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Journal of Materials Science & Technology

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Solid-state additive manufacturing and repairing by cold spraying:

A review
Wenya Li a,∗ , Kang Yang a , Shuo Yin b , Xiawei Yang a , Yaxin Xu a , Rocco Lupoi b
a
State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Friction Welding Technologies, School of Materials Science and Engineering,
Northwestern Polytechnical University, Xi’an 710072, China
b
Trinity College Dublin, The University of Dublin, Department of Mechanical and Manufacturing Engineering, Parsons Building, Dublin 2, Ireland

a r t i c l e i n f o a b s t r a c t

Article history: High-performance metal additive manufacturing (AM) has been extensively investigated in recent years
Received 17 February 2017 because of its unique advantages over traditional manufacturing processes. AM has been applied to form
Received in revised form 10 June 2017 complex components of Ti, Fe or Ni alloys. However, for other nonferrous alloys such as Al alloys, Mg
Accepted 19 June 2017
alloys and Cu alloys, AM may not be appropriate because of its melting nature during processing by laser,
Available online xxx
electron beam, and/or arc. Cold spraying (CS) has been widely accepted as a promising solid-state coating
technique in last decade for its mass production of high-quality metals and alloys, and/or metal matrix
Keywords:
composites coatings. It is now recognized as a useful and powerful tool for AM, but the related research
Cold spraying
Additive manufacturing
work has just started. This review summarized the literature on the state-of-the-art and problems for CS
Repairing as an AM and repairing technique.
Strength © 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science &
Ductility Technology.

1. Introduction and electro-chemical deposition. While some other processes are

based on solid-state welding, i.e. block joining (e.g. friction weld-
Unlike conventional material removal methods, additive ing), ultrasonic welding and cold spraying. Cold spraying (CS) seems
manufacturing (AM) is based on a novel-materials-incremental- the most popular solid-state process because it works like the “3D
manufacturing philosophy. AM implies layer by layer shaping and printing” process, which can be used to repair turbine and com-
consolidating of powders/wires to arbitrary configurations, nor- pressor blades without changing their highly complex underlying
mally through melting them by using computer controlled laser, crystal structure [6]. CS could build entire new parts with walls as
electron beam, and/or arc [1,2]. For example, laser based AM pro- thick as one inch or more, for example, a gear made by controlling
cesses have a complex non-equilibrium physical and chemical a spray nozzle motion and external motor-drive as shown in Fig. 2
metallurgical nature [1,2]. Generally, AM has been applied in pro- [7].
ducing complex components of Ti, Fe or Ni alloys. However, due to The phenomenon of CS was discovered in the early 1980s at
melting, it seems not so good for nonferrous alloys such as high- the Institute of Theoretical and Applied Mechanics of the Rus-
strength Al alloys, Mg alloys or Cu alloys [3,4]. Hence, an alternative sian Academy of Sciences in Novosibirsk (ITAM of RAS) [8,9]. As
AM technology is particularly necessary. an emerging solid-state coating technology for enhancing surface
Compared with traditional AM techniques, the generalized AM properties, the interest in CS has been increasing at a relatively
processes are outlinedin Fig. 1 [5]. The inner part of Fig. 1 rep- high rate in the past two decades, as reflected by the number of
resents in a narrow sense the traditional AM processes based on publications per year as shown in Fig. 3. Academic publications on
laser and electron beams to form parts layer-by-layer with the CS include books and review articles, covering various aspects of
aid of CAD/CAM. The outer part of Fig. 1 gives an idea to under- the process and its applications [8–17]. In this process, substrate is
stand the generalized AM processes based on allied energy sources exposed to a high-velocity (300–1200m/s) stream of small metallic
[5]. It should be noted that of the AM technologies, several pro- particles accelerated by a supersonic jet of compressed gas at a tem-
cesses are based on the deposition at atomic scale, e.g. PVD, CAD perature lower than the melting point of the sprayed material(s).
Beyond a critical velocity defined by the material properties and
process conditions, effective bonding can be obtained [18–21]. To
∗ Corresponding author. reveal the bonding mechanisms in CS, three different bonding phe-
E-mail address: liwy@nwpu.edu.cn (W. Li). nomena have been proposed: (1) bonding is by surface adhesion

http://dx.doi.org/10.1016/j.jmst.2017.09.015
1005-0302/© 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

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Fig. 1. Additive manufacturing (AM) in a broad sense. Noting that the inner part shows the AM processes in a narrow sense.

Fig. 2. A gear made by cold spraying [7].

resulted from an interface-instability-induced physical anchoring Fig. 3. Publications of cold spraying per year.
effect; (2) through partial melting and fusion of materials in a heav-
ily deformed region; and (3) through fracture of the surface oxide(s)
which covers sprayed particles and substrate [22]. In addition, from
another perspective, metallurgical bonding and mechanical inter- and even nanostructured metallic materials [26–36]. In addition,
locking are commonly perceived to be two mechanisms of the the thickness growth of CS coating has almost no limitation. There-
metallic bonding in CS [23]. As schematically shown in Fig. 4 [24], fore, CS is not only known as a solid-state process for applying
the compaction, deformation and plastic flow of sprayed particles coating to surface and/or repair structure, it is also anoption of
under high impact pressure will remove the oxide films from sur- sound AM techniques [8,9].
face, and thus a large area of fresh metal is exposed, which allows In the present review, therefore, cold spraying additive man-
intimate metal-to-metal contact. ufacturing (CS-AM) and cold spraying repairing (CS-repairing)
Because of low processing temperature, in comparison to the are summarized and discussed from a materials perspective. The
melting AM techniques and conventional thermal spraying, the current issues, problems and prospects existing in the CS-AM/CS-
deleterious effects of oxidation, phase transformation, decom- repairing are explored. This review can help researchers gain
position, grain growth, and other problems can be minimized in-depth knowledge of CS-AM/CS-repairing, and have a better
or eliminated [9,10,25]. Such significant advantages make CS a understanding of microstructure and mechanical properties of CS-
promising technique for fabricating a large variety of coatings, AMed/CS-repaired parts, which will facilitate the applications of CS
including most metals, alloys, metal matrix composites (MMCs) in industries.

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Fig. 4. Schematic diagram of the bonding process of cold-sprayed particles accompanying with the breaking-up and extruding of surface oxide films and the formation of
jetting [24].

Fig. 5. A CSed steel bracket, first cold sprayed onto a substrate, then released and
machined to the required shape [41].

Fig. 7. Overview of a typical fin array [42].

initially sprayed over a shaped mandrel substrate, and then the

substrate was subsequently removed, and finally the bracket was
finish machined. Fig. 6 shows a CS-AMed part of copper and tool
steel with a cooling channel (left: drawing, right: real part) from
Hermle Maschinenbau Gmbh, Ottobrunn, Germany [9]. Cormier
et al. [42,43] explored the manufacturability of pyramidal fin arrays
by CS. Near-net shaped pyramidal fin arrays of various sizes and fin
densities were manufactured using masks made of commercially
Fig. 6. Example of an additively manufactured part with Cu and tool steel with
cooling channel-drawing (left) and real part (right) [9]. available steel wire mesh as shown in Fig. 7. Additionally, CS-AM
has also been used in producing a sputtering target which is widely
applied in flat display, photovoltaic panels and other industries.
2. Cold spraying additive manufacturing Compared to plasma spraying, the quality of a cold-sprayed tar-
get is better, with a density of 95%-98%, a purity over 99.95% and
Although originally used as a coating technology, CS can achieve an oxygen content of lower than 1000 ppm. Some related patents
coatings without a limit in thickness produced. In comparison have been lodged in USA, Japan, etc. [44,45]. Fig. 8 shows a large
toother AM methods, CS involves neither high-temperature pro- rotation target fabricated by CS, as reported by Plasma Giken Co.,
cesses, such as Selective Laser Melting (SLM) [37,38] and Direct Ltd. [46].
Metal Deposition (DMD) [39], nor ecologically troublesome chem- During CS-AM, machining is usually required. Two different
ical processes, such as electroplating. Therefore, CS is capable of strategies can be considered. One is final machining of near-net
creating 3D shapes of various geometries and has been regarded as shape objects made by CS. In this case, the process could be divided
one of the standard AM processes (ASTM F2793-12A) as listed in in two stages: fabrication of the objects using CS, and final treat-
Table 1 [40]. ment by machining in order to achieve required dimensions and
For examples, Fig. 5 shows a steel bracket created by CS at tolerances. The otherone is the application of machining process
the United Technologies Research Center [41]. The structure was during CS. In this case, machining tool is integrated into the spray-

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Table 1
Names of AM processes described in the ASTM F2793-12A standard [40].

AM processes Description in ASTM F2793-12A standard

Material Extrusion Material is selectively dispensed through a nozzle or orifice

Material Jetting Droplets of build material are selectively deposited
Binder Jetting A liquid bonding agent is selectively deposited to join powder materials
Sheet Lamination Material sheets are bonded to form an object
Vat Photopolymerisation Liquid photopolymer in a vat is selectively cured by light-activated polymerisation
Powder Bed Fusion Thermal energy selectively fuses regions of a powder bed
Directed Energy Deposition Focused thermal energy is used to fuse materials by melting as the material is deposited
Cold Spraying Powdered material is propelled at a substrate at a sufficiently high velocity to cause adhesion and material build-up

bulks as shown in Fig. 11. It is clear that dense Cu bulk waspro-

duced by CS with helium as driving gas. But the as-sprayed bulk
has poor ductility and almost no elongation, while heat treatment
can effectively improve the mechanical properties. The heat treated
Cu at 400 ◦ C is similar to pure Cu bulk [48].
It should be noted that in addition toheat treatment, driving gas
is also one of the important factors affecting the tensile property of
CSed deposits. Huang et al. [48] reported a higher tensile strength of
Cu deposit sprayed with nitrogen compared to using air as driving
gas. Helium is much better than nitrogen as driving gas. They also
found an increase in adhesion strength with the increase in gas
temperature. In Coddet et al. [49], it even reached up to 300 MPa
using helium as the driving gas.
Coddet et al. [49] also assessed the mechanical and electri-
cal properties of Cu-Ag and Cu-Ag-Zr alloys as shown in Table 2
and Fig. 12. For Cu-3Ag-0.5Zr alloy, the ultimate tensile strength
of the as-sprayed bulk reaches 576 MPa with an elongation less
than 0.5%. After annealing (at 415 ◦ C for4 h), the ultimate tensile
strength decreases to 460 MPa and ductility increases to about 19%.
For Cu-Ag alloy, the ultimate tensile strength of the as-sprayed bulk
reaches 646 MPa with an elongation less than 0.5%. After annealing
(300 ◦ C × 4 h), the ductility becomes about 40%. The contents of Ag
and Zr have also a significant influence on the strength of bulks and
heat treatment effect.
In brief summary, dense Cu or Cu alloys bulks with high-quality
particle interface bonding and good mechanical properties can be
achieved by using expensive helium gas, but avoiding oxidation.
High strength Cu components can be produced by CS with opti-
Fig. 8. Large rotation target by cold spraying [46].
mized parameters. Fig. 13 shows a CSed Cu flange [46].

ing system in order to provide the finishing of each layer directly 2.2. Ti and its alloys
after deposition [21].
In the following, based on the existing literature, the applica- Because of the advantages of Ti and its alloys, such as low
tions of CS-AM aresummarized and discussed from a materials density, high strength-to-weight ratio and excellent corrosion
perspective. resistance, they have also been widely used in aerospace, chemical
and biological industries. Many researchers studied the microstruc-
2.1. Cu and its alloys ture and mechanical properties of CSed Ti or Ti-6Al-4V coatings
[32]. However, because of the active characteristics of Ti and its
Cu is one of the most widely studied materials because of its alloys and their unique reactions with surrounding air, it is very dif-
excellent cold-sprayability, i.e. good deformability. Previous results ficult to have a dense coating of Ti or Ti-6Al-4V [33]. Fig. 14 shows a
[15] show that dense Cu deposits can be easily produced by CS typical cross-sectional microstructure of CSed Ti and Ti-6Al–4V. For
(Fig. 9), with the interfaces between particles disappearing after pure Ti, the compactness of the deposits could be greatly improved
heat treatment. The tensile strength could be increased by 34.2% by increasing the air or nitrogen temperature, or using helium as
after heat treatment [15] (Fig. 10). Generally speaking, the hardness driving gas. Whilst for Ti-6Al-4V, a better choice is to use helium to
of as-cold-sprayed Cu films is higher than pure Cu bulk counter- get a relatively dense deposit.
parts, while annealing can effectively reduce hardness value [47]. Jahedi et al. [50] obtained dense CSed Ti deposits with nitrogen
Suchfacts can be attributed to: (1) grain refinement at the particle as driving gas. It is also very interesting that the tensile strength of
interfaces as a result of partial recrystallization during CS; (2) dis- CSed Ti can reach 800 MPa, but the ductility is very poor (∼0.01%)
appearance of the interfaces between particles and increment in as shown in Fig. 15. After heat treatment (550 ◦ C × 2 h), because
metallurgical bonding owning to the atomic diffusion during heat recrystallization occurs and residual stresses are released, the fail-
treatment. ure strain increases to ∼13% with a tensile strength of about
In some extreme spray conditions, strength of the deposits could 600 MPa. Vo et al. [51] reported the mechanical and microstruc-
be very high. Huang et al. [48] reported the effects of heat treatment tural characterizations of CSed Ti-6Al-4V and the effect of heat
on microstructure and mechanical properties of cold-sprayed Cu treatment. When using different driving gases, the porosities of

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Fig. 9. EBSD maps and inverse pole figures of the cold-sprayed Cu bulks (a) before and (b) after heat treatment. Noting that the figure in bottom-right corner of (a) shows
the corresponding ‘Band Contrast’ [15].

Table 2
Properties of Cu alloys deposits in the as-sprayed conditions [45].

Copper alloys (wt%) Oxygen content (ppm) Yield strength (MPa) Ultimate tensile strength (MPa) Elongation (%) Electrical conductivity (% IACS)

Pure copper <200 306 ± 10 320 ± 5 3.0 ± 0.5 96.9 ± 0.5

Cu-0.1Ag <200 438 ± 10 466 ± 5 4.04 ± 0.5 95.4 ± 0.5
Cu-5.7Ag <200 643 ± 10 701 ± 5 1.25 ± 0.5 74.3 ± 0.5
Cu-23.7Ag <200 646 ± 10 646 ± 5 0 ± 0.5 62.4 ± 0.5
Cu-0.1Ag-0.1Zr <200 442 ± 10 483 ± 5 7.03 ± 0.5 87.8 ± 0.5
Cu-3Ag-0.5Zr <200 576 ± 10 576 ± 5 0 ± 0.5 64.4 ± 0.5

the CSed Ti-6Al-4V deposits were greatly different (Fig. 16). The ing 765 MPa, with increasing ductility as well. Heat treatment can
porosity when using helium is lower by one order of magnitude effectively improve ductility and tensile strength of CSed Ti or Ti
than that using nitrogen due to the better particle acceleration. Ten- alloys deposits, but ductility is still lower than the bulk.
sile properties of the CSed Ti-6Al-4V deposits are shown in Fig. 17 It should be noted that based on the “tamping effect” of CS
[51]. After heat treatment, tensile strength increases by 72%, reach- [52], a novel procedure to get dense deposits with cheaper nitro-

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Fig. 10. Average tensile strengths of the as-sprayed and annealed Cu deposits [15].

Fig. 11. Tensile properties of cold-sprayed Cu [48].

gen gas was developed by Li et al. [53]. They invented anin-situ or hard particles were mixed with Ti or Ti-6Al-4V powder before
peening technique [53], which can effectively decrease the poros- spraying. During spraying, these large hard particles rebound with-
ity of deposit as shown in Fig. 18. In this procedure, large ceramics out deposition, while having a strong “shot peening” effect on

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Fig. 12. Ultimate tensile strength versus heat treating temperature for cold-sprayed copper alloys (full symbol for binary alloys Cu-0.1Ag (), Cu-5.7Ag and ( ) Cu-23.7Ag
( ) and open symbol for ternary alloys Cu-0.1Ag-0.1Zr (䊐) Cu-3Ag-0.5Zr ()) [49].

Fig. 13. Cold spraying manufacturing of Cu flange [46].

Fig. 14. Typical SEM microstructure of as-sprayed Ti (a, b) and Ti-6Al-4V (c, d) coating in the etched state [33].

the deposited layers. Therefore, relatively dense deposits could be BothCSed Ti and Ti-6Al-4V deposits have been used in AM of
obtained through adjusting the content of hard particles. With in- components. Fig. 19 shows the photos of bimetallic Ti-6Al-4V/Al
situ peening, porosity of Ti coatings decreases from 13.7% to 0.3%, and Ti/Cu plates fabricated by CS and machined after spraying [54].
and that of Ti-6Al-4V from 15.3% to 0.7%. Pattison et al. [55] also used CS to fabricate a Ti part and Ti hemi-
sphere with aidof a mould tool (Fig. 20).

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Fig. 15. (a) Tensile stress-strain curves for cold spraying fabricated commercial pure Ti before and after heat treatment, and corresponding microstructure for (b) as-sprayed
and (c) heat-treated deposits [50].

Fig. 16. Cross-section views of Ti-6Al-4V coatings sprayed with helium and nitrogen [51].

Fig. 17. Typical stress-strain curves for Ti-6Al-4V substrate, helium-sprayed Ti-6Al-4V coatings (as-sprayed and annealed at 600 ◦ C for 2 h), and nitrogen-sprayed Ti-6Al-4V
coatings (as-sprayed and annealed at 1000 ◦ C for 4 h) [51].

2.3. Al and its alloys density. Fig. 21 is a typical cross-section of CSed Al deposit [26].
For Al alloys, Rokni et al. [34,56,57] investigated the effect of heat
Al is another easily cold-sprayable material. Hence many treatment including annealing and solutionizing on CSed Al6061
researchers studied the deposition of Al and its alloys coatings. But and Al7075 deposits. Fig. 22 shows the typical microstructure of
the focus is on microstructure and corrosion performance of the Al7075 deposit [34]. It is found that MgZn2 precipitates do form
coatings, with only a few reports having been on the mechanical compared with the feedstock. Because of the high strain levels and
properties of CSed Al and its alloys [8,56,57]. temperatures during CS, particularly at particle interfaces, dynamic
Although Al powder is easily deposited, Al coatings compact- and static precipitation are expected in Al7075 [56]. Tensile prop-
ness is slightly lower than that of Cu coatings because of its lower erties of CSed Al7075 deposits are shown in Fig. 23. It is clear that

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Fig. 18. Cross sectional microstructures of the (a) Ti and (b) Ti-6Al-4V coatings deposited with pure powder and powder mixtures with different proportions of the shot
peening particles [53].

Fig. 19. Fragments of bimetal Ti-6Al-4V/Al and Ti/Cu plates fabricated by cold spray- Fig. 21. Typical cross-section of the CSed Al deposit [26].
ing and machined milling after spraying [55].

out on deposits. It was found that the annealed deposit has much
the as-deposited bulk has lower tensile strength and much lower higher fracture elongation but similar tensile strength compared
fracture elongation compared to 7075 substrate. Therefore, both to the as-deposited state. In addition, under different solutions
low-temperature and high-temperature treatments were carried and aging conditions, tensile strength is greatly increased and

Fig. 20. (a) Three sheathed thermocouples embedded within a Ti part and (b) a Ti hemisphere formed by a mould tool [55].

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Fig. 22. Typical microstructure of Al7075 deposit [34].

fracture elongation as well. In summary, the strength and duc-

tility of the CSed Al7075 deposits increase after both low- and
high-temperature treatments, which results in the precipitation of
strengthening phases and the better inter-particle bonding.
However, according to the present studies, there are some defi-
ciencies: (1) work on the effect of annealing treatment on Al alloys is
seriously lacking; (2) although the solution and aging treatment of
Al alloys bulks has been extensively investigated, the related work
on CSed Al alloys is very unsystematic; and (3) only a few Al alloys Fig. 23. Ultimate tensile strength (a) and elongation to fracture (b) of microtensile
in CS-AM have been investigated, and studies on other alloys such specimens of the CSed Al7075 deposits in the as-deposited and different heat-
treatment conditions. The tensile strength and elongation of bulk Al7075 substrate
as 2xxx and 5xxx Al alloys series need to be carried out. has also been added for comparison [56].

2.4. Stainless steel

mechanical and electrochemical properties. Therefore, CS coat-
Stainless steel coatings are generally used for protective applica- ings may become a suitable alternative to the conventional bulk
tions and represent a cost-effective method for wear and corrosion materials [60]. In general, it is difficult to produce dense stain-
protection [58,59]. Moreover, many components,like stents, are less steel deposits because of its relatively high strength. However,
mostly manufactured from stainless steel due to its well-suited a dense stainless steel deposit is readily sprayed at high particle

Fig. 24. Cross-sectional SEM micrographs and tensile properties of cold-sprayed 304L stainless steel [59].

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Fig. 26. Measured displacement of annealed cold-sprayed and conventional (bulk)

316L stainless steel with cyclic loading [60].

of Coddet et al. [59] and Meng et al. [61] can be attributed to the
different spraying parameters.
Fig. 25. Ultimate strength and fracture elongation of cold-sprayed and annealed
Fatigue behavior of CSed stainless steel has also been investi-
304L at different temperatures [61].
gated [60]. Fig. 26 shows the measured displacement of annealed
CSed and conventional 316L stainless steel deposits with cyclic
loading. Diffusion during annealing of CSed deposits can fuse par-
velocities, i.e. using high gas pressure and temperature, or with ticle/particle interfaces, which in turn, reduces the number of
helium. Several researchers have studied the deposition behavior potential crack initiation sites. The fatigue life of CSed deposits is,
and mechanical performances of stainless steel deposits consider- therefore, lower than that of the bulk material. Additionally, com-
ing in particular powder particle size, annealing post treatments pressive stresses generated from particles deposition are usually
and deposition parameters like particle velocity and gas tempera- associated with the increase in fatigue resistance. In some cases,
ture. compressive stresses within a coating may be insufficient to pre-
For examples, Coddet et al. [59] investigated the mechanical vent fatigue crack formation, leading to premature fracture [60].
properties of CSed 304L stainless steel deposits. Representative In brief summary, relatively dense stainless steel deposits can
micrographs and stress-strain curves of CSed 304L deposits are be obtained by CS. Although the mechanical properties are still not
shown in Fig. 24. A fairly dense microstructure is obtained with comparable to those of typical wrought materials, they can be effec-
low porosity (1.3%). But after heat treatment at 1050 ◦ C, the poros- tively improved through appropriate post-spray heat treatments.
ity increases up to about 2.3%, which is probably attributed to Therefore, the applications of CSed stainless steels are encouraged
the re-organization and coalescence of small defects generated at in the future work.
particle boundaries in the course of their deformation. In the as-
sprayed state, the tensile strength is 525 MPa with a brittle fracture
nature. When annealed at 400◦ C, the tensile strength is increased to 2.5. Superalloys
629 MPa without improved ductility. The high plastic deformation
of impact particles is certainly accounted for the high mechani- Superalloys are commonly used at high temperatures. However,
cal resistance of the material, however, at this stage, inter-particle during CS, the plastic deformation of hard-to-deform superalloys
bonding strength is not adequate andbrittle fractures occur. While upon particle impact, even at high gas temperatures, is believed
heat treatment at 1050 ◦ C leads to microstructure re-organization to be insufficient for a dense deposit to form. Consequently, such
and reduces the rate of stack dislocations. Thus elongation at rup- deposits would possess a low mechanical behavior similar to a
ture increases and reaches to about 22%. Meng et al. [21,61] also high-density/quality green compact, i.e. low strength without duc-
studied the influence of heat treatment on CSed 304L stainless steel tility. Therefore, in order to investigate the possibility of getting
(Fig. 25). But results show that with an increase in annealing tem- better physical and mechanical properties of CSed superalloys, pos-
perature, ultimate strength increases. The difference in the results sible post-spray heat treatments have been investigated [62–66].

Fig. 27. Optical micrograph of (a) as-sprayed, (b) sintered Inconel 718 (1250 ◦ C × 60 min), (c) flexural strength and strain results for the sintered CS samples [62].

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Fig. 28. Tensile test results and deposit porosity for as-sprayed and heat-treated Inconel 718 using nitrogen and helium as the propelling gas [64].

In summary, different spray materials exhibit different cold-

sprayability because of their physical, mechanical and chemical
properties. Tensile properties of CS-AMed deposits are summa-
rized in Table 3. Materials can be classified into three groups, 1)
hard-to-deform materials, like Ti-6Al-4V and In718; 2) relatively
soft materials, e.g. Al and its alloys, Cu and its alloys; and 3) mate-
rials of moderate strength, like stainless steel. It is very difficult to
achieve quality deposits of the first group, even with helium. It is
clear that using helium one can get the best deposit quality. All the
CSed deposits possess a very low ductility although the strength
could be optimized by careful selection of spray parameters. Post-
spray heat treatment is an effective way to improve strength and
ductility of the deposits.

Fig. 29. S-N curves obtained from fatigue tests [20].

3. Cold spraying repairing/remanufacturing

For examples, Levasseur et al. [62] studied pressureless sinter- Commercial use of CS-AM has been more recent, while com-
ing of CSed Inconel 718 deposits. Fig. 27a and b shows the typical ponents repairs by cold spraying are mostly applied. CS is a kind
cross-sections of as-sprayed and sintered Inconel 718 deposits, of special repairing technology applied to repairing or remanu-
respectively. For the as-sprayed state, density, measured by image facturing of failure parts. In spite of the development of advanced
analysis of the pore volume fraction, was 97.5% (porosity of 2.5%). composites, aerospace industry still makes an extensive use of Al,
After sintering, it is noticeable that porosity decreased to approx- Ti and Mg based alloys. Using CS to repair parts made of these
imately 0.2%. In addition, the flexural strength was measured as materials can overcome limitations of existing repairing technolo-
shown in Fig. 27c. As expected, the sintering of CS samples increases gies (such as traditional welding, electroplating, and 3D printing).
significantly both the strength and ductility. Wong et al. [64] stud- The advantages of using CS as repair tool are listed as follows: (1)
ied the tensile property of CSed Inconel 718 (Fig. 28). Firstly, it can high repairing efficiency; (2) in-site repairing can be realized with
be clearly seen that the tensile strength of CSed Inconel 718 using a portable CS system; (3) the powders employed can be of the same
helium as the driving gas is obviously higher than that using nitro- material as the worn metal parts, making parts return to their initial
gen. Secondly, irrespective of the driving gases used to produce the state; (4) since the effect of heating gas on substrate is very limited,
Inconel 718 deposits, the deposit becomes more ductile (increasing it can be used for precision processing; and (5) CS can result in resid-
engineering strain) as the heat treatment temperature increases. ual compressive stresses, avoiding crack initiation and propagation,
This may be explained by an improved interparticle bonding as therefore improving the fatigue properties of base metals.
aresult of sintering at high temperatures. Based on the existing literature, CS repairing has been used
Because of the hard-to-deform behavior, the mechanical per- extensively, mostly with Al, Al alloys and Al-based MMCs, Cu, Cu
formance of CSed superalloy deposits is poor. Although the role of alloys and Cu-based MMCs, and Ni, stainless steel and their com-
low-temperature heat treatment is very limited, using helium as posites. However, few publications have focused on the detailed
driving gas and high-temperature post-spray heat treatment (like microstructure and properties of CS repaired parts. Furthermore,
sintering) can greatly improve its performance. although the static properties of deposits are important for repair-

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Table 3
Tensile properties of representative cold-sprayed deposits.

Material Gas As-sprayed Heat-treated Reference

Tensile strength (MPa) Elongation (%) Tensile strength (MPa) Elongation (%)

Cu Air 125 – 168 – Authors

N2 295 0.35 220 34 [48]
He 300 3 – – [49]
Cu-Ag-Zr He 442 7.03 500 8 [49]
Cu-Ag He 438 4.04 280 48 [49]
Ti He 800 0.8 600 13.8 [50]
Ti-6Al-4V N2 150 1.5 460 5.5 [51]
He 480 3 765 6 [51]
Al7075 He 415 3.2 560 5.6 [56]
Al6061 He 340 3 200 17 [8]
304L N2 70 0 360 3.5 [61]
He 530 7.5 420 22 [59]
In718 N2 240 0.2 570 2.4 [64]
He 650 1 800 33 [64]

Fig. 30. Fatigue life results of cold sprayed Al on Al2024 specimens at stress level of (a) 180 MPa and (b) 210 MPa [29].

ing, the dynamic mechanical properties are crucial in service. In

the following, we will give a brief summary on fatigue properties
of CSed deposits.

3.1. Fatigue property

CS, as a potential candidate for repairing damaged structural

parts, is emerging in the aerospace industry [22]. However, the
challenge of CS repairing is that the repaired parts must retain
the bulk material properties to withstand the service loads. Fatigue
causes the majority of mechanical failures in service and therefore
it should be fully understood for reliable design and repairing [29].
In theory, the high velocity impact of particles plays the most
important role in CS material deposition. The high-velocity impact
causes compressive residual stresses in substrate, which also leads
to high plastic deformation producing surface nanocrystallization.
As such, CS has a great potential to increase the fatigue endurance Fig. 31. Fatigue strength of cold sprayed Al6082 on Al6082 specimens [67]. (AR: As
of its treated structures [20]. Ghelichi et al. [20] studied the fatigue received, CS: Cold sprayed, SP + CS: Shot peening followed by cold spraying, SSP + CS:
behavior of CS coated Al5052 specimens using Al and Al7075 as Severe shot peening followed by cold spraying, CS + SP: Cold spraying followed by
feedstock powders (Fig. 29), proposing that CS can increase the shot peening, CS + SSP: Cold spraying followed by severe shot peening.).

substrate fatigue life noticeably. Fatigue strength is improved sig-

nificantly by 30% in CSed Al7075 coatings. Ziemian et al. [29] ment. It was found that shot peening is more efficient if performed
investigated the effect of substrate surface roughening and CS prior to CS. Fatigue strength is improved by 26% under the condi-
coatings on the fatigue life of Al2024 specimens (Fig. 30). Results tion of severe shot peening prior to CS. In addition, Sansoucy et al.
indicate that fatigue strength is improved on average, up to 50% at [68] studied the bending fatigue of CSed Al-Co-Ce coatings (Fig. 32).
180 MPa and up to 38% at 210 MPa, by the deposition of CSed Al Results show that Al-Co-Ce coatings improved the fatigue behavior
coatings. Moridi et al. [67] studied the effect of conventional and of Al2024-T3 specimens compared to the as-received ones. How-
severe shot peening as pre-/post-treatment on fatigue behavior of ever, some researchers have found that CS deposition of a Ti layer
CS Al6082 coatings (Fig. 31). The CSed coating increases the fatigue leads to deterioration in the fatigue lives [69] (Fig. 33), possibly due
strength by 15% compared to the as-received specimens. Further to a vertical cracking in the coatings followed by crack transfer into
improvement can be achieved by hybrid peening and coating treat- the substrates.

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Fig. 34. Field repairing of corrosion surface damage by portable cold spraying [71].
Fig. 32. Mean number of cycles prior to failure as a function of the alternating stress
obtained from the bending fatigue tests of the bare, alclad, and cold sprayed Al-Co-Ce
coating on Al2024-T3 specimens [68].
of warships at a cost of millions of dollars every year. Villafuerte
et al. [72] reported CS repairing of Al-Mg alloy component of a
In brief summary, there is a strong dependency of the fatigue utility engine (Fig. 35). After surface pretreatment, thick deposits
strength on the deposited materials, spray parameters, and were generated in the corroded area, and then machining was used
pre/post surface treatments. to recover parts to the initial dimensions. Researchers [73] from
U.S. Army Research Laboratory also repaired aircraft actuator and
3.2. Repairing practices Mg alloy gearbox with CSed Al coatings. Results show that bond-
ing strength of the coating is higher than 70 MPa and hardness
Up to now, many research institutions or companies have car- reaches 57 HB. Salt-spray corrosion experiments (in 5% NaCl solu-
ried out CS repairing/remanufacturing of components, such as tion) show that the repaired parts do not present obvious corrosion
turbine blades, engine blocks, helicopter propellers and landing phenomenon after more than 7000 h. With CS repairing, the num-
gears, pistons, valves, camshafts and gearboxes. Fig. 34 shows an ber of parts that need to be replaced can be reduced down to 40%
example of in-site repairing of aircraft landing gear using a portable [73]. In addition, portable CS equipment has been supplied to 11
CS system [71]. nuclear-powered aircraft carriers to ensure that the ship and other
U.S. military has done pioneer work on CS repairing. For exam- equipment can be operational at all time [70].
ple, U.S. military were plagued with the corrosion of Mg alloy parts, Lee et al. [74] repaired a worn Al6061 mould by CS (Fig. 36). Fric-
and the corroded parts had to be replaced to prolong the lifespan tion and wear experiments show that higher wear resistance can

Fig. 33. Fatigue test results of cold sprayed Ti-6Al-4V on Ti specimens. “Delaminated set” consisted of plasma and cold-sprayed specimens which delaminated spontaneously
during the start of fatigue testing [69].

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Fig. 35. Repairing of an Al component in a utility engine for a private jet: (a) before with extensive corrosion damage, (b) as-sprayed with aluminum, (c) as-machined and
(d) finished part [72].

be obtained (Fig. 36a) than that of an undamaged mould, which ings and found that the erosion property can be improved greatly
is favorable for prolonging service life. However, the dimensional by following FSP. Overall, there is great room for further applica-
accuracy is relatively poor, which necessitates machining after tions of CS to repair mould, which warrants further investigation.
spraying. Combination of CS with other techniques is an effective method to
In addition, according to the report funded by the European increase the performance of the repaired parts.
Commission, service life of blades can be extended by 25–30%
together with 25% cut in maintenance cost, by CS nanometer struc- 4. Summary and prospectives
ture of high strength, waterproof coatings on wind turbine blades
[75]. CS has a great potential in solid-state forming of coatings/parts
For other applications, for example, cast iron engine block [76] and repairing. Fortunately, it has shown a large trend of worldwide
and nuclear steel containment [77] have also been repaired with moving from R&D trials to industrial applications. With the devel-
CS. In such cases, CS is used to restore the needed dimension, and opment of high performance CS equipment, an increasing list of
subsequent machining creates flat mating surfaces. materials (including Mg alloys, Al alloys, Ti alloys, even superalloys
and metal matrix composites, etc.) are expected to be deposited for
3.3. Hybrid repairing by CS together with other advanced manufacturing and in-situ fast repairing. Furthermore, more types
processes of damages will be repaired by CS, such as scratches, wear, cracks
and holes. However, up to now, there exist a certain number of
With the development of advanced manufacturing technolo- key issues that limit its applications as additive manufacturing and
gies, innovative repairing techniques which combine CS with other repairing technique, including:
technologies are emerged. Take hybrid process CS + friction stir
processing (FSP) as an example. As another solid-state technique, 1) Low ductility. Both CSed bulk and coating show an extremely
FSP has been proposed to obtain high-strength ultrafine-grained low ductility. Strengthening and toughening methods such as
CSed deposits [36,78–81]. Huang et al. [80] found that significant heat treatment require further investigations.
enhancement in mechanical properties can be achieved in CSed Cu- 2) Control over inter-particle bonding. The main bonding mech-
Zn alloy coatings with post-FSP (Fig. 37). Additionally, Peat et al. anisms of CS are mechanical and metallurgical. Therefore,
[81] has investigated the erosion performance of CSed MMC coat- realizing changes from mechanical bonding to metallurgical

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Fig. 36. (a) Wear rates of repaired specimen and original Al6061-T6, and (b) Comparison of fabricated channels (top view) and cross sections of polystyrol parts made by
injection molding [74].

bonding would be of great benefit to the improvement of bond- bulk/coating of different materials, the process parameters and
ing strength and ductility. control method should be studied in a systematic manner.
3) Method of regulation and control. Processing parameters 4) Cost saving. Although using compressed air as the driving gas can
(including gas and powder parameters) are significant factors greatly reduce expenses, the corresponding performance is rel-
that influence the bonding strength (including particle/particle atively lower when compared to using helium. For CS, spraying
and particle/substrate) and porosity. To achieve a good CSed powder should be produced in accordance with the require-
ments (size and shape) while the preparation is expensive.

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Fig. 37. (a) Typical stress-strain curves and (b) ultimate tensile strength of as-sprayed and friction-stirred coating [80].

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