Effects of Shot Peening Treatment on the Properties of Additive and Conventionally Manufactured Ti6Al4V Alloy: A Review

1. Introduction

Nowadays, the demand for improving materials from titanium alloys has increased tremendously. The reason for this is that titanium alloys are characterised by low density, high specific strength, corrosion resistance and good process performance with wide applications in the world economy in industries such as aerospace, marine, automobile, biomedical, chemical, energy and many others [1,2,3]. In evidence, in Figure 1. there are shown results from research centres of Web of Science and Scopus and the number of articles on the subject of shot penning, additive manufacturing, Ti-6Al-4V titanium alloy.

As it can be observed from Figure 1, Additive Manufacturing is a prominent subject of the today’s studies. In addition, it is worth noting that these numbers only include Additive Manufacturing phrase. There are definitely more articles which use specific AM terminology like SLM, DMLS or its full names without mentioning the phrase Additive Manufacturing or AM abbreviation. Noteworthy is also the fact that there are numerous studies about shot peening treatment which means that this type of surface treatment is still to be fully exploited. Moreover, it can be observed that even though recent reports suggest an increase interest in new β-phase titanium alloys, Ti-6Al-4V titanium alloy is continuously considered as a valuable subject of the research.

Figure 1. Searching results for selected phrases relating to papers scope. Search done in title, abstract and keywords of papers published in years 2019-2023, and indexed in Web of Science Core Collection and Scopus.

Figure 1. Searching results for selected phrases relating to papers scope. Search done in title, abstract and keywords of papers published in years 2019-2023, and indexed in Web of Science Core Collection and Scopus.

Increased interest in titanium alloys is caused by many factors. One of the examples can be seen in medical applications: world societies focus more on their health and start being more active. That results in much younger patients, who do not want to change their lifestyle and place more physical strain on the implants. It means that implants have to serve longer with more efficiency [4]. The quality of implants is crucial because as wear and corrosion processes contribute negatively on the properties of the implant, they might also have a direct negative biological impact on the periprosthetic environment [5,6]. Table 1 shows a comparison of widely used materials in biomedical applications in terms of their physicomechanical properties.

Titanium alloys are very sensitive to surface integrity, which is directly related to their performance. This generates the need of surface treatment of Ti-6A1-4V components in engineering in order to improve the surface integrity. The well-established post-producing surface treatment technologies for modifying surface layer of Ti-6Al-4V are shot peening and electropolishing [11,12]. There are numerous studies which indicate that combining those technologies on steels is profitable [13,14]. However, there is very little information about the impact on titanium alloys, especially, on additively manufactured Ti-6Al-4V titanium alloy. So, the main purpose of this review is to summarize all of the gathered data about the influence of shot peening and electropolishing processes on Ti-6Al-4V titanium alloy as well as gather information from the literature data about the influence of combined treatments of shot peening and electropolishing.

2. Properties of Ti6Al4V Manufactured Using Conventional Methods and Additive Manufacturing Methods

2.1. Microstructural Phases

Conventional microstructure of titanium alloys can be structurally classified by the size and organization of dual-phase α and β. Titanium alloys usually experience thermomechanical treatments [15]. Ti–6Al–4V contains 6% of aluminium element, which stabilizes hexagonal close-packed α-phase, and 4% of vanadium element, which stabilizes β-phase. Both phases i.e., α phase (hcp) and β phase (bcc), coexists at room temperature [16]. The control and the optimization of the microstructure of Ti-6Al-4V alloy is one of the most important issues to achieve desired properties. During the heating process, α-phase transforms into β phase approximately at temperature 1000 °C and it is called β transus temperature (Tβ). The heat treatment below β transus is referred to as sub-transus whereas above β transus is referred to as super-transus [17]. The cooling rate determines the transformation of β into → α (α′)( α′′). The transformation of unit cells is shown on Figure 2 Results

This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.

Figure 2. Martensitic transformation of unit cells in Ti6Al4V: β phase (a), α and α′ phases (b) based on [18,19].

Figure 2. Martensitic transformation of unit cells in Ti6Al4V: β phase (a), α and α′ phases (b) based on [18,19].

The basic microstructure of Ti6Al4V alloy is bimodal of interconnected equiaxed primary α grains and lamellar α+β colonies (transformed β). An example of this structure shows Figure 3a. On the other hand, martensitic structure is lamellar, which can be obtained as a Widmanstätten structure (Figure 3b) [20] or martensite plates.

Martensitic two-phase α + β alloys in the hardened state depends on the quenching temperature. The phase constituents could be in the form of martensitic phase precipitations of the α′ and α_M, as well as grains of metastable β_M phase [18]. The scheme of emerging phases depending on the cooling rate is shown in Figure 4.

The impact on microstructure has also cooling medium which then results in differences in Ti6Al4V properties. Table 2 describes the expected microstructure which is classified according to the cooling medium used and the phases in which the transformation occurs, irrespective of the material soaking time [22].

2.2. Manufacturing Methods of Ti6Al4V Martensitic Structure

2.2.1. Subtractive Manufacturing

Transformation of basic α+β into martensitic α′+ β could be made in conventional heat-treatment processes: quenching and annealing. In industry applications, the combination of solution heat-treatment and ageing operation is used in aerospace and automobile industry [23]. The main reason is that microstructure consists of soft α-phase and grain boundary β [24]. The effect on mechanical properties of Ti6Al4V is shown in Table 3

However, the conventional methods (CM) are slowly being replaced by additive manufacturing methods (AM). The main reason for that is hard machining material which is caused by the low thermal conductivity. There are certain challenges in refining casting, forging or rolling leading to notable amount of material waste, long lead time and high fabricating cost [30]. As a solution to this challenging scenario, additive manufacturing gained attention as it has immanent advantages, such as unrivalled design freedom and short lead times [31]. The extensive researches in this area implicate that achievable characteristics of Ti6Al4V are satisfying [32] and even could be superior for certain characteristics, like ductility [33].

2.2.2. Additive Manufacturing

There are 7 main powder bed technologies as per ASTM F2792 norm shown in Figure 5 [32]

The power of the laser and the feed rate of powder have direct impact on homogeneity of additively manufactured structure [34]. The laser beam can be described as Gaussian moving heat source as Equation (1):

$$Q_{laser}=\frac{2A{p}_{laser}}{\Pi r_0^2}\exp \left(\frac{-2{(x_i-x_0-v_{laser}t)}^2}{r_0^2}\right)$$

(1)

where A is the laser absorption coefficient, P_laser is the laser power, r₀ is the focus radius, x₀ is the laser’s beginning beam location, x_i is the position of laser focus, and v_laser is the laser’s scanning speed, and the heat transfer equation according to Fourier’s law can be described as (2) [35]:

$$Q_{laser}={\rho }_m{C}_m\left(\frac{\partial T}{\partial t}+u·\nabla \mathrm{T}\right)-\nabla ·\left(\lambda \nabla \mathrm{T}\right)$$

(2)

where ρ_m is the density, C_m is the specific heat, λ is the base thermal, conductivity, and u is the fluid flow velocity.

Literature indicates that powder bed fusion (PBF) technologies are used for industrial manufacturing of Ti6Al4V alloys [36,37]. In Table 4. comparison of the properties between particular PBF AM technologies and Direct Energy Deposition (DED) technology is presented.

2.2.3. SLM—Selective Laser Melting

During the SLM process, a product is formed by selectively melting successive layers of Ti–6Al–4V powder by the interaction of a laser beam. The powder material is heated and after applying sufficient power, it melts and forms a liquid pool. The molten pool consolidates and cools down quickly [51]. During the cooling down process, decomposition of 𝛽-phase proceeds into diffusionless martensitic α′ [52]. Afterwards, the cross-section of a layer is scanned, the building platform is lowered by an amount equal to the layer thickness and the process continues until the final product is formed [51]. Figure 6. shows scheme of SLM process.

Due to the high reactivity of Ti alloys, the process needs to be carried under an inert argon atmosphere [51]. SLM possesses several advantages such as a high material use efficiency, a high level of flexibility and production of geometrically complexed shape of parts, close to the final product [54]. The main disadvantages are mainly: higher cost, large residual stresses caused by steep thermal gradients and defects such as in conventional manufacturing like deformation [54] delamination [55], porosities [56] or even cracking of the parts in the form of hot cracking [57] or initiated by micro-sized defects [58].

2.2.4. DMLS—Directive Metal Laser Sintering

During the DMLS process, a moderately low intensity of laser power is used to sinter but not fully melt the metallic powders of Ti-6Al-4V alloy like in SLM process. DMLS process is initiated with placing a titanium alloy powder layer on the substrate, kept on the building platform, and then the laser beam is allowed to scan over the entire surface of the powder layer. The laser fuses the powder selectively or partially melts it in this process and then the powder bed is solidified to form a dense component [59,60]. Further, the platform is lowered down by a distance equal to layer thickness. A fresh layer of powder is again placed and the method is repeated in the same manner to get the final product [61]. The heat treatment and high cooling rate lead to the formation of dendritic martensite or fully acicular α’ martensite in Ti6Al4V titanium alloy [62]. Figure 7. shows the scheme of the DMLS process.

DMLS operation needs very careful observation since the occurrence of melting, re-solidification, heat transfer mechanism (convection, radiation) and volume shrinkage make it more challenging [65]. Process defects such as pores, cracks, high residual stresses and other microstructural defects can be noticed in DMLS operation [66].

2.2.5. EBM—Electron Beam Melting

During the EBM process, the electron beam or focused laser are the heat sources applied to melt the titanium powder. Each layer is manufactured by the following steps: spreading the titanium powders on a base plate, preheating of the powder, sintering of the powder with a defocused beam, melting the powders by using a focused beam, and then lowering the building platform by the thickness of one layer [67,68]. Using the electron beam necessitates the powder to be sintered or, otherwise, the electrostatic forces lead to clouds of charged particles in the build chamber [69]. The entirety of the EBM fabrication process operates under a vacuum environment. This implies that highly reactive materials, such as titanium alloys, can be produced using the EBM method, without oxidation and contamination of the parts throughout the process [70]. Figure 8. shows the scheme of the EBM process.

The defects after EBM manufacturing process are similar to the defects after other AM technologies. There are different types of defects after EBM producing Ti-6Al-4V titanium alloy, which have different origins in the building process [73]. Distinct defect types show differences in both size and shape, which are the factors that influence crack initiation behaviour. Lack of fusion defects are also common anomalies which are a result of under-melting during processing [74,75,76]. On the other hand, porosity is a result of over-melting during production and partially vaporizing the titanium metal [76,77]. Another defect type is gas pores which are coming from the powder or the building process. Many defect types can be avoided or minimized by appropriately selected processing parameters, but gas pores are generally considered unavoidable [78]. Population of these defects has direct influence on fatigue life of EBM produced Ti-6Al-4V titanium alloy [79].

2.2.6. DED—Direct Energy Deposition Processes

During Direct Energy Deposition (DED) a stream of metallic powder or wire is fed into a melt pool that is created by a laser beam and melts it as it is deposited [80]. It varies from Powder Bed Fusion technologies (PBF) where thermal energy is used to selectively fuse regions of a powder bed. Main application of DED processes are typically used on existing parts of arbitrary geometry with a relatively high deposition rate [81]. During the DED process, the electron or laser beam creates a molten pool on the surface. Then, the material transfer is conducted using a nozzle (laser as powder and beam as wire). The nozzle and the beam move along the path determined by the CAD data. The consecutive layers are melted and frozen on each other until the process is completed. The whole process takes place in a chamber filled with inert gas when using the laser method or vacuum environment when electron beam is used [67,82,83,84,85,86]. The scheme of building layers in DED technology using laser is shown in Figure 9.

It has been well established that products of DED technology cannot be completely free from defects (i.e., pores, un-melted powder and Lack of fusion (LOF)). LOFs are developed when the molten material in one layer does not completely fill the space between each DED pass, forming crack-like features perpendicular to BD [88,89,90,91]. Other typical defects after DED process are shrinkage, residual stresses and deformations after local temperature difference occurring, which means post-fabrication machining is often required. Utilizing wires and metallic sheets as printing feedstock material in DED processes usually lead to more defects, lower geometry precision, high surface roughness and limitations for the production of complex shapes rather than using powder feedstock in PBF technologies However, often better static and dynamic mechanical properties are obtained in the DED-deposited condition rather than in PBF-deposited parts [92,93,94].

3. Post-Process Surface Treatment Methods Applied to Modifying Ti6Al4V Surface Layer

3.1. Post-Processing after Additive Manufacturing

Metal parts fabricated directly by additive manufacturing are usually not ready for the service in their as-built state. Once the building process is completed, parts often have to be subjected to post-processing treatment including support material removal, surface finishing, colouring, coating and heat treatment. Existing heat treatment standards for CM metals and alloys are not specifically designed for AM parts and may differ in many cases depending on the initial microstructures and desired properties for specific applications. That is why it is crucial to determine optimal parameters of the post-processing treatment as it would not only improve properties of this materials but also be beneficial in reducing the cost of the process [95]. However, the application of optimal 3D-priniting parameters recommended by the producers does not avoid exhibition of surface defects, anomalies and undesired residual stresses in Ti-6Al-4V products. These lead to unsatisfactory properties for their application and are a justification for the use of surface finishing treatment like the shot peening process [96]. The scheme of a typical surface layer after AM is shown in Figure 10, and SEM microphotographs, illustrating the morphology of specimens surface after AM technology of DMLS, is shown Figure 11

3.2. Shot Peening

One of the most popular methods of modifying the surface layer is shot peening (SP). Shot peening is a mechanical surface treatment performed by the repeated impact of shots at high impact velocities onto surfaces of materials which causes plastic deformation of this material [101]. The amount of the plastic deformation of shot peening process influences failure mechanisms such as fatigue [102,103,104] and corrosion [105]. It also has an impact on important mechanical properties of material such as surface roughness [106], hardness [107] and residual stresses [108]. The scheme of the effect of shot peening process is shown on Figure 12 and modified structure of surface layer obtained after the process is shown on Figure 13.

The overall favourable outcomes of shot peening process can be summed up according to [99] as:

• Grain refinement
• Increase in dislocation density
• Formation of passive layer
• Decrease in porosity
• Formation of compressive residual stresses

The According to the collection and the researches of AM DMLS manufactured samples of Ti-6Al-4V carried out by R. Żebrowski, M. Walczak and their team [20,98,100,110,111], the main advantage of shot peening is the improvement of working parameters. There is a relevant increase of hardness after that type of process from approximately 10% up to 25% for highest peening pressure of 0.4 MPa which is shown in Figure 14 [110]. In comparison to substractively manufactured Ti-6Al-4V, a similar increase was obtained for AM according to [112] from approximately 10% to 25% for higher peening pressure of 0.5 MPa which is shown in Table 5.

The increase of surface hardness can be explained by the fact that the creation of nanocrystalline layer after shot peening treatment on the surface of treated specimen leads to material strengthening effect [113,114].

The similar effect of increase was achieved for the group of specimens from ultimate tensile strength test as from hardness test, which means that the Ti-6Al-4V demonstrates similar correlations between hardness and UTS [98]. Results of tensile tests are shown in Figure 15

In terms of wear and tribological performance of Ti-6Al-4V, there is decrease of surfaces friction coefficient for surfaces. There are changes associated with the use of balls which are made of Al2O3 constituting the counter-body. The comparable friction coefficient was obtained for soft surface subjection. For some surfaces, the friction coefficient increased, which is shown in Figure 16 [100]

Similar decrease of COF was obtained for substractively made Ti-6Al-4V according to [115] which is shown in Figure 17

The signs of wear of Ti-Al-4V are typical for metallic materials with significant hardness and high ductility. Prevailing wear mechanism relies on abrasion and groove forming is caused by the presence of β phase which is the cause of such an effect. The plasticity of β phase is higher than the plasticity of α phase which contributes to locally increased plastic deformation, which was confirmed by Faria et al. [116]. The wear mechanisms for AM and SM of Ti-6Al-4V after shot peening are similar as it also depends on the uniformity and quality of the surface layer which was confirmed by Airao et. al. [117]. After shot peening, the comparable structure is obtained for both AM and SM.

In terms of corrosion behaviour, there is a decrease of surfaces impedance (Figure 18), but, on the other hand, an improvement of electrochemical Ti-6Al-4V properties after shot peening process as seen in Table 6

Impedance spectra shown in Figure 18 are representing the impedance module versus frequency displays close impedance values after shot peening to untreated specimens. The obtained results after shot peening are high and situated between the range of 105÷106 Ω·cm2 at low frequencies, which means that these surfaces are sufficient for bioengineering purposes with enough corrosion resistance in body fluids [113,118,119].

Table 6. Electrochemical corrosion tests in 0,9 % NaCl for various peened surfaces of Ti-6Al-4V [110].

Conditions		Current density, Icorr (mA/cm2)	Potential, Ecorr (mV)	Polarization resistance Rp (kΩcm2)
Untreated		0,064	-318,6	2291
Untreated mechanically polished		0,067	-141,1	328,5
Steel CrNi	0.2 MPa	0,421	-173,5	210,5
	0.3 MPa	0,561	-207,4	138,8
	0.4 MPa	0,682	-337,1	81,2
Nuts	0.2 MPa	0,124	-106,6	346,5
	0.3 MPa	0,275	-228,5	367,4
	0.4 MPa	1,469	-279,2	349,5
Ceramic Beads	0.2 MPa	0,026	-123,8	170,8
	0.3 MPa	0,045	-151,4	206,2
	0.4 MPa	0,063	-174,3	432,8

Table 6. Electrochemical corrosion tests in 0,9 % NaCl for various peened surfaces of Ti-6Al-4V [110].

Conditions		Current density, Icorr (mA/cm2)	Potential, Ecorr (mV)	Polarization resistance Rp (kΩcm2)
Untreated		0,064	-318,6	2291
Untreated mechanically polished		0,067	-141,1	328,5
Steel CrNi	0.2 MPa	0,421	-173,5	210,5
	0.3 MPa	0,561	-207,4	138,8
	0.4 MPa	0,682	-337,1	81,2
Nuts	0.2 MPa	0,124	-106,6	346,5
	0.3 MPa	0,275	-228,5	367,4
	0.4 MPa	1,469	-279,2	349,5
Ceramic Beads	0.2 MPa	0,026	-123,8	170,8
	0.3 MPa	0,045	-151,4	206,2
	0.4 MPa	0,063	-174,3	432,8

As increase of surface hardness, the increase of corrosion resistance can be explained by the spontaneous creation of nanocrystalline layer after shot peening treatment. This passive layer created on Ti-6Al-4V surgical alloys is stable in Ringer fluid solution and is rich mainly in amorphous TiO2 [113,114]. However, more factors should be taken into account. The disorientation of the topography is reported to play a crucial role in corrosion resistance [120] as can be seen in Table 6 Mechanically polished surfaces obtained favourable electrochemical parameters. It is favourable due to low roughness values (especially of Sa parameter) and the lack of structural discontinuities in surface layer created in course of DMLS process [110]. Comparing AM to CM [121] indicates that corrosion behaviour, for polished surfaces after shot peening treatment, could be more favourable for 3D printed surfaces rather than the conventional one.

Surface roughness also influences coating adhesion such as droplet impact, wetting and solidification [122]. Another factor which has a direct impact on coating adhesion is stiffness and hardness. Comparing DMLS technology and conventional manufacturing indicates that the formation of martensitic structure during additive manufacturing like DMLS technology makes the substrate more stiff and hard of approximately 20% higher nanohardness and elastic modulus of the surface layer than the alloy fabricated by the conventional methods. This results in, for instance, a better fit between the Ecoating/Esubstrate for PVD nitride coatings which are shown in Table 7

3.3. Electropolishing

Electropolishing (EP) is a damage-free and highly efficient electrochemical surface finishing method for metal components. During electropolishing, the material works as an anode, and connects with the positive electrode of a DC power. The anode metal is oxidized into metal ions due to the loss of electrons. Then, it dissolves into the electrolyte, resulting in the removal of surface materials in order to polish, passivate and deburr the metal parts [123,124,125,126,127]. This method does not cause mechanical interaction, damage or leave any residual stress [128]. The schematic example of typical electropolishing setup and mechanism of electropolishing for titanium alloys are shown in Figure 19 and Figure 20.

Electrolytes used in electrochemical processes of electropolishing titanium and titanium alloys are:

• perchloric acid-based electrolytes, such as perchloric acid/acetic acid and perchloric acid/methanol/ethylene glycol systems [132,133];
• perchloric acid-free electrolytes, for instance, methanol/sulfuric acid and ethylene glycol/NaCl [134,135];
• deep eutectic solvents (DES) such as ChCl [136,137].

Comparing mechanical polishing to electropolishing, surface layer characteristics [138] and corrosion behaviour properties [139] are more favourable for electropolishing of conventionally made Ti-6Al-4V. The literature [138,139,140,141] also indicates that surface roughness (Ra parameter) is affected by the time, current density and temperature of the process and depending on parameter selection can result in an increase (Table 9) [140] or a decrease (Table 10) [138] of roughness. Influence of temperature on material roughness after electropolishing treatment is shown in Table 8.

Table 8. Surface roughness parameters (Ra) and empty pits diameters of Ti-6Al-4V titanium alloy electrochemically etched in Ethaline at different current densities (t = 25°C, T = 20 min). According to [140].

Conditions	Current density of treatment (mA·cm-2)	Ra (nm)	Pit diameter (µm)
Untreated	-	321	-
Electrochemically etched	5	651	25
	10	967	24
	15	504	27

Table 8. Surface roughness parameters (Ra) and empty pits diameters of Ti-6Al-4V titanium alloy electrochemically etched in Ethaline at different current densities (t = 25°C, T = 20 min). According to [140].

Conditions	Current density of treatment (mA·cm-2)	Ra (nm)	Pit diameter (µm)
Untreated	-	321	-
Electrochemically etched	5	651	25
	10	967	24
	15	504	27

Table 9. Comparison of roughness for mechanical and electropolishing treatment [138].

Roughness	Microstructure at various cooling medium
	Untreated	Electropolishing 100s	Electropolishing 200s	Electropolishing 300s	Mechanical #1000SiC	Mechanical #1500SiC
Micro [nm]	120,05 ± 7,89	58,72 ± 3,68	12,63 ± 0,81 0,75 ± 0,05	10,33 ± 1,14	98,30 ± 3,79	86,42 ± 2,05
Macro [µm]	2,34 ± 0,07	1,68 ± 0,02		0,68 ± 0,03	2,04 ± 0,03	1,82 ± 0,09

Table 9. Comparison of roughness for mechanical and electropolishing treatment [138].

Roughness	Microstructure at various cooling medium
	Untreated	Electropolishing 100s	Electropolishing 200s	Electropolishing 300s	Mechanical #1000SiC	Mechanical #1500SiC
Micro [nm]	120,05 ± 7,89	58,72 ± 3,68	12,63 ± 0,81 0,75 ± 0,05	10,33 ± 1,14	98,30 ± 3,79	86,42 ± 2,05
Macro [µm]	2,34 ± 0,07	1,68 ± 0,02		0,68 ± 0,03	2,04 ± 0,03	1,82 ± 0,09

Table 10. Temperature impact on Ti6Al4V specimens roughness during electropolishing (current density = 0,19 A·cm-2, T = 8 min) according to [141].

Temperature	Roughness
	Ra (nm)	Rms (nm)	Rz (nm)
7°C	7,4	10,0	39,7
18°C	6,1	8,1	27,5
25°C	6,4	8,1	26,6

Table 10. Temperature impact on Ti6Al4V specimens roughness during electropolishing (current density = 0,19 A·cm-2, T = 8 min) according to [141].

Temperature	Roughness
	Ra (nm)	Rms (nm)	Rz (nm)
7°C	7,4	10,0	39,7
18°C	6,1	8,1	27,5
25°C	6,4	8,1	26,6

Although many previous investigations have reported the properties and electrochemical behaviours of traditional titanium alloys after electropolishing based on different bath components and process parameters, the special surface state of additively manufactured titanium alloys has limited their usage [142]. This can be ascribed to the reason that the titanium alloys fabricated by AM often have incomplete melting powder, which generates an oxide film-like ceramic with high hardness and low chemical activity [143].

According to Zhang research [131] on electropolishing of additively manufactured Ti-6Al-4V specimens by selective laser melting, optimal electrochemical treatment is able to improve roughness (Table 11) and impedance (Table 12) of the titanium alloy.

4. Properties of Ti6Al4V Manufactured Using Conventional Methods and Additive Manufacturing Methods

Many fields of application could benefit from the properties provided by shot peening process but they are overshadowed by the unfavourable characteristics from the roughened surface. The example of this is the automotive branch where reports suggests that the contact fatigue resistance of shot peened austempered ductile iron gears has poor performance due to the roughness of the surface layer [144]. Another issue which challenges its successful application in some fields is the potential presence of locked shot fragments on the treated surfaces introducing potential crack nucleation sites (Figure 21). In biomedical applications, these particles could be responsible for early implant failure [145].

The topography of the surface treated Ti-6Al-4V alloy modified by shot grains penetrates into surface layer. The effect of this process is visible in Figure 22. Penetrating shot is located in structural discontinuities or directly on the surface as a result of its high kinetic energy because the properties of material being printed contribute to formation of these lamellar structures [111].

In this regard, it is necessary to control the peening processes to remove embedded remains of peening media in order to meet the requirements of smooth and defects-free surface for certain applications. This suggested method could provide further improvement of material properties such as wear and corrosion resistance [146].

Some research was done by M. Kiel; J. Szewczenko’s team et. al. [147,148,149,150] of combining shot peening and electropolishing for conventionally made Ti-6Al-4V which show promising result. The effect of combining shot peening and electrolytical polishing on corrosion resistance is shown in Table 12

Table 12. Corrosion behaviour after different surface treatment of Ti-6Al-4V ELI alloy [150].

Surface treatment processes	Ecorr [mV]	Icorr [µA·cm-2]	Rp [kΩ·cm2]
Shot peening, sandblasting	-266	0,047	551
Shot peening, electropolishing	-95	0,053	489
Shot peening, mechanical polishing, electropolishing	-172	0,069	377,15

Table 12. Corrosion behaviour after different surface treatment of Ti-6Al-4V ELI alloy [150].

Surface treatment processes	Ecorr [mV]	Icorr [µA·cm-2]	Rp [kΩ·cm2]
Shot peening, sandblasting	-266	0,047	551
Shot peening, electropolishing	-95	0,053	489
Shot peening, mechanical polishing, electropolishing	-172	0,069	377,15

5. Summary

This paper reviews the basic information regarding shot peening methods and the effects of the shot peening (SP) and electropolishing (EP) treatments on the surface layer properties of Ti6Al4V components and compares its effects depending on manufacturing process (AM or CM). In addition, the literature survey done regarding the shot peening and electropolishing treatments allows one to indicate the research gaps and further directions of the surface treatment process of the titanium alloys.

There is a research gap since, as far as the authors’ knowledge goes, there are no studies describing a hybrid treatment consisting of a combination of shot peening and electropolishing methods on additively manufactured objects including Ti6Al4V titanium alloy.

A research area in this direction could include, for instance, the application of EP treatment after SP, or EP treatment followed by SP. In addition, the optimisation of hybrid process parameters such as different peening times, intensities or shot sizes in the SP process and different polishing times, the type of electrolytes and voltage in the EP process are also required.

Although many previous investigations have reported the properties and electrochemical behaviours of traditional titanium alloys after electropolishing based on different bath components and process parameters, the special surface state of additively manufactured titanium alloys signifies the need for further investigations on that matter.

Metal parts fabricated directly by AM technology, even though they seem as a good quality product after the manufacturing process, are usually not ready for the service in their as-built state. For about 60-70% 3D-printed objects, additional processes of surface modifying treatment are needed as a result of surface defects and discontinuities in their surface layer.

The results obtained by the peening of additive technologies may differ depending on the method used for printing and the parameters or gas used in the shot peening process and employed peening method. Increasing the intensity and parameters of the peening process does not always lead to positive results as roughness and structural integrity can suffer in particular.

Few reports suggest that electropolishing reduces the presence of embedded fragments after shot peening treatment and smooths the surface of peened samples, which could be beneficial for properties of titanium alloys.

The literature gap leading to the scope of future work seems to be in need of the investigation of the effects of shot peening and combined peening and electropolishing on the anti-wear and corrosion performance of additively manufactured titanium alloys.