Hybrid Additive Manufacturing via Wire Arc Metal Deposition and Deformation for Microstructure Refinement and Performance Enhancement

1. Introduction

Wire Arc Additive Manufacturing (WAAM), a Directed Energy Deposition (DED)-based process, has attracted growing attention as a viable technique for producing medium- to large-scale metallic components with high deposition rates and excellent material utilization. Compared with powder-based additive manufacturing, WAAM offers advantages in cost efficiency, scalability, and deposition speed, making it particularly attractive for aerospace, marine, energy, and tooling applications [1,2,3,4]. Despite these advantages, conventional WAAM is associated with several metallurgical and geometric limitations that restrict its broader industrial adoption.

A primary challenge in WAAM is the formation of coarse columnar grains aligned along the build direction, driven by steep thermal gradients and epitaxial solidification during layer-by-layer deposition. These columnar structures are typically accompanied by strong crystallographic textures, leading to pronounced mechanical anisotropy and reduced structural reliability [5,6,7]. Repeated thermal cycling and high heat input promote residual stress accumulation, which can result in distortion, cracking, and reduced fatigue performance [8,9,10]. Process-induced defects—including porosity, lack of fusion, and surface waviness—further degrade component quality and necessitate extensive post-processing [11,12,13].

To overcome these limitations, significant research has been directed toward hybrid WAAM processes that integrate mechanical deformation, and in some cases subtractive machining, into the additive manufacturing workflow. Deformation-assisted techniques—including interlayer rolling, hammer peening, FSP, and ultrasonic vibration—introduce controlled plastic strain during or between deposition passes, promoting grain refinement, texture randomization, defect closure, and improved mechanical performance [14,15,16,17,18].

Hybrid WAAM approaches have demonstrated the ability to activate fundamental metallurgical mechanisms such as dynamic recrystallization (DRX) and severe plastic deformation (SPD). Interlayer rolling introduces sufficient stored strain energy to promote recrystallization during subsequent thermal cycles, leading to fine equiaxed grain structures and reduced anisotropy [19]. FSP applied to WAAM deposits eliminates porosity and produces ultrafine grains through intense shear deformation and DRX [20]. Ultrasonic vibration-assisted WAAM, where acoustic cavitation and streaming effects enhance melt pool convection and nucleation, has also shown promising grain refinement results [21,22].

This review provides a unified framework for understanding deformation-assisted WAAM, focusing on hybrid processing strategies incorporating deformation, fundamental microstructure evolution mechanisms under different thermomechanical conditions, and their implications for mechanical performance across major alloy systems. Unlike previous reviews, this work establishes a unified thermomechanical framework explicitly linking deformation modes, processing conditions, and microstructure evolution mechanisms across multiple alloy systems, providing a systematic basis for hybrid WAAM process design and optimization.

This review is organized as follows. Section 2 introduces the fundamentals of WAAM thermophysics. Section 3 describes hybrid deposition-deformation processes. Section 4 covers alloy-specific microstructure evolution. Section 5 presents the physical mechanisms of microstructure evolution. Section 6 discusses mechanical property improvements. Section 7 addresses the integration of auxiliary processes. Section 8 presents challenges and future outlook. Section 9 provides conclusions.

2. Fundamentals of Wire Arc Additive Manufacturing and Thermophysical Characteristics

Wire Arc Additive Manufacturing is a Directed Energy Deposition process that uses an electric arc as a heat source and a continuously fed metallic wire as feedstock. It is typically implemented using gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), or plasma arc welding (PAW) variants, including advanced modes such as Cold Metal Transfer (CMT) and pulsed arc techniques. The thermophysical characteristics of WAAM are governed by high heat input, relatively low cooling rates, and steep directional thermal gradients, which collectively dictate solidification behavior and microstructure evolution.

During deposition, the melt pool experiences strong thermal gradients (G) and solidification rates (R), which determine the solidification mode according to the classical G/R criterion. High G/R ratios promote epitaxial columnar growth along the maximum thermal gradient direction, typically aligned with the build direction. Consequently, WAAM components frequently exhibit coarse columnar dendritic microstructures with pronounced crystallographic textures, particularly <100> or <110> orientations in cubic alloys [23,24,25].

Repeated thermal cycling inherent to WAAM further complicates microstructure evolution. Each newly deposited layer reheats previously solidified material, leading to complex thermal histories characterized by partial remelting, solid-state phase transformations, and cyclic recovery processes. These cycles can result in heterogeneous microstructures, including banded structures, heat-affected zones, and variations in grain size and morphology along the build height [26,27,28].

From a thermomechanical perspective, WAAM generates significant residual stresses due to non-uniform thermal expansion and contraction. Tensile residual stresses, particularly in the longitudinal direction, can lead to distortion, cracking, and reduced fatigue performance [29,30,31]. In addition, WAAM is susceptible to process-induced defects including porosity and surface irregularities such as waviness and stair-stepping. These defects act as stress concentrators and can significantly degrade mechanical performance under cyclic loading [32,33,34]. Advanced process control strategies have been shown to reduce defect formation and improve microstructural uniformity; however, purely thermal control approaches are often insufficient to eliminate columnar grain growth and anisotropy, necessitating deformation-based hybrid processing strategies [35,36,37].

3. Hybrid WAAM Processes with Interlayer Deformation

Hybrid WAAM processes incorporate controlled plastic deformation into the layer-wise deposition sequence to modify both solidification behavior and subsequent solid-state microstructure evolution. Unlike conventional WAAM, where microstructure is primarily governed by thermal gradients and cooling rates, hybrid approaches introduce an additional degree of freedom through strain, strain rate, and stress, enabling thermomechanical control of grain structure, defect evolution, and residual stress state. The effectiveness of these processes arises from the interplay between strain-induced defect generation and thermally activated restoration mechanisms, including recovery and recrystallization.

Deformation mechanisms in hybrid WAAM can be broadly categorized by mode of strain application: compressive deformation (e.g., interlayer rolling), which introduces relatively uniform plastic strain across the deposited layer; localized impact deformation (e.g., hammer peening and ultrasonic impact), which generates high strain rates and near-surface plastic deformation; severe plastic deformation (e.g., FSP), characterized by intense shear deformation and large accumulated strains; and oscillatory or high-frequency deformation (e.g., ultrasonic vibration), which introduces cyclic stresses and modifies melt pool dynamics and solidification behavior. Each mode results in distinct strain distributions, strain rates, and thermal-mechanical interactions, leading to different microstructural outcomes.

3.1. Interlayer Rolling: Compressive Thermomechanical Processing

Interlayer rolling is one of the most extensively studied hybrid WAAM techniques, characterized by the application of compressive plastic deformation using rollers immediately after or between deposition passes. The rolling process introduces relatively uniform plastic strain across the deposited bead, with strain magnitudes typically in the range of 5–30%, depending on roller geometry, applied load, and interpass temperature.

From a mechanistic standpoint, interlayer rolling increases dislocation density within the as-deposited microstructure, leading to significant strain hardening and accumulation of stored energy that serves as the driving force for subsequent recrystallization during thermal cycling. In alloys such as Ti-6Al-4V, steels, and aluminum alloys, rolling-induced deformation has been shown to disrupt epitaxial grain growth, promoting the transformation from columnar to equiaxed grain structures [34,35]. Gornyakov et al. [34] developed a coupled thermo-mechanical finite element model for in-process interlayer rolling during WAAM of a steel wall, showing that a slotted roller reduces tensile residual stress from 500 MPa to approximately 3 MPa in the 9th layer while also limiting wall widening and reducing shape distortion. Rolling also contributes to defect mitigation by mechanically closing pores and enhancing interlayer bonding, reducing lack-of-fusion defects and improving structural integrity.

Figure 1. Hybrid interlayer hot rolling and WAAM: (a) schematic diagram; (b) temperature distribution [34]. Reprinted under a Creative Commons CC-BY-NC-ND license.

Figure 1. Hybrid interlayer hot rolling and WAAM: (a) schematic diagram; (b) temperature distribution [34]. Reprinted under a Creative Commons CC-BY-NC-ND license.

3.2. Hammer Peening and Mechanical Impact: High Strain-Rate Deformation

Hammer peening and related mechanical impact techniques introduce localized plastic deformation through repeated high-frequency impacts applied to the deposited surface. Unlike rolling, hammer peening generates high strain-rate deformation (typically 10² to 10⁴ s⁻¹) concentrated in the near-surface region. The high strain rates lead to rapid dislocation multiplication and formation of dense dislocation networks that can evolve into refined subgrain structures. A key advantage is the ability to introduce compressive residual stresses in the surface region, counteracting tensile stresses generated during WAAM deposition and significantly enhancing fatigue performance by delaying crack initiation and propagation [37,43]. Mechanical impact processes can also influence solidification behavior—impact-induced vibrations can disrupt dendritic growth and promote grain refinement, particularly in alloys with high susceptibility to hot cracking [38].

Figure 2. MHP setup and its influence on WAAM Fe-based shape-memory alloy deposits: (a) hammer peening machine attached to Kuka robot; (b) peening tool close-up; (c) peening on the substrate surface; (d) peening indentation marks; (e) Fe-based shape memory alloy deposit showing vertical cracks; (f) front and top views of the wall fabricated with interlayer MHP-assisted WAAM. Adapted with permission from Elsevier [38].

Figure 2. MHP setup and its influence on WAAM Fe-based shape-memory alloy deposits: (a) hammer peening machine attached to Kuka robot; (b) peening tool close-up; (c) peening on the substrate surface; (d) peening indentation marks; (e) Fe-based shape memory alloy deposit showing vertical cracks; (f) front and top views of the wall fabricated with interlayer MHP-assisted WAAM. Adapted with permission from Elsevier [38].

3.3. Friction Stir Processing (FSP): Severe Plastic Deformation and Material Flow

Friction stir processing represents a severe plastic deformation technique successfully integrated into WAAM workflows. A rotating tool traversed across the deposited layer generates intense shear deformation and frictional heating. The combined effects of high strain (typically greater than 100%) and elevated temperature result in extensive microstructural modification through continuous dynamic recrystallization (CDRX), leading to the formation of ultrafine equiaxed grains, often in the range of 1–10 µm [26,27]. FSP also promotes significant microstructural homogenization—the intense material flow redistributes second-phase particles, breaks up intermetallic networks, and eliminates porosity, resulting in improved strength, ductility, and fatigue resistance. The effectiveness of FSP is highly sensitive to tool rotational speed, traverse speed, and plunge depth, which control the strain rate and temperature field that in turn determine the extent of recrystallization and grain refinement [26,61].

Figure 3. Schematic diagram of friction stir additive manufacturing. Adapted from Mishra and Ma (2005) [27].

Figure 3. Schematic diagram of friction stir additive manufacturing. Adapted from Mishra and Ma (2005) [27].

Figure 4. Process chain of WAAM followed by interlayer FSP: (a) robotic deposition system; (b) welding gun positioning during WAAM; (c) resulting as-deposited Al-Cu wall structures; (d) dedicated FSP machine; (e) tool and fixture arrangement for post-deposition stirring; (f) comparison of FSP-processed surface regions. Reprinted with permission from Elsevier [32].

Figure 4. Process chain of WAAM followed by interlayer FSP: (a) robotic deposition system; (b) welding gun positioning during WAAM; (c) resulting as-deposited Al-Cu wall structures; (d) dedicated FSP machine; (e) tool and fixture arrangement for post-deposition stirring; (f) comparison of FSP-processed surface regions. Reprinted with permission from Elsevier [32].

3.4. Laser Shock Peening (LSP): Deep Compressive Residual Stress Induction

High-energy pulse laser beams generate plasma shock waves that propagate into the deposited layer, inducing deep compressive plastic deformation (up to 1–2 mm depth) without solid tool contact. The strain rate can be extremely high (greater than 10⁶ s⁻¹) while the temperature remains near ambient. Thangamani et al. [44] studied hybrid WAAM-LSP processing of NiTi shape memory alloys and reported that LSP progressively refined grains, reduced porosity from 13.08% to 4.10%, and significantly increased UTS from 217 MPa to 426 MPa at the highest peening intensity. Sun et al. [45] demonstrated that LSP applied to WAAM-fabricated 2319 aluminum transformed tensile residual stresses into beneficial compressive stresses, resulting in greatly enhanced fatigue performance.

3.5. Ultrasonic Vibration-Assisted WAAM: Coupled Solidification-Deformation Control

Ultrasonic vibration-assisted WAAM represents an emerging hybrid processing approach that simultaneously influences liquid-phase solidification and solid-state deformation. High-frequency vibrations (typically 20–40 kHz) are introduced into the melt pool or substrate during deposition. The primary mechanisms include acoustic cavitation, acoustic streaming, and oscillatory stress fields, which collectively promote the columnar-to-equiaxed transition during solidification. Experimental studies have shown that ultrasonic vibration can significantly refine grain size in WAAM deposits, with reductions exceeding 50–80% in certain alloy systems [39,40]. The cyclic nature of ultrasonic loading can also accelerate dislocation rearrangement and promote dynamic recovery and recrystallization under appropriate conditions.

3.6. Comparative Perspective on Hybrid Deformation Techniques

Each deformation-assisted WAAM technique offers distinct advantages: interlayer rolling delivers uniform bulk deformation and effective residual stress control; hammer peening offers high strain-rate surface modification and improved fatigue resistance; FSP produces severe plastic deformation and ultrafine grain structures; LSP achieves deep compressive residual stress fields without thermal damage; and ultrasonic vibration enables coupled melt pool and solid-state control. Despite their demonstrated benefits, each technique presents inherent limitations that must be carefully considered based on material system, component geometry, required performance, and production constraints. Table 1 summarizes the key characteristics of each approach.

4. Microstructure Evolution in Major Alloy Systems

4.1. Microstructure Evolution Mechanisms

In hybrid WAAM, the thermodynamics and physical metallurgy principles remain the same as in traditional manufacturing, but several distinctive characteristics apply. The processing zone is often highly localized, producing steep temperature and deformation field gradients. Materials experience multiple thermomechanical cycles spanning a wide domain of strain, temperature, and time, and the final microstructure is the integrated result of all cycles. The stacking-fault energy (SFE) governs the dominant deformation mechanism and post-deformation microstructural changes.

The classical metallurgical phenomena active in hybrid WAAM, in order of increasing processing temperature, are: strain hardening at low deformation temperatures (characterized by increased dislocation density and stored energy described by Es = ½Gb²ρ, where G is shear modulus, b is the Burgers vector, and ρ is dislocation density); annealing (recovery) at approximately 0.2 to 0.4 Tm; static recrystallization (SRX) above approximately 0.5 Tm with sufficient prior strain, with kinetics following X = 1 − exp[−(kt)ⁿ]; and dynamic recrystallization (DRX) in hot deformation above 0.5 Tm, subdivided into discontinuous (dDRX) and continuous (cDRX) variants, the latter prevalent in high-SFE metals such as aluminum.

Figure 5. Schematic strain-temperature framework illustrating dominant microstructure evolution mechanisms in hybrid WAAM. Different deformation processes occupy distinct regions of the map, leading to varying contributions of strain hardening, recovery, static recrystallization (SRX), and dynamic recrystallization (DRX, d-discontinuous and c-continuous).

Figure 5. Schematic strain-temperature framework illustrating dominant microstructure evolution mechanisms in hybrid WAAM. Different deformation processes occupy distinct regions of the map, leading to varying contributions of strain hardening, recovery, static recrystallization (SRX), and dynamic recrystallization (DRX, d-discontinuous and c-continuous).

4.2. Aluminum Alloys

Aluminum alloys in WAAM frequently develop coarse columnar grains and high porosity due to their high thermal conductivity, fluid melt pools, and sensitivity to hydrogen pickup [52]. CMT-based WAAM walls of 2319 aluminum alloy exhibit mixed columnar and equiaxed grains with theta/theta’ precipitates, retaining approximately 72% of the strength and about 65% of the fatigue strength of wrought AA2219-T62 [53,54]. Combining WAAM with interlayer FSP on 2319 aluminum generates a vertical gradient microstructure with ultrafine grains near the processed surface, increasing the DRX fraction to approximately 62% and substantially refining grains by approximately 87% [46].

The present authors (Elalem and Wu [61]) compared MIG-WAAM of 4043 aluminum alloy with a hybrid MIG-WAAM plus FSP route, in which FSP was applied behind the MIG molten pool over the deposited layer. In the WAAM-only wall, interlayer heat accumulation promoted coarse, partially dendritic grains that coarsened with build height. In contrast, MIG-FSP imposed severe plastic deformation dominated by shear strain at a high strain rate; DRX produced consistently fine equiaxed grains (a few micrometers in diameter) across all layers with a more isotropic morphology, resulting in approximately 46% higher hardness compared with the WAAM-only wall.

Figure 6. (a) Experimental setup consisting of MIG welder (spool gun) and FSP tool mounted on a CNC machining center (HAAS VF3); (b) comparative probability density distributions of 2D grain area for MIG only and MIG+FSP (hybrid AM), showing a strong shift toward a narrow distribution of fine grains; (c) representative infrared thermographic temperature contour maps comparing thermal fields during fabrication. Reproduced under a CC BY license [61].

Figure 6. (a) Experimental setup consisting of MIG welder (spool gun) and FSP tool mounted on a CNC machining center (HAAS VF3); (b) comparative probability density distributions of 2D grain area for MIG only and MIG+FSP (hybrid AM), showing a strong shift toward a narrow distribution of fine grains; (c) representative infrared thermographic temperature contour maps comparing thermal fields during fabrication. Reproduced under a CC BY license [61].

Ma et al. [65] applied FSP to WAAM-fabricated 205A Al-Cu alloy, refining grains from approximately 22.8 µm to less than 5 µm, largely dissolving eutectic structures, and eliminating pore defects; FSP at 800 min⁻¹ provided yield strength +32.7%, UTS +20.6%, and elongation +56.7%. Multi-pass FSP on 2319 Al walls refined grains in the stir zone to 3.4 µm [26]. For high-strength Al-Zn-Mg-Cu alloys, interlayer FSP combined with high-entropy alloy particle reinforcement achieved grain refinement to approximately 2.3 µm and simultaneous improvements in hardness (~152 HV), UTS (~374 MPa), and elongation (~10.6%) [59].

Figure 7. WAAM + FSP applied to AA6061: (a) experimental WAAM-milling-FSP robotic system; (b) EBSD inverse pole figure map of the WAAM 6061 thin-wall showing coarse columnar grains; (c) EBSD map of the WAAM + interlayer FSP stir zone showing a refined equiaxed grain structure. Adapted with permission from Elsevier [72].

Figure 7. WAAM + FSP applied to AA6061: (a) experimental WAAM-milling-FSP robotic system; (b) EBSD inverse pole figure map of the WAAM 6061 thin-wall showing coarse columnar grains; (c) EBSD map of the WAAM + interlayer FSP stir zone showing a refined equiaxed grain structure. Adapted with permission from Elsevier [72].

Figure 8. FSP post-treatment of WAAM-fabricated 205A Al-Cu alloy: (a) schematic of the WAAM process; (b,c) WAAM Al-Cu-Mn alloy component after FSP post-treatment; (d) stress-strain curves showing increased strength and elongation after FSP at 800 and 1200 min⁻¹; (e) comparison of YS, UTS, and elongation demonstrating substantial improvements over the as-deposited condition. Adapted under a CC BY license [65].

Figure 8. FSP post-treatment of WAAM-fabricated 205A Al-Cu alloy: (a) schematic of the WAAM process; (b,c) WAAM Al-Cu-Mn alloy component after FSP post-treatment; (d) stress-strain curves showing increased strength and elongation after FSP at 800 and 1200 min⁻¹; (e) comparison of YS, UTS, and elongation demonstrating substantial improvements over the as-deposited condition. Adapted under a CC BY license [65].

4.3. Titanium Alloys

Titanium alloys, particularly Ti-6Al-4V, exhibit coarse, epitaxially grown beta-columnar grains in plain WAAM, followed by martensitic alpha’ or basketweave alpha formation during cooling [10]. This morphology yields pronounced anisotropy, high residual stress, and limited ductility. Maurya et al. [73] optimized a WAAM-forging hybrid route for Ti-6Al-4V, identifying a stable, high-efficiency deformation window around 900–950 °C at strain rates below 1 s⁻¹. Forging near 920 °C to strains of 0.6–0.9 significantly reduced texture intensity and improved both tensile strength and elongation. Hicks et al. [74] extended the hybrid WAAM concept to landing gear components by depositing WAAM Ti-6Al-4V features onto forged Ti-5Al-5Mo-5V-3Cr substrates, demonstrating acceptable microstructural continuity and mechanical performance. Overall, titanium alloys respond exceptionally well to deformation-assisted hybridization, with reported reductions in beta-grain width, texture intensity, and residual stress [72].

Figure 9. (a) WAAM-built Ti-6Al-4V deposit indicating traveling direction and build direction; (b) engineering stress-strain curves comparing the as-received WAAM condition, stress-relieved state, and hot-forged conditions produced at 920 °C with imposed strains of 0.6, 0.8, and 0.9. Adapted with permission from Elsevier [73].

Figure 9. (a) WAAM-built Ti-6Al-4V deposit indicating traveling direction and build direction; (b) engineering stress-strain curves comparing the as-received WAAM condition, stress-relieved state, and hot-forged conditions produced at 920 °C with imposed strains of 0.6, 0.8, and 0.9. Adapted with permission from Elsevier [73].

4.4. Steels and Stainless Steels

In steels and stainless steels, microstructure evolution during WAAM is governed by solidification mode, phase transformations, and stacking-fault energy. Plain WAAM often leads to columnar ferrite, austenite, or martensite depending on alloy content, with noticeable texture and high residual stress [13,73,74]. Panchenko et al. [77] showed that repeated reheating and slow cooling promote grain coarsening, texture sharpening, and heterogeneous hardness, confirming that poorly controlled thermal cycling can strongly degrade toughness and isotropy. Koley and Ganguly [38] demonstrated that interlayer MHP effectively suppressed hot cracking in Fe-based shape memory alloys by modifying the residual stress state and redistributing strain concentration in the solidifying weld pool. Overall, steels exhibit moderate but consistent microstructural refinement under hybrid processing, with degree of refinement generally lower than in titanium or aluminum due to higher recrystallization temperatures.

4.5. Nickel-Based Superalloys

Nickel-based superalloys such as Inconel 625 and Inconel 718 are prone to columnar solidification, segregation, Laves phase formation, and residual stress accumulation in WAAM. Their high gamma’/gamma’’ strengthening precipitate content reduces recrystallization kinetics, limiting spontaneous grain refinement during cooling [75]. Interlayer rolling introduces plastic strain that fragments the as-deposited columnar dendritic structure and, aided by reheating from subsequent deposition, promotes recrystallization to produce finer equiaxed grains. Chen et al. [78] showed that raising the solution treatment temperature from 950 °C to 1150 °C dramatically reduced the Laves phase volume fraction and re-partitioned Nb into strengthening gamma’’ precipitates, improving tensile strength and elongation by approximately 35%. Grain refinement in superalloys is often less pronounced than in titanium and aluminum due to sluggish recrystallization and high hot strength.

Table 2. Summary of deformation influence on hybrid WAAM microstructure across alloy families.

Alloy Family	As-Deposited Microstructure	Deformation Response	Key Microstructural Evolution	Key References
Al alloys	Columnar grains (often mixed), porosity/hydrogen defects, coarse intermetallic networks	High	cDRX/DRX, ultrafine/equiaxed grains; porosity elimination; improved homogeneity	[24,25,26,32,46,52,54,60,61,62,66,67,68]
Ti alloys	Coarse epitaxial prior-beta columnar grains; alpha’ martensite/lamellar alpha; anisotropy + residual stress	Very high	DRX/recrystallization; finer/more equiaxed microstructure; texture reduction; improved isotropy/ductility	[10,37,69,70,71,72,82]
Steels / stainless steels	Columnar ferrite/austenite/martensite; texture; residual stress; thermal-history sensitivity	Moderate	Moderate grain refinement; peening/LSP near-surface refinement + compressive residual stresses	[13,35,36,38,40,73,74,76]
Ni-based superalloys	Columnar gamma/dendritic solidification; segregation; Laves formation; sluggish recrystallization	Limited to moderate	Limited bulk grain refinement; rolling/FSP reduce segregation; peening/LSP deep compressive residual stress	[6,75,76,77,78,79,80,81,83,84,85]

Table 2. Summary of deformation influence on hybrid WAAM microstructure across alloy families.

Alloy Family	As-Deposited Microstructure	Deformation Response	Key Microstructural Evolution	Key References
Al alloys	Columnar grains (often mixed), porosity/hydrogen defects, coarse intermetallic networks	High	cDRX/DRX, ultrafine/equiaxed grains; porosity elimination; improved homogeneity	[24,25,26,32,46,52,54,60,61,62,66,67,68]
Ti alloys	Coarse epitaxial prior-beta columnar grains; alpha’ martensite/lamellar alpha; anisotropy + residual stress	Very high	DRX/recrystallization; finer/more equiaxed microstructure; texture reduction; improved isotropy/ductility	[10,37,69,70,71,72,82]
Steels / stainless steels	Columnar ferrite/austenite/martensite; texture; residual stress; thermal-history sensitivity	Moderate	Moderate grain refinement; peening/LSP near-surface refinement + compressive residual stresses	[13,35,36,38,40,73,74,76]
Ni-based superalloys	Columnar gamma/dendritic solidification; segregation; Laves formation; sluggish recrystallization	Limited to moderate	Limited bulk grain refinement; rolling/FSP reduce segregation; peening/LSP deep compressive residual stress	[6,75,76,77,78,79,80,81,83,84,85]

5. Microstructure Evolution Mechanisms under Deformation-Assisted WAAM

Microstructural evolution in hybrid WAAM processes is governed by the coupled effects of temperature (T), strain (ε), strain rate (ε̇), and applied stress (σ). A useful framework is based on the Zener-Hollomon parameter: Z = ε̇ exp(Q/RT), where high Z values favor work hardening and limited recovery, whereas low Z values promote dynamic restoration processes such as recrystallization.

5.1. Dislocation Accumulation, Work Hardening, and Substructure Formation

Plastic deformation introduced by hybrid WAAM generates a high density of dislocations within the crystal lattice. The evolution of dislocation density (ρ) during deformation can be described by dρ/dε = k₁ − k₂ρ, where k₁ represents dislocation generation and k₂ accounts for dynamic recovery. At high strain rates or low temperatures (high Z), dislocation generation dominates, leading to rapid work hardening and the formation of dislocation cells and subgrain structures characterized by low-angle grain boundaries, which serve as precursors to recrystallized grains. Rolling and hammer peening significantly accelerate dislocation accumulation, while FSP and ultrasonic vibration can produce extremely high dislocation densities due to severe plastic deformation and cyclic loading.

5.2. Recovery, Polygonization, and Annealing

At elevated temperatures, dislocation structures undergo recovery through thermally activated processes such as climb and cross-slip, reducing stored strain energy by rearranging dislocations into more stable configurations. The rate of recovery is strongly dependent on temperature and stacking fault energy. High SFE materials (e.g., aluminum alloys) exhibit rapid recovery, while low SFE materials (e.g., austenitic steels and Ni-based alloys) retain higher dislocation densities, favoring subsequent recrystallization. In WAAM, recovery processes are frequently activated during interpass thermal cycling, resulting in partial stress relaxation and subgrain growth without fully eliminating deformation-induced heterogeneity.

5.3. Static Recrystallization (SRX)

Static recrystallization occurs when plastically deformed material is exposed to elevated temperatures without additional deformation. Its kinetics can be described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation: X = 1 − exp(−ktⁿ), where X is the recrystallized fraction, k is a temperature-dependent rate constant, and n is the Avrami exponent. In hybrid WAAM, SRX is particularly relevant when deformation is applied at relatively low temperatures, followed by reheating during subsequent deposition passes, with new strain-free grains nucleating preferentially at high-energy sites such as grain boundaries and triple junctions.

5.4. Dynamic Recrystallization (DRX): Coupled Deformation-Recrystallization Processes

Dynamic recrystallization occurs when deformation is applied at elevated temperatures, allowing recrystallization to proceed concurrently with plastic deformation. The onset of DRX is typically defined by a critical strain (εc ≈ 0.5εp). Two primary DRX mechanisms are relevant in hybrid WAAM.

5.4.1. Continuous Dynamic Recrystallization (CDRX)

CDRX is characterized by the gradual transformation of subgrain structures into recrystallized grains through progressive lattice rotation. This mechanism dominates in high SFE materials such as aluminum alloys and is the dominant mechanism in FSP-assisted WAAM, where large shear strains and moderate temperatures promote continuous grain refinement without distinct nucleation events.

5.4.2. Discontinuous Dynamic Recrystallization (DDRX)

DDRX involves the nucleation and growth of new grains during deformation, typically at pre-existing grain boundaries. This mechanism is common in low- to medium-SFE materials such as steels and nickel-based alloys, characterized by distinct nucleation of strain-free grains, grain boundary bulging and migration, and cyclic flow behavior in stress-strain curves. In hybrid WAAM, DDRX is often activated during high-temperature deformation processes such as interlayer rolling at elevated interpass temperatures.

5.5. Severe Plastic Deformation (SPD) and Ultrafine/Nanocrystalline Grain Formation

Severe plastic deformation occurs under conditions of very high strain (ε >> 1), leading to extensive grain subdivision and the formation of ultrafine-grained or nanocrystalline structures. The relationship between grain size (d) and strength follows the Hall-Petch relation: σy = σ0 + k·d⁻½, where σy is the yield strength, σ0 is the friction stress, and k is the Hall-Petch coefficient [47,48,49]. In hybrid WAAM, FSP produces ultrafine grains (1–10 µm or smaller), while hammer peening and ultrasonic impact produce nanostructured surface layers, and ultrasonic vibration can enhance grain refinement through cyclic plasticity and cavitation-assisted nucleation.

5.6. Integrated Thermomechanical Pathways and Process-Structure-Property Implications

In practical hybrid WAAM processes, these mechanisms operate simultaneously and interact dynamically. The microstructure evolution pathway depends on the local thermomechanical history: high temperature with moderate strain leads to DRX; low temperature with high strain produces dislocation accumulation followed by delayed SRX; repeated thermal cycling activates recovery and SRX; and very high strain (SPD) results in ultrafine or nanocrystalline grains. The integration of deformation into WAAM fundamentally transforms the process-structure-property relationship: grain refinement enhances strength via Hall-Petch strengthening; texture randomization improves isotropy; defect closure improves fatigue resistance; and residual stress redistribution enhances dimensional stability.

6. Mechanical Property Improvements and Performance Correlations

6.1. Strength-Ductility Synergy

In conventional WAAM, coarse columnar grains and strong crystallographic textures result in anisotropic mechanical behavior and limited ductility. Hybrid deformation processes disrupt these structures and promote equiaxed grain formation, enabling simultaneous improvements in strength and ductility. Grain refinement enhances strength through the Hall-Petch relationship, while the reduction of defects such as porosity increases the effective load-bearing area. Texture randomization reduces directional dependence of slip systems, improving uniform plastic deformation. Increases of 20–40% in yield strength and 10–30% in elongation have been reported in hybrid WAAM materials compared with as-deposited counterparts [73,74,75,76]. Zhang et al. [91] showed that inter-pass rolling improved isotropic elongation by 35–54% after heat treatment while maintaining strength through theta’/theta’’ precipitation. Zhou et al. [88] demonstrated that adding 0.15 wt.% Sc to WAAM Al-Cu walls combined with interlayer FSP increased UTS from 256 MPa to 303 MPa after FSP.

Figure 10. (a) CMT platform; (b) schematic of deposition strategy for WAAM + FSP; (c,d) FSP system; (e) stress-strain curves; (f) tensile strength histogram. Reprinted with permission from Elsevier [88].

Figure 10. (a) CMT platform; (b) schematic of deposition strategy for WAAM + FSP; (c,d) FSP system; (e) stress-strain curves; (f) tensile strength histogram. Reprinted with permission from Elsevier [88].

6.2. Fatigue Performance

Fatigue performance in WAAM components is highly sensitive to surface condition, microstructural heterogeneity, and residual stress state. Hybrid deformation processes improve fatigue resistance through multiple mechanisms: grain refinement reduces slip localization; compressive residual stresses delay crack initiation; porosity closure removes critical stress concentrators; and surface integrity is enhanced through deformation-assisted smoothing. Interlayer rolling and hammer peening have been shown to significantly improve fatigue life in titanium and aluminum alloys by introducing beneficial compressive residual stresses and refining surface microstructure [37,82]. Zhang et al. [83] showed that cold rolling combined with heat treatment improves fatigue life in Inconel 718 due to gamma’’ precipitation and refined Laves distribution. Wang et al. [95] reported nearly isotropic fatigue crack growth behavior in hybrid additive manufactured Ti-6Al-4V ELI, with crack growth rates comparable to wrought material.

6.3. Creep and High-Temperature Performance

Creep resistance in WAAM materials is strongly influenced by grain structure, phase stability, and defect distribution. Coarse columnar grains in conventional WAAM can accelerate creep deformation due to grain boundary sliding and anisotropic behavior. Hybrid deformation processes improve creep performance by refining grain structures, reducing segregation and second-phase instability, and eliminating defects that act as creep cavities. In nickel-based superalloys, deformation-assisted WAAM combined with heat treatment has been shown to improve high-temperature strength and reduce creep rate by stabilizing gamma’/gamma’’ precipitates and reducing Laves phase content [78,85].

6.4. Reduction of Mechanical Anisotropy

Mechanical anisotropy is a critical limitation of conventional WAAM, arising from directional solidification and columnar grain growth. Hybrid deformation processes significantly reduce anisotropy by transforming columnar grains into equiaxed structures, randomizing crystallographic texture, and homogenizing phase distribution. In titanium alloys, hybrid WAAM processes have been shown to reduce anisotropy in elongation and strength to less than 5%, compared with 20–40% in conventional WAAM [10]. Table 3 summarizes representative hybrid WAAM-based studies and their key outcomes.

7. Integration of Auxiliary Processes in Hybrid WAAM

7.1. Integration of Subtractive Machining

Due to the inherent characteristics of arc-based deposition, WAAM components typically exhibit limited dimensional accuracy and high surface roughness [62,81]. The present authors developed a patented ADM process and system [103] that combines WAAM with FSP and milling in one platform cell, allowing both inner and outer edge surfaces to be machined at the layer level, resolving tool accessibility problems for complex geometries and preventing accumulation of geometric inaccuracies over multiple layers [119]. Huang et al. [108] demonstrated quantitatively that as-built surface undulations reduce the fatigue endurance limit by approximately 35% and shorten fatigue life by approximately 60% relative to machined specimens. Table 4 summarizes the synergistic effects of post-deformation machining in hybrid WAAM.

Figure 11. Effect of subtractive machining on the performance of WAAM ER70S-6 steel plates: (a) as-built and machined plate conditions and coupon extraction scheme; (b) representative fracture surface; (c) engineering stress-strain curves for machined and as-built coupons; (d) S-N comparison of as-built versus machined coupons. Adapted under a CC BY 4.0 license [108].

Figure 11. Effect of subtractive machining on the performance of WAAM ER70S-6 steel plates: (a) as-built and machined plate conditions and coupon extraction scheme; (b) representative fracture surface; (c) engineering stress-strain curves for machined and as-built coupons; (d) S-N comparison of as-built versus machined coupons. Adapted under a CC BY 4.0 license [108].

7.2. In-Situ Heat Treatment in Hybrid WAAM

Thermal management through in-situ heat treatment provides a complementary pathway for controlling phase stability, residual stress evolution, and recrystallization kinetics alongside deformation-assisted processing. In contrast to conventional ex-situ heat treatment applied after fabrication, in-situ thermal control occurs during or between deposition passes and is inherently coupled with the transient thermal cycles of the WAAM process. Its primary role is to regulate the balance between strain hardening, recovery, and recrystallization.

For Ti-6Al-4V, mill annealing (sub-transus, 700–785 °C) produces a fine equiaxed microstructure with superior balance of tensile strength and ductility, while beta annealing (super-transus, 1015–1050 °C) forms a lamellar microstructure that improves fracture toughness and fatigue crack growth resistance. For Inconel 718, solution treatment at approximately 980–1050 °C dissolves secondary phases, followed by two-stage aging (e.g., 720 °C then 620 °C) to precipitate gamma’ and gamma’’ phases. For heat-treatable aluminum alloys, T6 treatment (solutionizing, quenching, and artificial aging) maximizes strength by precipitating fine strengthening particles. Ex-situ and in-situ heat treatments are best viewed as complementary strategies: in-situ thermal control governs microstructure evolution during fabrication, while ex-situ treatments are applied selectively to optimize final properties.

8. Challenges and Future Outlook

8.1. Process Integration and Scalability

Scaling from laboratory feasibility studies to industrial-scale production remains a primary obstacle. Maintaining consistent thermal histories and uniform microstructural evolution across large build volumes is still difficult, and integrating deformation processes such as rolling, FSP, or ultrasonic vibration into large-scale WAAM systems presents challenges in equipment design, tool accessibility, and process synchronization [21,99].

8.2. Process Control, Optimization, and Closed-Loop Monitoring

Hybrid WAAM introduces additional process variables including deformation timing, strain magnitude, and temperature control. Moving from open-loop fixed parameters to closed-loop control is a critical requirement for next-generation hybrid WAAM systems. AI-driven closed-loop systems with machine learning and digital twins offer promising avenues for developing adaptive control strategies capable of responding dynamically to bead height variations, thermal fluctuations, or local microstructural conditions during the build [111,112].

8.3. In-Situ Monitoring and Defect Detection

Real-time monitoring of microstructure evolution and defect formation remains a major challenge. Techniques such as thermal imaging, acoustic emission monitoring, and laser scanning are being explored, but integration into a unified control system is still limited. Future research should focus on integrated hybrid WAAM platforms combining multiple deformation techniques; real-time closed-loop control strategies supported by machine learning and high-fidelity digital twins; expanding hybrid WAAM to emerging material systems including high-entropy alloys, advanced magnesium alloys, and functionally graded bimetallic structures; and developing standardized qualification frameworks incorporating deformation and machining steps to accelerate industrial adoption for fatigue-, creep-, and fracture-critical applications.

8.4. Extension of Materials and Hybrid WAAM Processes

High-entropy alloys (HEAs) containing five or more principal elements in near-equal composition represent a growing frontier for hybrid WAAM. Fe-Mn-Cr-Ni material systems have been processed by hybrid WAAM for in-situ alloying using multiple elemental wires, creating complex solid solutions with exceptional ductility (80% elongation) [113]. Advanced magnesium alloys also offer significant opportunity, with WAAM of scandium-modified magnesium alloys achieving yield strength of at least 200 MPa [114]. Copper alloys and bimetals present particular fabrication challenges; Joshi et al. [115] used wire WAAM to process low-alloyed steels, copper-aluminum alloys, and their bimetals, creating functionally graded materials for heat exchanger applications.

8.5. Toward Fully Integrated Additive-Deformation-Machining Systems

The future of hybrid WAAM lies in fully integrated manufacturing platforms that combine additive deposition, deformation processing, subtractive machining, and real-time monitoring. Such systems will enable adaptive process control, defect mitigation during fabrication, and near-net-shape manufacturing with tailored properties. Achieving this vision requires digital twins capable of predicting thermal, mechanical, and microstructural evolution in real time. The present authors’ patented ADM process [103] represents one practical step toward this fully integrated vision. As standards organizations begin to include deformation and machining steps within qualification frameworks for fatigue-, creep-, and fracture-critical parts, the path toward widespread industrial adoption will become clearer.

9. Conclusions

Hybrid WAAM processes incorporating deformation-assisted techniques represent a significant advancement in additive manufacturing of metallic materials. By introducing controlled thermomechanical conditions, these processes overcome the inherent limitations of conventional WAAM, including coarse columnar microstructures, anisotropy, and defect formation. The following conclusions can be drawn from this review:

(1) Deformation-assisted WAAM processes—including interlayer rolling, hammer peening, FSP, laser shock peening, ultrasonic impact treatment, and ultrasonic vibration—provide effective and complementary pathways for microstructure refinement and defect mitigation across major alloy systems. Each technique has distinct strengths and limitations that must be considered in relation to material system, component geometry, and production scale.

(2) Microstructure evolution in hybrid WAAM is governed by coupled thermomechanical mechanisms, including dislocation accumulation, recovery, static and dynamic recrystallization, and severe plastic deformation. The Zener-Hollomon parameter provides a useful framework for predicting the operative mechanism under given deformation conditions. High-SFE alloys (e.g., aluminum) favor cDRX under FSP, while low-SFE alloys (e.g., steels and Ni-based superalloys) tend toward dDRX.

(3) Hybrid WAAM significantly improves mechanical performance, with yield strength gains of 20–40%, elongation gains of 10–30%, and substantially enhanced fatigue resistance compared with as-deposited WAAM. These improvements are mechanistically linked to Hall-Petch strengthening from grain refinement, texture randomization, porosity closure, and beneficial compressive residual stresses.

(4) Ultrasonic-assisted WAAM offers unique capabilities for simultaneously controlling solidification microstructure and introducing solid-state deformation, representing a promising direction for future research, particularly for alloys susceptible to hot cracking and hydrogen-related porosity.

(5) The integration of subtractive machining into hybrid WAAM workflows provides dimensional accuracy and surface integrity improvements that are synergistic with deformation-induced microstructural benefits, with fatigue life improvements of up to 60% demonstrated relative to as-built WAAM surfaces.

(6) Future progress requires the development of closed-loop control systems, machine learning-based process optimization, high-fidelity digital twins, and expansion of hybrid WAAM to emerging materials including high-entropy alloys, advanced magnesium alloys, and bimetallic structures. Standardized qualification frameworks incorporating deformation and machining steps are also needed to support industrial adoption.

Overall, hybrid WAAM enables the transition from purely thermal additive manufacturing to thermomechanically engineered microstructures, opening new opportunities for high-performance structural applications in aerospace, energy, defense, and transportation. Continued progress in process integration, monitoring, and qualification will be essential to realize the full industrial potential of this technology.