Hybrid directed energy deposition process coupled with plastic deformation

Laser directed energy deposition (LDED) process has unique advantage in rapid forming of large-sized metal components, gradient material/structural components, or repairing/remanufacturing worn parts. However, the high residual stress and strong anisotropy in mechanical properties of the as-deposit components limit the application of LDED technology in the manufacturing of key structural components. To overcome these problems, various hybrid additive manufacturing (HAM) technologies have been developed, such as plastic deformation, ultrasonic or magnetic field assisted LDED processes to improve the quality and the mechanical properties, where these coupled processes are carried out either simultaneously or cyclically with the LDED process. The hybrid additive manufacturing, while retaining the advantages of individual forming process, avoids the mutual interference between each process and reducing the adverse effects generated if used separately. Hybrid additive manufacturing processes fundamentally change the underlying physical mechanisms of molten pool dynamics, microstructural evolution, temperature and thermal stress gradient in additive manufacturing, thereby optimizing the microstructure and performance of the manufactured components. In this paper, the key technical features of the hybrid additive manufacturing process coupled with plastic deformation were described in details, and the resulting differences in microstructure, residual stress, and mechanical properties of the prepared samples were systematically analyzed. The developing trend of hybrid additive manufacturing processes in coupling mechanisms, parameter optimization, and equipment have been discussed.


Introduction
Laser directed energy deposition (LDED) uses a laser beam to melt raw material powders and depositing the materials layer by layer along a preset path to achieve the near-net formation of three-dimensional components.Argon or other protective gases are normally used during the manufacturing process to protect from oxidation [1,2].LDED process has a large degree of freedom in the fabrication of complex components, including the in-situ deposition of multi-materials [3,4] and the production of large-size components, such as large-scale mirror brackets and frames [2].In addition, LDED process can also be used to repair high-value-added worn parts, such as turbine blades and bearings [5].LDED is a fast, strong non-equilibrium metallurgical process coupled with multiple physical fields.The quality of the as-deposited samples is highly dependent on the equipment and processing parameters, such as powder particle size, laser spot diameter and gas flow rate, and small changes could bring great differences to the final products [6].The extremely high temperature gradient and nearly onedimensional heat dissipation during the LDED printing process leads to the epitaxial growth of columnar grains with relatively high residual stress, which limits the application of LDED technology in largesized structural parts [7].Excessive residual stress will lead to deformation, poor dimensional accuracy and the delamination of the deposited layers, resulting in the reduction of fatigue performance and fracture toughness.Although post-treatment, such as heat treatment and hot isostatic pressing, can alleviate the above problems, they will increase costs and uncertainties [8,9].Therefore, how to realize the high-performance of LDED metal components has been the key research interests in the industry for a long time.With the rapid development of LDED technology, researchers have gradually realized that it is difficult to solve the problems by optimizing the process parameters or adjusting the printing strategies.To resolve the above constraints, hybrid additive manufacturing (HAM) processes have been developed by adding auxiliary processes to AM to modify or eliminate manufacturing defects and residual stress.HAM was defined as a process that incorporates one or more conventional or other types of processing technologies into AM in a single-step process or a step-by-step process [10][11][12].The HAM process avoids or reduces the mutual interference between various processes and the adverse effect generated if used separately, while retaining the advantages of these processes.The HAM is more widely applicable in LDED, and the process advantages if brings also vary [13][14][15].Due to its high processing flexibility, the LDED process has become the most common platform integrated in HAM [16][17][18][19].In this paper, the classification and the research progress of HAM process coupled with plastic deformation are reviewed, and the equipment characteristics and underlaying mechanisms of the different processes are compared and analyzed.Finally, the development of HAM is presented.[20][21][22][23][24][25][26].

HAM processes coupled with plastic deformation
Plastic deformation applied to solid or semi-solid metals at medium/high temperatures can introduce a large number of dislocations, and the grain structure can be refined through dynamic recrystallization during layer-by-layer thermal cycling, and reducing the generation of defects, such as pores and cracks.The HAM process coupled with plastic deformation processes, which can be categorized as rolling, forging, shot peening and hammering, regulates the stress distribution and microstructure of asdeposited materials by inducing a certain depth of deformation into the deposition materials, consequently to influence the mechanical properties.Figure 1 summarizes the various types of HAM processes coupled with plastic deformation and their major influence on LDED materials.

In-situ rolling assisted LDED
The rolling process has the advantages of refining grain structure and regulating stress distribution, thereby improving the mechanical properties of LDED components and reducing cracking.The rolling process introduced in the LDED mainly includes interlayer rolling, i.e. the rolling is carried out after each pass/layer deposition, and in-situ rolling, i.e. the rolling is conducted along with the deposition.Interlayer rolling is often carried out at room temperature or relatively low temperature to obtain the best grain refinement effect, while in-situ rolling belongs to warm rolling and the deposition layer is in a medium temperature state when plastic deformation occurs.The rolling process was first introduced to the wire-arc additive manufacture (WAAM).Colegrove et al. [27][28][29][30] have carried out research on the interlayer high-pressure cold rolling in WAAM and investigating the effect of roll shape and cold rolling passes on the microstructure, properties and stress of components.A hydraulically loaded roll was used to apply plastic deformation on the deposited material after each finished layer.The results showed that the columnar grains formed during the rapid solidification were broken into equiaxed grains by interlayer rolling and the grain size was significantly reduced.The co-deformation of the fine grains at the surface and the coarse grains within delayed the occurrence of necking.The tensile strength and plasticity increased with the rolling load while the residual stress decreased [31].A HAM process coupled with in-situ micro-rolling was developed by Zhang et al. [22,[32][33][34] by adding a roller after the welding torch to realize the warm rolling of the deposited material.The deformation temperature could be determined by the distance between the welding torch and the roller, as well as the heat exchange and heat conduction of the material.As the distance between the welding torch and the roller reaches a certain value, the deposited material could undergo plastic deformation near the recrystallization temperature.The roller is water cooled and has a chilling effect on the deposition materials during rolling, which helps to cool the deposited layer below 200 C rapidly and solving the problem of low efficiency caused by interlayer cold rolling.Due to the high rolling load required for cold deformation, the degree of rolling deformation in WAAM cannot completely refine the whole layer of the deposited material, resulting in microstructural inhomogeneity and bringing uncertainty to the service stability of the components.Huang et al. [35][36][37] have studied the effect of in-situ micro-rolling during LDED process, as shown schematically in figure 2   Bi et al. from Jihua Laboratory have developed an in-situ rolling assisted LDED process by adding a roller next to the deposition head.The rolling unit is integrated into the machine tool and the collaborative movement of the machine tool is synchronized with a robot mounted laser/powder deposition head.The industrial sized in-situ rolling assisted LDED system is capable of manufacturing large-size components up to 2000 mm 800 mm 800 mm with a maximum rolling load of 12 kN, as shown in figure 3(a).The rolling axis is kept perpendicular to the moving direction of the deposition head and a pressure sensor is used to adjust the load change with real-time.In addition, the system is equipped with infrared temperature measurement to monitor the temperature change of a specific position during the printing process.The distance between the mini-roller and the molten pool can be adjusted from 20 mm to 60 mm to achieve different deformation temperatures.So far, various alloy systems, such as Ti6Al4V, Inconel 718, 316L, 15-5PH and Invar 36 alloys, have been investigated with this HAM process.As shown in figure 3(b), the columnar grains of Ti6Al4V grown along the building direction during normal LDED process have been changed to smaller equiaxed grains after in-situ rolling.With the increase of rolling force from 4 kN to 8 kN, the yield strength of Ti6Al4V samples increased from 905 MPa to 941 MPa in addition to the increase of the elongation from 8.79% to 10.89%, achieving a synergistic improvement in strength and plasticity mainly due to the grain refinement and defects reduction.

Synchronous-hammer-forging assisted LDED
Rolling assisted LDED process has a positive effect on the control of microstructure and mechanical properties of as-deposited materials, however, this process has limitations in fabricating weakly rigid metallic parts due to in-plane rolling.Synchronous-hammer-forging-assisted LDED (SHLDED) process has been developed by Niu et al. [3] by coupling a mechanical hammering device into LDED system for manufacturing weakly rigid metallic components.This HAM equipment [see figure 4(a)] mainly consists of a mini-hammering unit and a control/measuring unit.A bending structure of the hammer head is designed to realize the controllable distance between the hammer head and the deposition position, to achieve plastic deformation within different temperature zones.The deposition head and the hammering unit are installed on two separate manipulators, which maintain relative movements during the printing process to complete the collaborative manufacturing.In-situ plastic deformation of the deposited material is achieved by synchronously applying high-frequency hammering, while the output hammering force is controlled by changing the input voltage of the hammering power supply.shows the microstructure of the SHLDED processed 316L sample.The distance between the hammering unit and the molten pool was set at 20 mm to control the deformation temperature at about 700 C, closed to the recrystallization temperature of 316L [38], so that a small load could generate relatively large plastic deformation.The grain structure of the deposited material was deformed with severe plastic deformation resulting in the breakdown of the original irregular grains into fine equiaxed grains [39].The microstructure of the pre-solidified layers recovered and recrystallized by the heat generated during the subsequent deposition.Although the grain morphology of the SHLDED fabricated 316L samples is dominated by columnar grains and dendrites, the length of the columnar grains is reduced compared with that fabricated by LDED.In the region near the free surface, the microstructure shows typical fibrotic characteristics, while the middle part of the sample consists of equiaxed dendrites.Synchronous hammer forging has an obvious grain refinement effect and the average grain size of SHLDED samples is about 17 m (versus 55 m without synchronous-hammer-forging).SHLDED samples showed significant improvement in yield strength mainly due to the strengthening of grain refinement and dislocation strengthening.As shown in figure 4, the yield strength of SHLDED 316L samples has increased by 38% in the vertical direction and by 41% in the horizontal direction.

Shot Peening assisted LDED
Shot peening (SP) as a strengthening process uses a high-speed projectile flow to impinge on material surface to undergo elastic-plastic deformation and generating residual compressive stress to improve the fatigue resistance and the surface integrity of parts [40][41][42].By introducing compressive stress and gas cooling to the components to reduce heat accumulation and generating plastic deformation [42], HAM process coupled with shot peening can effectively improve the surface quality, microhardness and wear resistance of the printed metal components [43][44][45][46].
Comparing with room temperature shot peening, the in-situ shot peening in the HAM process is carried out at a medium temperature, therefore it is easy to generate large compressive stress and plastic deformation.Moreover, the surface oxide film of the sample can be peeled off by the in-situ shot peening during the printing process [47], making the surface of the component smoother and improving the mechanical properties of the component to a certain degree.Since some shots inevitably enter the molten pool to form defects, shots made of the raw powder material is generally recommended.By coupling shot peening process on a LDED system, Li et al. [48] developed shot peening LDED, combining the advantages of surface strengthening with AM process.The shot peening nozzle is fixed next the deposition head through an adjustable clamping device, and the shots gas flow is generated by the air compressor, as shown in figure 5(a).The collaborative improvement in surface performance and the internal properties is achieved layer-by-layer through the deposited layer surface strengthening.As the subsequent layer is deposited on the shot peening LDED Fe-based alloys, the austenitic dendrite in the pre-solidified layer is refined by recrystallization.This recrystallization process is triggered by the thermal heat from the subsequent deposition and the strain energy induced by the shot peening, and resulting in more uniform equiaxed grain structure of 10 ~ 100 m, as shown in figure 5(b)-(c).Moreover, the cold gas brought by shot peening reduces heat accumulation and inhibiting the coarsening of columnar grains.The mechanical properties of Fe-based alloy samples fabricated by shot peening are significantly improved comparing with those by LDED.As shown in figure 5(d), the tensile strength, yield strength and microhardness are increased by 12%, 53% and 7% respectively, whereas the elongation is reduced by 38% primarily due to the increased dislocations and micro strain leading to the increased strain hardening [49].By applying in-situ shot peening, the residual tensile stress on the sample free surface changes to compressive stress.When shot peening is used as a post-treatment process, its strengthening depth is about 0.1 ~ 0.8 mm, while the influence depth of HAM can reach 2.5 mm or above.

Ultrasonic impact treatment assisted LDED
Ultrasonic impact treatment (UIT) adopts the combined action of ultrasound and high-frequency mechanical impact to cause severe plastic deformation on the component surface [20,[50][51][52][53], resulting in grain refinement and introducing a large amount of residual compressive stress.The fatigue, corrosion and tribological properties of the components after UIT can be significantly improved, while the surface roughness and internal porosity are reduced [54][55][56][57].Jiang et al. [58] introduced UIT to LDED by a converter, as shown in figure 5(a).The LDED and UIT processes are carried out alternately, i.e. the deposition head is converted into an ultrasonic impact head after each layer deposited, as shown in figure 6(a).A similar ultrasonic micro-forge treatment (UMT) has been developed by Zhao et al. [59], where an ultrasonic strengthening device is used to conduct high-frequency impact on material surface to produce plastic deformation.This process has been successfully used for residual stress relief, grain refinement, performance enhancement and defect repair [60][61][62].The dynamic impact pressure generated by ultrasonic device creates large compressive stress and shear strain, and effectively controlling the formation of defects during the HAM process by promoting the closure of micropores in materials [63].The top layer of the sample consists of fibrous microstructure and a large number of dislocations [50].The dislocations are rearranged under the combined action of strain and temperature, while some of the dislocations are consumed due to the transformation from low angle grain boundaries (LAGB) to high angle grain boundaries (HAGB) to form new fine grains, which may coarsen rapidly as subsequent layers are deposited, as shown in figure 6(b).The large numbers of dislocations introduced by UIT also promote the recrystallization.Since the degree of deformation decreases with the increase of depth, the grain refinement also decreases with the depth resulting in gradient structure of equiaxed grains.The improved mechanical properties of the ultrasonic impact treatment are mainly due to grain refinement and dislocation strengthening [62,64].With UIT applied on each deposited layer, the generated fine equiaxed grains effectively improve the strength of the samples.When a tensile stress is perpendicular to the printing direction, micro-cracks will be activated at the columnar grain boundaries where high density of dislocations accumulate, resulting in a decrease in plasticity.The gradient grain structure of UIT samples can effectively enhance the co-deformation between grains and improving the plasticity of the samples.Although the high-density of dislocations enhances the compressive strength, the dislocation hardening effect has detrimental effect on materials' plasticity [57].As the contribution from grain refinement is greater than the detrimental effect of dislocation strengthening, the total elongation of the samples fabricated by HAM is improved comparing with that fabricated by LDED [See figure 6(c)-(d)].

Laser shock peening assisted LDED
As a surface treatment process, laser shock peening (LSP) uses a short pulse laser beam with a highenergy density to act on the component surface, so that the material absorbs laser energy and rapidly heating up and vaporizing to generate a strong transient laser shock wave, causing local plastic deformation with high strain rates [65][66][67][68].The residual compressive stress generated can reach up to several millimeters in maximum depth, to offset part or all of the tensile stress on the component surface.The effect helps to reduce crack growth and increasing the critical stress of crack growth, thus improving the strength, fatigue resistance, wear resistance and corrosion resistance of the metal components [24,[69][70][71].LDED process coupled with LSP can be divided into layer-by-layer LSP at medium temperature and laser micro-forging process acting directly on the high temperature molten pool.Layer-by-layer LSP process is developed by Liu et al. [23] to improve the surface quality and the mechanical properties of AlSi10Mg alloy.The device consists of an in-situ LSP unit and LDED system, where the LSP unit is installed next to the deposition head to achieve the synchronized LSP during printing, as show in figure 7(a).The distance between the pulsed laser and the continuous laser is about 5 mm, which is larger than the size of the molten pool formed by LDED to ensure the LSP act on the solidified matter.Grains near the free surface are significantly refined with the LSP treatment, whereas the grain morphologies farther away from the treated surface show little change due to the limited penetration depth of the LSP.With layer-by-layer LSP treatment, the grain structure shows less preferred orientation due to the combined action of heat flow and compressive impact pressure.The upper surface treated by LSP is mainly consisted of ultrafine equiaxed grains, as shown in figure 7  Based on LSP, Zhang et al. [72] proposed laser shock forging (LSF) process, which uses two laser beams simultaneously and cooperatively to manufacture metal parts, as shown in figure 8(a).The first continuous laser beam is used for AM, while the second short pulse laser beam acts directly on the surface of the deposited layer at high-temperatures.The distance between the two laser beams can be adjusted to ensure the material is cooled to the forging temperature range.Hu et al. [25] developed a HAM process, named as laser shock modulation of molten pool (LSMMP), in which the action region of pulsed laser moves from solidified or semi-solidified surface to the molten pool, and greatly reducing the impact pressure required for plastic deformation [See figure 8(b)].The shock wave induced by pulsed laser propagates in the liquid medium of the molten pool, leading to further enhance the convection, homogenizing the solute distribution and improving the cooling rate.By disturbing the molten pool, the solidified microstructure could be greatly refined and the residual stress is regulated, where the residual stress can be further reduced under condition of low laser energy input.The laser forging process, different from LSP, does not need a protective layer and constraint layer.The laser beam directly irradiates the medium or high temperature deposited materials, where shock waves are generated as materials vaporize and ionize, as shown in figure 8(c).LSP generally strengthens the components at room temperature, while laser forging acts on the medium or high temperature deposition materials in-situ during printing.Moreover, the main functions of LSP are to regulate residual stress, followed by microstructural modification, and it is difficult to use LSP to control internal defects during printing.The purpose of laser forging is to eliminate pores, microcracks and other defects inside the deposited materials at medium or high temperatures, enhancing the mechanical properties and altering the residual stress state [73].The residual stress distribution of as-deposited components can be adjusted by varying the distance between the continuous and the pulsed lasers.Plastic deformation is more likely to occur as the pulsed laser acts on the semi-solidified region than the fully solidified area.The value of the compressive stress at the top region of deposited layers decreases with the increasing distance between the two laser beams, as shown in figure 8(d).Comparing with the traditional post-treatment process, laser forging process is regarded as efficient method in residual stress control.

Conclusions and perspectives
LDED coupled with plastic deformation either synchronously or on each pass/layer can introduce severe plastic deformation to achieve defect control, residual stress regulation and performance strengthening on the deposited materials.HAM processes can fundamentally change the underlying physical mechanisms of molten pool dynamics, microstructural evolution, temperature/stress field.The use of different types of HAM processes is based on the materials, the geometric characteristics of the forming parts and the functional requirements.
Overall, LDED processes coupled with plastic deformation have unique advantages in microstructure refinement, defects reduction and residual stress control.The degree of the influence is summarized in Table 1, where disadvantages of each technique is also listed.With the fast development of additive manufacturing technologies, researches on new types of hybrid AM techniques will continue attract wide interests, as well as practical applications.Nevertheless, we still face many challenges in HAM processes, as discussed below: (1) The mechanical properties of metal components fabricated by HAM are affected by surface roughness, grain morphology, microstructure, residual stress and other factors, therefore, any parameters acting on the above factors can affect the properties of the final parts.The collaborative control between the auxiliary processes and AM process, the process sequence and its acting mechanism in manufacturing large-sized metal component are to be addressed urgently.
(2) HAM processes introduce new degrees of freedom on the basis of complex temperature/stress field inherent in LDED process, which brings huge difficulties to process control and process parameter optimization.Therefore, establishing a numerical model of temperature/stress field to accurately predict the stress distribution within the deposited layers and on the interfaces will be crucial for applying HAM in manufacturing high-quality and high-performance components.
(3) The microstructure of the deposited material is very sensitive to the processing parameters, and also affected by the heat conduction of the deformation roller.Therefore, there is need to further study the evolution of the microstructural stability and the grain morphology in the process of HAM by establishing the internal relationship between the process parameters and the temperature field.It is important to study the relationship between the process characteristics, materials, microstructural evolution and mechanical properties for the promotion and application of HAM.
(a).The results indicated that an in-situ micro-roller could in generally refine the grain structure.The yield strength of Inconel 718 has increased by nearly 50% after in-situ micro-rolling, as shown in figure2(b) while both the yield strength and the plasticity of Ti6Al4V were improved and the anisotropy of the mechanical properties was reduced [See figure2(c)-(d)].Microstructural examination showed that the prior- grains were broken into equiaxed grains and the average grain size decreased from 536 m to 130 m after the in-situ micro-rolling.

Figure 3 .
Figure 3. (a) Schematic diagram of in-situ rolling assisted LDED process.Microstructure of Ti6Al4V fabricated by HAM process under (b) 0 kN and (c) 4 kN, (d) tensile properties of Ti6Al4V under different rolling force.

Figure 4 (
Figure 4(b)  shows the microstructure of the SHLDED processed 316L sample.The distance between the hammering unit and the molten pool was set at 20 mm to control the deformation temperature at about 700 C, closed to the recrystallization temperature of 316L[38], so that a small load could generate relatively large plastic deformation.The grain structure of the deposited material was deformed with severe plastic deformation resulting in the breakdown of the original irregular grains into fine equiaxed grains[39].The microstructure of the pre-solidified layers recovered and recrystallized by the heat generated during the subsequent deposition.Although the grain morphology of the SHLDED fabricated 316L samples is dominated by columnar grains and dendrites, the length of the columnar grains is reduced compared with that fabricated by LDED.In the region near the free surface, the microstructure shows typical fibrotic characteristics, while the middle part of the sample consists of equiaxed dendrites.Synchronous hammer forging has an obvious grain refinement effect and the average grain size of SHLDED samples is about 17 m (versus 55 m without synchronous-hammer-forging).SHLDED samples showed significant improvement in yield strength mainly due to the strengthening of grain refinement and dislocation strengthening.As shown in figure4, the yield strength of SHLDED 316L samples has increased by 38% in the vertical direction and by 41% in the horizontal direction.

Figure 5 .
Figure 5. (a) Schematic diagram of HAM process coupled with shot peening, (b)-(c) microstructure and (d) tensile properties of Fe-based alloy samples fabricated by HAM and LDED [48].

Figure 6 .
Figure 6.(a) Schematic diagram of HAM process coupled with UIT, (b) microstructure, (c) Vickers hardness and (d) mechanical properties of 304 stainless steel samples fabricated by HAM and LDED [58].
(b).The compressive stress after micro-forging LSP treatment extends up to ~500 m in depth and the value of the residual stress remains relatively stable.The tensile residual stress after layer-by-layer LSP treatment is largely suppressed, and the depth of the compressive residual stress affected zone is almost twice than that of micro-forging LSP treated samples [See figure 7(c)].The trade-off between the strength and the ductility is solved through grain refinement, dislocation strengthening and residual compressive stress.The feasibility of LSP assisted LDED in the fabrication of large-sized aluminum alloy components has been verified by others [See figure 7(d)].

Table 1
Comparison of LDED process coupled with different plastic deformation techniques.