challenges due to high cost and energy consumption	Adoption of hybrid thermomechanical and laser-assisted diffusion routes enabling rapid, energy efficient processing	High	[57,60,68,71,76,84,93]
Integration of hardened surfaces with bulk alloys	Implementation of consolidation techniques such as hot isostatic pressing (HIP), spark plasma sintering (SPS/FAST), diffusion bonding, and thermal spraying for robust component assembly	Medium	[18,19,20,21,87,94,95]
Sustainability and circular economy considerations	Development of machine-learning-guided alloy design and recycling compatible alloys aligned with circular economy principles	Medium High	[17,84,85,86,93]

.

These directions converge on three overarching research themes: (1) systematic mitigation of fundamental barriers (Al₂O₃ diffusion barrier, strength–ductility trade-offs, long-term environmental stability), (2) synergistic hybrid strategies combining diffusion-assisted, thermomechanical, and composite approaches to transcend individual method limitations, and (3) integrated computational and experimental characterization of durability, scalability, and sustainability across alloy families and processing routes. Strategic investment in these integrated directions will accelerate translation of emerging scientific advances into scalable, economically viable, and environmentally responsible industrial practices, enabling next-generation aluminum components optimized for modern lightweight structures in aerospace, automotive, marine, and emerging mobility sectors. From a sustainability perspective, thermomechanical surface treatments (e.g., shot peening) generally consume less energy than high-temperature diffusion cycles, although the production of ceramic reinforcements for MMCs entails a higher initial carbon footprint.

4. Conclusions

This review consolidates current knowledge on diffusion-assisted aging, thermomechanical processing, and composite/hybrid reinforcement routes for strengthening aluminum alloys. By positioning these strategies within a unified comparative framework, the study clarifies their mechanisms, processing constraints, and performance outcomes—an integrated perspective largely absent in prior literature.

Thermomechanical treatments remain the most industrially viable and cost-effective approach for achieving balanced hardness–ductility responses through grain refinement, dislocation multiplication, and deep compressive residual stresses. These routes are already widely deployed across 2xxx, 6xxx, and 7xxx alloy systems. Composite and hybrid reinforcements—including SiC, B₄C, Al₂O₃, and graphene-based phases—offer high wear resistance, stiffness, and thermal stability through Orowan strengthening, load transfer, and interface-controlled mechanisms [4,5,6,7,8]. Additional studies further confirm their strong performance in tribological environments [68].

Diffusion-assisted Zn/Mg enrichment provides localized precipitation hardening but remains constrained by the native Al₂O₃ barrier, slow diffusion kinetics, and limited solubility in commercially pure alloys such as AA1100; thus, its relevance is primarily mechanistic rather than industrial [13,14,17,18,19], with additional insights from diffusion studies [20,21,22,23,34,90,95].

These three pathways are best viewed as complementary rather than competing strategies. Diffusion-assisted and thermomechanical routes provide gradient, surface-focused hardening (10–1000 µm), whereas composite and hybrid reinforcements deliver through-thickness strengthening and intrinsically multifunctional performance. Hybrid strategies—such as shot-peened or laser-shock-peened AMCs—combine deep compressive residual stresses with reinforcement-mediated load transfer, offering highly promising routes for multifunctional aerospace, automotive, and marine components. In this context, composite reinforcements serve as supplements, not replacements, for diffusion and thermomechanical approaches when deep wear resistance must coincide with fatigue durability and corrosion stability.

Corrosion and stress corrosion cracking (SCC) remain critical design variables across all routes. SCC susceptibility in high-strength aluminum alloys is governed by anodic dissolution, hydrogen-assisted cracking, PFZ evolution, and grain boundary chemistry [35,36,47,88]. Grain boundary segregation and Q phase evolution dominate SCC behavior in Al–Mg–Si–(Cu) alloys, while near-atomic-scale studies reveal hydrogen-driven dissolution of η phase precipitates as a key accelerator of SCC in 7xxx systems. Recent multistage aging strategies that optimize PFZ width and grain boundary precipitate chemistry have significantly improved SCC resistance, reinforcing the imperative to treat corrosion as a primary design constraint during method and process selection.

Method selection is fundamentally guided by dominant service requirements:

• High-cycle fatigue or vibration-dominated loading: thermomechanical shot peening or laser shock peening combined with optimized aging, selected for aerospace linkages, wing structures, and landing gear assemblies [15,16,59,61].
• Severe wear or tribological environments: composite and hybrid reinforcement routes preferred for pistons, brake rotors, sliding bearings, and heat exchanger components [6,7,24,25,26,27,28].
• Combined wear–fatigue–corrosion demands: synergistic hybrid approaches such as peened or LSP-treated AMCs or multistage-aged 7xxx alloys, suitable for marine propeller hubs and vibration-loaded structural assemblies [4,5,6,7,8,36,70].

Industrial qualification and adoption rely on standardized test protocols—ASTM B557 (tensile strength), ASTM G99 and G65 (tribological wear), ISO 9227 (salt spray corrosion), and ASTM G47 (SCC)—which serve as critical enablers for regulatory certification and design confidence across aerospace, automotive, marine, and structural applications.

From a sustainability perspective, thermomechanical peening coupled with optimized aging remains the most energy- and material-efficient route. AMC development must balance superior wear and thermal performance against the high embodied energy of reinforcement production and limited recyclability. Machine-learning-based frameworks increasingly support optimization of reinforcement chemistry and process variables [17,45,66], building on broader ML-driven alloy design studies [70,81,82,83,84,85,86] and emerging sustainability-focused methodologies [93].

In summary, the combined application of solute-driven precipitation hardening, deformation-induced grain refinement, and reinforcement-mediated strengthening—supported by advanced surface activation, hybrid processing routes, and predictive data-driven design—provides a robust foundation for next-generation aluminum components with tailored gradient hardening, superior tribological performance, and long-term environmental stability. Strategic selection among diffusion-assisted, thermomechanical, and composite/hybrid approaches, guided by standardized testing, enables scalable and sustainable solutions for aerospace, automotive, marine, and emerging engineering sectors. This unified framework is expected to accelerate industrial adoption and direct future research toward hierarchical, multiphase aluminum architectures.