Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF)

*Massimo Lorusso*

### **Abstract**

Laser powder bed fusion (LPBF) is an additive manufacturing technique for the production of parts with complex geometry, and it is especially appropriate for structural applications in aircraft and automotive industries. Wear is the most important cause of malfunction of mechanical systems. Abrasive wear accounts for 50% of wear in industrial situations, and it is most common in components of machines. LPBF is very attractive due to its extremely high melting and solidification rates that make possible to obtain materials with particular tribological and wear behavior than those by traditional manufacturing routes. The aim of this chapter is to investigate the different behaviors of principal metallic alloys by LPBF.

**Keywords:** additive manufacturing (AM), laser powder bed fusion (LPBF), metallic alloys, wear

### **1. Introduction**

According to ASTM F2792-10, additive manufacturing (AM) is defined as "The process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing technologies." The fundamental principle of AM is that a geometric representation, originally generated using 3D-CAD system, can be manufactured directly without a need to process planning [1].

Today AM is receiving a very high attention from the mainstream media, investment community, national governments, and scientific communities. Nearly 10 years ago (2008), only 231 articles were published with AM topic, 5 years ago (2013) about 800 articles, and in 2018 about 4900 articles; in 10 years the number of articles per year is increased more than 20 times (**Figure 1**).

AM technologies have a strong potential to change the characteristic of manufacturing process, away from mass production in large factories with dedicated tooling and with high costs, to a world of mass customization and distributed manufacture.

Everyday new and innovative applications are emerging for the additive manufacturing [2]:


**93**

**Figure 1.** *Number of scientific articles per years with AM topic (source: Scopus).*


In many application of the AM, the wear resistance is important to guarantee the efficacy and the safety. Wear is the most important cause of malfunction of mechanical systems; for this reason, it is important to study the effect of wear and generally, the tribological characteristic of material used and processed by AM [3].

The aim of this chapter is to investigate the different behaviors of principal metallic alloys processed by laser powder bed fusion (one of the most diffused AM technologies) in terms of tribological properties, with a particular focus on the wear resistance and the coefficient of friction (COF). At the moment, few studies are available about tribological properties of metallic alloys produced by LPBF; this chapter searches to organize the works present.

### **2. Additive manufacturing**

### **2.1 Introduction**

Seven different technologies are classified for additive manufacturing agreed by the AM SIG (special interest group) as can be seen in detail in **Table 1** [4].

### **2.2 Laser powder bed fusion (LPBF)**

The SLM process has been defined as the laser powder bed fusion process (LPBF), according to ISO/ASTM 52900. It is also known by the trade names LaserCUSING or DMLS (Direct Metal Laser Sintering), which directly produces homogenous metal objects, layer by layer, from 3D CAD data, by selectively melting very fine layers of metal powder (**Figure 2**) with a laser beam.

**95**

**Figure 2.**

*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF)*

**Classification Description Technology Materials**

Laser deposition Laser consolidation Direct metal deposition Electron beam direct Metals

Metals, polymers, and ceramics

Polymers

wax

Metals, polymers, and ceramics

Metals, ceramics, and hybrids

Photopolymers and ceramics

Photopolymers,

melting

3D printing Ink-jetting S-print M-print

modeling

Selective laser melting Laser powder bed

Electron beam melting

Stereolithography Digital light processing

Polyjet Ink-jetting Thermojet

fusion Selective laser sintering

Ultrasonic consolidation Laminated object manufacturing

Builds parts using focused thermal energy and wire to fuse materials and they are deposited on a substrate

of a binding agent to join powdered material

Material extrusion Fused deposition modeling Fused deposition

small droplets of build material, which are then cured by exposure to light

of a powder bed

thermal energy to fuse regions

sheets of material and binding them together in layers

Builds parts by using a vat of liquid photopolymer resin, out of which the model is constructed layer by layer. An ultraviolet (UV) light is used to cure or harden the resin where required.

*Scanning electron magnification (SEM) observation of typical powder used in LPBF process (AlSi10Mg).*

*DOI: http://dx.doi.org/10.5772/intechopen.85167*

Binder jetting Creates objects by deposition

Material jetting Builds parts by depositing

Powder bed fusion Creates objects by using

Sheet lamination Builds parts by trimming

*Classification of AM adapted from ASTM AM classification.*

Direct energy deposition

VAT

**Table 1.**

photopolymerization


*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF) DOI: http://dx.doi.org/10.5772/intechopen.85167*

**Table 1.**

*Friction, Lubrication and Wear*

• Medical and dental

chapter searches to organize the works present.

*Number of scientific articles per years with AM topic (source: Scopus).*

**2. Additive manufacturing**

**2.2 Laser powder bed fusion (LPBF)**

**2.1 Introduction**

In many application of the AM, the wear resistance is important to guarantee the efficacy and the safety. Wear is the most important cause of malfunction of mechanical systems; for this reason, it is important to study the effect of wear and generally, the tribological characteristic of material used and processed by AM [3]. The aim of this chapter is to investigate the different behaviors of principal metallic alloys processed by laser powder bed fusion (one of the most diffused AM technologies) in terms of tribological properties, with a particular focus on the wear resistance and the coefficient of friction (COF). At the moment, few studies are available about tribological properties of metallic alloys produced by LPBF; this

Seven different technologies are classified for additive manufacturing agreed by

the AM SIG (special interest group) as can be seen in detail in **Table 1** [4].

very fine layers of metal powder (**Figure 2**) with a laser beam.

The SLM process has been defined as the laser powder bed fusion process (LPBF), according to ISO/ASTM 52900. It is also known by the trade names LaserCUSING or DMLS (Direct Metal Laser Sintering), which directly produces homogenous metal objects, layer by layer, from 3D CAD data, by selectively melting

• Automotive

**Figure 1.**

• Aerospace

**94**

*Classification of AM adapted from ASTM AM classification.*

where required.

**Figure 2.**

*Scanning electron magnification (SEM) observation of typical powder used in LPBF process (AlSi10Mg).*

Laser powder bed fusion (LPBF) is an additive manufacturing technique for the fabrication of near net-shape parts directly from computer-aided design data by melting together different layers with the help of a laser source. LPBF process produces parts with good surface quality, high accuracy and detail resolution, and excellent mechanical properties. The components are built layer by layer; it is possible to project internal channel and features that are impossible to obtain by casting or machining. LPBF does not require special tooling like casting, so it is more convenient for not so big production. It is a good alternative to conventional machining for complex metallic parts [5].

It has been demonstrated, in recent literature, that SLM can also be used to fabricate metal matrix composites (MMCs). These could have applications in automotive and aerospace industries, where it is necessary to improve mechanical properties (stiffness and hardness) and specially the wear resistance [6].

### **3. Aluminum alloys**

### **3.1 Introduction**

The most used Al alloys are Al-Si alloys, which represent 80% of aluminum casting alloys, thanks to their high fluidity, high weldability, good corrosion resistance, and low coefficient of thermal expansion. The binary Al-Si system is a eutectic alloy when the amount of Si is 11–13 wt%, a hypoeutectic alloy when Si is less than 11 wt%, and a hypereutectic alloy when Si is more than 13 wt%. The strengthening of these alloys is generally possible, through the addition of other alloying elements such as Cu and Mg that make the Al-Si alloys hardenable either by means of a heat treatment. There is a large demand for Al-Si-Mg alloys for different applications, such as the aerospace industry, and for automotive and heat exchangers, due to their high mechanical properties, like strength and hardness, in the heat-treated state [7–9].

The most popular Al-Si alloy processed by LPBF is AlSi10Mg alloy (similar to A360). Other Al-Si alloys by LPBF are AlSi7Mg (called also A357) [10–11] and AlSi12Mg [12].

Despite this growing interest in the AM processability of Al-Mg-Zn-Cu alloys, to date, few studies are available on the AM process of high mechanical properties' (harness and strength) aluminum alloys. It is well-known that the alloys belonging to the Al-Mg-Zn-Cu alloys are appropriate for different applications in aerospace as they are characterized by toughness and high strength reached mainly through the precipitation of the MgZn2 phase. These alloys are not well weldable because they suffer strongly from liquation cracking [13–14].

### **3.2 AlSi10Mg**

In the literature, it is demonstrated that AlSi10Mg alloy produced by casting has a coefficient of friction (against a WC cemented with CO pin) lower than AlSi10Mg alloy by LPBF since their microstructure and hardness are different. The typical microstructure of metallic alloys by LPBF without heat treatments is characterized of a small grain size. At higher magnification after hatching, it can be seen as a fine cellular-dendritic structure made by agglomerates of grains with mean diameters of a hundred of nanometers or less. It is generally observed that materials with large grains have a COF lower than materials with a fine microstructure; this is one of the most important reasons of higher COF of AlSi10Mg by LPBF [15].

The different sizes of microstructure influence the hardness very strong. The hardness is higher for the finer grain size. As suggested by the theoretical considerations, the material with the highest hardness has the highest wear resistance. The

**97**

**Figure 3.**

*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF)*

difference between the wear resistance of the AlSi10Mg alloy produced by casting and by LPBF is immediately evident. During pin on disc test, the volume per meter loss of the AlSi10Mg produced by LPBF is 35% less than the volume per meter loss of

In general, for the conventional casted alloys, the Al-Si alloys with small primary silicon phase present a higher wear resistance than that of the alloys with large silicon phase, due to their high surface-volume fraction. The aluminum alloys by LPBF show the inverse results that could be attributed to their ultrafine microstructure. During the wear process, the fine primary silicon particles form a full contacted wear layer; the primary silicon is directly pressed into Al-matrix and then forms the full contacted wear layer [7]. For this reason alloys with small primary silicon have a relative poor wear resistance. The A357 aluminum alloy has less silicon (6.5–7.56%)

For the Al-Zn-Mg alloys, the microstructure has a strong influence on the wear behavior that is due to higher content and the higher amount of MgZn2 precipitate that is harder than α-aluminum matrix and helps to protect the surface of material [13].

Aluminum matrix composites (AMCs) have generally excellent mechanical properties such as improved stiffness, strength, and hardness when compared with the aluminum matrix. AMCs attract much attention because they are characterized by low density and high specific strength and good tribology properties. The limits of this material are the high difficulty in the process of production and in the post-processing phases. The principal problem when a ceramic is used as reinforcement is the clustering and agglomeration caused by the poor wettability and a large

The LPBF process seems to be particularly suitable for the production of AMCs because near net-shape complex components can be made, which reduces the postprocessing phases. The most used ceramic reinforcements in AMCs produced by LBPF are magnesium spinel (MgAl2O4) and titanium diboride (TiB2) [15].

The sufficiently high densification rate combined with the homogeneous incorporation of nanoscale TiC reinforcement throughout the matrix led to the consider-

surface-to-volume ratio that does not promote a homogenous dispersion.

ably low coefficient of friction (COF) and resultant wear rate [17].

*Example of detached particles that have an effect of solid lubrication (third part).*

*DOI: http://dx.doi.org/10.5772/intechopen.85167*

than AlSi10Mg (9–11%) but higher COF and wear [16].

**3.4 Aluminum matrix composites (AMCs)**

the AlSi10Mg produced by casting.

**3.3 Other aluminum alloys**

*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF) DOI: http://dx.doi.org/10.5772/intechopen.85167*

difference between the wear resistance of the AlSi10Mg alloy produced by casting and by LPBF is immediately evident. During pin on disc test, the volume per meter loss of the AlSi10Mg produced by LPBF is 35% less than the volume per meter loss of the AlSi10Mg produced by casting.

### **3.3 Other aluminum alloys**

*Friction, Lubrication and Wear*

**3. Aluminum alloys**

**3.1 Introduction**

machining for complex metallic parts [5].

suffer strongly from liquation cracking [13–14].

Laser powder bed fusion (LPBF) is an additive manufacturing technique for the fabrication of near net-shape parts directly from computer-aided design data by melting together different layers with the help of a laser source. LPBF process produces parts with good surface quality, high accuracy and detail resolution, and excellent mechanical properties. The components are built layer by layer; it is possible to project internal channel and features that are impossible to obtain by casting or machining. LPBF does not require special tooling like casting, so it is more convenient for not so big production. It is a good alternative to conventional

It has been demonstrated, in recent literature, that SLM can also be used to fabricate metal matrix composites (MMCs). These could have applications in automotive and aerospace industries, where it is necessary to improve mechanical

The most used Al alloys are Al-Si alloys, which represent 80% of aluminum casting alloys, thanks to their high fluidity, high weldability, good corrosion resistance, and low coefficient of thermal expansion. The binary Al-Si system is a eutectic alloy when the amount of Si is 11–13 wt%, a hypoeutectic alloy when Si is less than 11 wt%, and a hypereutectic alloy when Si is more than 13 wt%. The strengthening of these alloys is generally possible, through the addition of other alloying elements such as Cu and Mg that make the Al-Si alloys hardenable either by means of a heat treatment. There is a large demand for Al-Si-Mg alloys for different applications, such as the aerospace industry, and for automotive and heat exchangers, due to their high mechanical properties, like strength and hardness, in the heat-treated state [7–9].

The most popular Al-Si alloy processed by LPBF is AlSi10Mg alloy (similar to A360). Other Al-Si alloys by LPBF are AlSi7Mg (called also A357) [10–11] and AlSi12Mg [12]. Despite this growing interest in the AM processability of Al-Mg-Zn-Cu alloys, to date, few studies are available on the AM process of high mechanical properties' (harness and strength) aluminum alloys. It is well-known that the alloys belonging to the Al-Mg-Zn-Cu alloys are appropriate for different applications in aerospace as they are characterized by toughness and high strength reached mainly through the precipitation of the MgZn2 phase. These alloys are not well weldable because they

In the literature, it is demonstrated that AlSi10Mg alloy produced by casting has a coefficient of friction (against a WC cemented with CO pin) lower than AlSi10Mg alloy by LPBF since their microstructure and hardness are different. The typical microstructure of metallic alloys by LPBF without heat treatments is characterized of a small grain size. At higher magnification after hatching, it can be seen as a fine cellular-dendritic structure made by agglomerates of grains with mean diameters of a hundred of nanometers or less. It is generally observed that materials with large grains have a COF lower than materials with a fine microstructure; this is one of the

The different sizes of microstructure influence the hardness very strong. The hardness is higher for the finer grain size. As suggested by the theoretical considerations, the material with the highest hardness has the highest wear resistance. The

most important reasons of higher COF of AlSi10Mg by LPBF [15].

properties (stiffness and hardness) and specially the wear resistance [6].

**96**

**3.2 AlSi10Mg**

In general, for the conventional casted alloys, the Al-Si alloys with small primary silicon phase present a higher wear resistance than that of the alloys with large silicon phase, due to their high surface-volume fraction. The aluminum alloys by LPBF show the inverse results that could be attributed to their ultrafine microstructure.

During the wear process, the fine primary silicon particles form a full contacted wear layer; the primary silicon is directly pressed into Al-matrix and then forms the full contacted wear layer [7]. For this reason alloys with small primary silicon have a relative poor wear resistance. The A357 aluminum alloy has less silicon (6.5–7.56%) than AlSi10Mg (9–11%) but higher COF and wear [16].

For the Al-Zn-Mg alloys, the microstructure has a strong influence on the wear behavior that is due to higher content and the higher amount of MgZn2 precipitate that is harder than α-aluminum matrix and helps to protect the surface of material [13].

### **3.4 Aluminum matrix composites (AMCs)**

Aluminum matrix composites (AMCs) have generally excellent mechanical properties such as improved stiffness, strength, and hardness when compared with the aluminum matrix. AMCs attract much attention because they are characterized by low density and high specific strength and good tribology properties. The limits of this material are the high difficulty in the process of production and in the post-processing phases. The principal problem when a ceramic is used as reinforcement is the clustering and agglomeration caused by the poor wettability and a large surface-to-volume ratio that does not promote a homogenous dispersion.

The LPBF process seems to be particularly suitable for the production of AMCs because near net-shape complex components can be made, which reduces the postprocessing phases. The most used ceramic reinforcements in AMCs produced by LBPF are magnesium spinel (MgAl2O4) and titanium diboride (TiB2) [15].

The sufficiently high densification rate combined with the homogeneous incorporation of nanoscale TiC reinforcement throughout the matrix led to the considerably low coefficient of friction (COF) and resultant wear rate [17].

**Figure 3.** *Example of detached particles that have an effect of solid lubrication (third part).*

The presence of reinforcements causes a reduction of COF. If the reinforcements have a micro-size, the effect is bigger than with nano-sized reinforcement. The reduction of COF is probably due to the detachment from the aluminum matrix of micro- or nanoparticles of ceramic reinforcements that can act as a third body (**Figure 3**).

### **4. Nickel alloys**

### **4.1 Introduction**

The most used nickel alloys produced through LPBF for their high weldability are Inconel 718 and 625. Inconel 718 and 625 have been used in high-temperature applications, such as nuclear reactors, pumps, molds, and gas-turbine engine aircraft. These nickel alloys are endowed with high-temperature strength, high creep, and oxidation resistance. These two alloys can be used depending on the applications, but the production of objects with a complex shape is expensive with conventional manufacturing technologies. Therefore, the ability to produce complex components without using molds makes LPBF process particularly interesting. The microstructure created during LPBF process are out-of-equilibrium, and it is necessary to perform some heat treatments in order to homogenize the microstructural features. Depending to application it would be necessary to carry out a simple stress relieving to reduce residual stress induced by thermal gradients during LPBF process. As for other applications, annealing or solution treatment to allow the grain recrystallization and growth, thus improving the creep resistance, is typically requested [18–21].

### **4.2 Inconel**

Inconel 625 is a nickel-chromium alloy designed as solid-solution-strengthened. The heat treatments favor the formation of metastable Ƴ" phase that further improves the mechanical properties. Inconel 625 thanks to higher concentration of Cr and Mo with respect to Inconel 718 has higher corrosion resistance.

Inconel 718 is an age-hardenable nickel-chromium alloy mainly due to the presence of aluminum, titanium, and niobium that leads to precipitation of gamma prime Ƴ' Ni3(Al,Ti) phase and metastable gamma double prime Ƴ" Ni3Nb phase [18]. Inconel by LPBF is more difficult to be machined than the same materials produced by extruding or rolling processes. During milling process of Inconel by LPBF, the cutting speed and chip load are lower due the presence of hard precipitated particles. In general, the Inconel produce by LPBF exhibited relatively good wear performance. Such microstructures indicated that the presence of severe adhesive wear in turn resulted a relatively higher wear rate. The clustered Ƴ dendrites gave rise to the fluctuations of COF. The formed protective adherent tribolayer on worn surfaces made considerable contributions to the further improved wear performance of LPBF-produced parts. The combined influence of elevated microhardness and the formation of adherent tribolayer contributed to the improvement of wear performance [20].

Different studies, in particular about Inconel 718, show that addition of tungsten carbide (WC) or titanium carbide (TiC) particles significantly increased the hardness, friction resistance, and wear performance. The composite acquired a considerably low COF. The existence of a gradient interface has a very important role in improving the wear performance of LPBF-processed WC/Inconel 718 and TiC/Inconel 718 composites [22].

**99**

*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF)*

Titanium and its alloys have good mechanical properties, good corrosion resistance, and excellent biocompatibility. These alloys are the most interesting metallic biomaterials for orthopedic and dental implants. Until few years ago, titanium processing via AM technologies was given little consideration by the medical industry due to the high cost of production. However, in recent years, AM metal technologies are becoming popular in biomedical field because of the ability to build metals with customized porous architectures and shape. The titanium alloy Ti6Al4V (the most popular titanium alloy) has been widely used in various industrial applications due to its mechanical and physical properties. Beyond the biomedical field, Ti6Al4V has been commonly employed in producing aircraft engine airframe parts owing to its high strength to mass ratio and good performance at high temperature (up to

Ti6Al4V has very good mechanical properties, but it has also been reported to exhibit poor tribology properties, such as a high COF and low wear resistance. The poor tribological property of Ti6Al4V is attributable to its low resistance to plastic shearing, low work hardening, and the low protection afforded by surface oxidation. No significant differences are present between TI6Al4V produced by the different processing technologies. Generally on Ti6Al4V produced by LPBF, less oxidized

During the LPBF process, the high cooling rate of laser melting leads to higher amount of α and α' harder phases on Ti6Al4V alloy than the traditional process. The presence of harder microstructural constituents on Ti6Al4V produced by LPBF leads to a higher wear resistance. The heat-treatment Ti6Al4V generates a protective tribolayer containing oxygen without plastic deformation in the bulk material, which has

Investigation of reinforced Ti6Al4V with TiB2 shows that nano-sized TiB whiskers are formed by the in situ reaction between Ti and TiB2. The interface between matrix and TiB is a very strong interface bonding. During the wear test, this avoids the possibility of easy detachment of TiB whiskers. This reduces the wear rate significantly but not the COF, because the detached particles are few and it is not

The 316L austenitic stainless steel has numerous application in different fields for its high resistance at oxidation and corrosion. The most popular applications are in marine, nuclear, oil and gas, and biomedical industry. 316L austenitic stainless steel which comprises iron alloyed with chromium of mass fraction up to 18%, nickel up to 14%, molybdenum up to 3% , manganese down 2%, silicon down 0.75%, copper down 0.5% and carbon down 0.03% along with minor elements [28]. During wear test on 316L stainless steel produced by LPBF, the passive layer made by chromium and nickel oxidation is removed and leaves iron exposed to the air, which easily gets oxidized especially at high temperature. Regarding the wear mechanisms, the worn surfaces of 316L stainless steel exhibited plastic deformation due to adhesive wear as well as grooves aligned along the sliding direction due to the

present enough third part that reduces significantly the friction [27].

*DOI: http://dx.doi.org/10.5772/intechopen.85167*

areas are found during the wear tests [25].

**5. Titanium**

**5.1 Introduction**

400–500°C) [23–24].

the lowest wear rate [26].

**6. Stainless steel (316L)**

**5.2 Ti6Al4V**

*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF) DOI: http://dx.doi.org/10.5772/intechopen.85167*

### **5. Titanium**

*Friction, Lubrication and Wear*

body (**Figure 3**).

**4. Nickel alloys**

**4.1 Introduction**

requested [18–21].

performance [20].

TiC/Inconel 718 composites [22].

**4.2 Inconel**

The presence of reinforcements causes a reduction of COF. If the reinforcements have a micro-size, the effect is bigger than with nano-sized reinforcement. The reduction of COF is probably due to the detachment from the aluminum matrix of micro- or nanoparticles of ceramic reinforcements that can act as a third

The most used nickel alloys produced through LPBF for their high weldability are Inconel 718 and 625. Inconel 718 and 625 have been used in high-temperature applications, such as nuclear reactors, pumps, molds, and gas-turbine engine aircraft. These nickel alloys are endowed with high-temperature strength, high creep, and oxidation resistance. These two alloys can be used depending on the applications, but the production of objects with a complex shape is expensive with conventional manufacturing technologies. Therefore, the ability to produce complex components without using molds makes LPBF process particularly interesting. The microstructure created during LPBF process are out-of-equilibrium, and it is necessary to perform some heat treatments in order to homogenize the microstructural features. Depending to application it would be necessary to carry out a simple stress relieving to reduce residual stress induced by thermal gradients during LPBF process. As for other applications, annealing or solution treatment to allow the grain recrystallization and growth, thus improving the creep resistance, is typically

Inconel 625 is a nickel-chromium alloy designed as solid-solution-strengthened.

The heat treatments favor the formation of metastable Ƴ" phase that further improves the mechanical properties. Inconel 625 thanks to higher concentration of

Inconel 718 is an age-hardenable nickel-chromium alloy mainly due to the presence of aluminum, titanium, and niobium that leads to precipitation of gamma prime Ƴ' Ni3(Al,Ti) phase and metastable gamma double prime Ƴ" Ni3Nb phase [18]. Inconel by LPBF is more difficult to be machined than the same materials produced by extruding or rolling processes. During milling process of Inconel by LPBF, the cutting speed and chip load are lower due the presence of hard precipitated particles. In general, the Inconel produce by LPBF exhibited relatively good wear performance. Such microstructures indicated that the presence of severe adhesive wear in turn resulted a relatively higher wear rate. The clustered Ƴ dendrites gave rise to the fluctuations of COF. The formed protective adherent tribolayer on worn surfaces made considerable contributions to the further improved wear performance of LPBF-produced parts. The combined influence of elevated microhardness and the formation of adherent tribolayer contributed to the improvement of wear

Different studies, in particular about Inconel 718, show that addition of tungsten carbide (WC) or titanium carbide (TiC) particles significantly increased the hardness, friction resistance, and wear performance. The composite acquired a considerably low COF. The existence of a gradient interface has a very important role in improving the wear performance of LPBF-processed WC/Inconel 718 and

Cr and Mo with respect to Inconel 718 has higher corrosion resistance.

**98**

### **5.1 Introduction**

Titanium and its alloys have good mechanical properties, good corrosion resistance, and excellent biocompatibility. These alloys are the most interesting metallic biomaterials for orthopedic and dental implants. Until few years ago, titanium processing via AM technologies was given little consideration by the medical industry due to the high cost of production. However, in recent years, AM metal technologies are becoming popular in biomedical field because of the ability to build metals with customized porous architectures and shape. The titanium alloy Ti6Al4V (the most popular titanium alloy) has been widely used in various industrial applications due to its mechanical and physical properties. Beyond the biomedical field, Ti6Al4V has been commonly employed in producing aircraft engine airframe parts owing to its high strength to mass ratio and good performance at high temperature (up to 400–500°C) [23–24].

### **5.2 Ti6Al4V**

Ti6Al4V has very good mechanical properties, but it has also been reported to exhibit poor tribology properties, such as a high COF and low wear resistance. The poor tribological property of Ti6Al4V is attributable to its low resistance to plastic shearing, low work hardening, and the low protection afforded by surface oxidation. No significant differences are present between TI6Al4V produced by the different processing technologies. Generally on Ti6Al4V produced by LPBF, less oxidized areas are found during the wear tests [25].

During the LPBF process, the high cooling rate of laser melting leads to higher amount of α and α' harder phases on Ti6Al4V alloy than the traditional process. The presence of harder microstructural constituents on Ti6Al4V produced by LPBF leads to a higher wear resistance. The heat-treatment Ti6Al4V generates a protective tribolayer containing oxygen without plastic deformation in the bulk material, which has the lowest wear rate [26].

Investigation of reinforced Ti6Al4V with TiB2 shows that nano-sized TiB whiskers are formed by the in situ reaction between Ti and TiB2. The interface between matrix and TiB is a very strong interface bonding. During the wear test, this avoids the possibility of easy detachment of TiB whiskers. This reduces the wear rate significantly but not the COF, because the detached particles are few and it is not present enough third part that reduces significantly the friction [27].

### **6. Stainless steel (316L)**

The 316L austenitic stainless steel has numerous application in different fields for its high resistance at oxidation and corrosion. The most popular applications are in marine, nuclear, oil and gas, and biomedical industry. 316L austenitic stainless steel which comprises iron alloyed with chromium of mass fraction up to 18%, nickel up to 14%, molybdenum up to 3% , manganese down 2%, silicon down 0.75%, copper down 0.5% and carbon down 0.03% along with minor elements [28].

During wear test on 316L stainless steel produced by LPBF, the passive layer made by chromium and nickel oxidation is removed and leaves iron exposed to the air, which easily gets oxidized especially at high temperature. Regarding the wear mechanisms, the worn surfaces of 316L stainless steel exhibited plastic deformation due to adhesive wear as well as grooves aligned along the sliding direction due to the abrasive wear. The wear rate and the friction of 316L stainless steel by LPBF were lower than the 316L traditionally processed; the LPBF-processed steel has a very fine austenite grains, the size of which was much smaller than in the traditionalprocessed 316L stainless steel. These fine grains in the 316L stainless steel by LPBF increase the wear resistance, and the surface is subjected to slight plastic deformation [29–30].

Investigation of the wear resistance of reinforced 316L stainless steel (with TiB2 or TiC) shows that the wear resistance increases with the increasing TiB2 content due to combined effects of grain refinement and grain-boundary strengthening [31].

### **7. Lubrication condition and heat treatment**

Metallic alloys by LPBF have generally pores and cracks that influence the wear under lubricated condition. Few studies are available in literature under boundary lubrication regime.

In those studies [26, 30], when a lubricating film is not yet formed, the metal alloys by LPBF have better wear performance than metal alloys by traditional processes.

Surface pores may positively influence the formation of the lubricating film.

The effect of lubricant is critical in reducing friction and wear. The choice of oil needs to be carefully considered before applying LPBF process to hydraulic components.

In general, heat treatment (used to reduce the stress in material after LPBF process) reduces the wear resistance of metallic alloys. The wear resistance is reduced because the heat treatment changes the microstructure of metallic alloys by LPBF and lost the very fine microstructure. The most sensibility materials at heat treatment are aluminum alloys [15, 32]. The only metallic alloy that increases the wear resistance and reduces COF after heat treatments is Ti6Al4V because the oxidation of surface (if the heat treatment is realized in the presence of oxygen) produces a protective tribo-oxide layer [26].

### **8. Conclusions**

In conclusion, metallic alloys by LPBF generally have higher wear resistance and less COF than metallic alloys produced by traditional processes under dry condition and boundary lubrication mainly due to the fine grains and high hardness.

The LPBF processing parameters are fundamental for wear rate since a fully densified part usually has high wear resistance and COF.

The existence of pores reduces the bonding between molten pools, resulting in cracks. These cracks can further cause material shell off which greatly increases wear.

In general, the metallic alloys produced by LPBF are more difficult to machine than the same metallic alloys produced by traditional processes. For this, it is important to reduce at the minimum the post-process machining.

The presence of ceramic particle reinforcements in MMCs causes generally a reduction of COF; this effect is due to the detachment from the metallic matrix of ceramic particles that can act as a third body. The interfacial bond between the matrix and the reinforcements has a fundamental role in wear process; a strong interfacial bond guaranties a low wear rate; a weak interfacial bond causes a low COF and sometimes high wear rate.

**101**

**Author details**

Massimo Lorusso

provided the original work is properly cited.

Istituto Italiano di Tecnologia (IIT), Torino, Italy

\*Address all correspondence to: massimo.lorusso@iit.it

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF)*

The author is grateful for the moral and scientific support of PhD Diego

Manfredi, PhD Flaviana Calignano, and Professor Matteo Pavese.

The heat treatment in general reduces the wear resistance and increases the COF.

*DOI: http://dx.doi.org/10.5772/intechopen.85167*

**Acknowledgements**

*Tribological and Wear Behavior of Metal Alloys Produced by Laser Powder Bed Fusion (LPBF) DOI: http://dx.doi.org/10.5772/intechopen.85167*

The heat treatment in general reduces the wear resistance and increases the COF.

### **Acknowledgements**

*Friction, Lubrication and Wear*

[29–30].

strengthening [31].

lubrication regime.

processes.

components.

**8. Conclusions**

increases wear.

protective tribo-oxide layer [26].

COF and sometimes high wear rate.

**7. Lubrication condition and heat treatment**

abrasive wear. The wear rate and the friction of 316L stainless steel by LPBF were lower than the 316L traditionally processed; the LPBF-processed steel has a very fine austenite grains, the size of which was much smaller than in the traditionalprocessed 316L stainless steel. These fine grains in the 316L stainless steel by LPBF increase the wear resistance, and the surface is subjected to slight plastic deformation

Investigation of the wear resistance of reinforced 316L stainless steel (with TiB2 or TiC) shows that the wear resistance increases with the increasing TiB2 content due to combined effects of grain refinement and grain-boundary

Metallic alloys by LPBF have generally pores and cracks that influence the wear under lubricated condition. Few studies are available in literature under boundary

In those studies [26, 30], when a lubricating film is not yet formed, the metal alloys by LPBF have better wear performance than metal alloys by traditional

Surface pores may positively influence the formation of the lubricating film. The effect of lubricant is critical in reducing friction and wear. The choice of oil needs to be carefully considered before applying LPBF process to hydraulic

In general, heat treatment (used to reduce the stress in material after LPBF process) reduces the wear resistance of metallic alloys. The wear resistance is reduced because the heat treatment changes the microstructure of metallic alloys by LPBF and lost the very fine microstructure. The most sensibility materials at heat treatment are aluminum alloys [15, 32]. The only metallic alloy that increases the wear resistance and reduces COF after heat treatments is Ti6Al4V because the oxidation of surface (if the heat treatment is realized in the presence of oxygen) produces a

In conclusion, metallic alloys by LPBF generally have higher wear resistance and less COF than metallic alloys produced by traditional processes under dry condition

The LPBF processing parameters are fundamental for wear rate since a fully

The existence of pores reduces the bonding between molten pools, resulting in cracks. These cracks can further cause material shell off which greatly

In general, the metallic alloys produced by LPBF are more difficult to machine

The presence of ceramic particle reinforcements in MMCs causes generally a reduction of COF; this effect is due to the detachment from the metallic matrix of ceramic particles that can act as a third body. The interfacial bond between the matrix and the reinforcements has a fundamental role in wear process; a strong interfacial bond guaranties a low wear rate; a weak interfacial bond causes a low

than the same metallic alloys produced by traditional processes. For this, it is

important to reduce at the minimum the post-process machining.

and boundary lubrication mainly due to the fine grains and high hardness.

densified part usually has high wear resistance and COF.

**100**

The author is grateful for the moral and scientific support of PhD Diego Manfredi, PhD Flaviana Calignano, and Professor Matteo Pavese.

### **Author details**

Massimo Lorusso Istituto Italiano di Tecnologia (IIT), Torino, Italy

\*Address all correspondence to: massimo.lorusso@iit.it

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[19] Marchese G, Colera XG, Calignano F, Lorusso M, Biamino S, Minetola P, et al. Characterization and comparison of Inconel 625 processed by selective

laser melting and laser metal deposition. Advanced Engineering Materials. 2016;**19**:3. DOI: 10.1002/

adem.201600635

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[15] Lorusso M, Aversa A, Manfredi D, Calignano F, Ambrosio EP, Ugues D, et al. Tribological behavior of aluminum ally AlSi10Mg-TiB2 composites produced by direct metal laser sintering (DMLS). Journal of Materials Engineering and Performance. 2016;**25**:3152-3160. DOI: 10.1007/s11665-016-2190-5

[16] Lorusso M, Trevisan F, Aversa A, Calignao F, Amborsio EP, Pavese M, et al. Macro-, micro- and nano-hardness and macro- and nano- wear behavior of aluminum alloys by laser powder bed fusion. In: Proceeding of the European Congress and Exhibition on Advanced Materials and Process (EUROMAT2017); Thessaloniki, Greece. 2017

[17] Gu D. Nanoscale TiC particlereinforced AlSi10Mg bulk-form nanocomposites by selective laser melting (SLM) additive manufacturing (AM): Tailored microstructures and enhanced properties. In: Laser Additive Manufacturing of High-Performance Materials. Berlin: Springer; 2015. pp. 175-199. DOI: 10.1007/978-3-662-46089-4\_6

[18] Marchese G, Bassini E, Calandri M, Ambrosio EP, Calignano F, Lorusso M, et al. Microstructural investigation of as fabricated and heat-treated Inconel 625 and Inconel 718 fabricated by direct metal laser sintering: Contribution of Politecnico di Torino and Istituto Italiano di Tecnologia (IIT) di Torino. Metal Powder Report. 2016;**71**:273-278. DOI: 10.1016/j.mprp.2016.06.002

[19] Marchese G, Colera XG, Calignano F, Lorusso M, Biamino S, Minetola P, et al. Characterization and comparison of Inconel 625 processed by selective laser melting and laser metal deposition. Advanced Engineering Materials. 2016;**19**:3. DOI: 10.1002/ adem.201600635

[20] Jia Q, Gu D. Selective laser melting additive manufacturing of Inconel 718 superalloy parts: Densification, microstructure and properties. Journal of Alloys and Compounds. 2014;**585**:712-721. DOI: 10.1016/j. jallcom.2013.091.171

[21] Zhang DY, Niu W, Cao XY, Liu Z. Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloys. Materials Science and Engineering A. 2015;**644**:32-40. DOI: 10.1016/j. msea.2015.06.021

[22] Rong T, Gu D, Shi Q, Cao S, Xia M. Effects of tailored gradient interface on wear properties of WC/Inconel 718 composites using selective laser melting. Surface & Coating Technology. 2016;**307**:418-427. DOI: 10.1016/j. surfcoat.2016.09.011

[23] Trevisan F, Calignano F, Aversa A, Marchese G, Lombardi M, Biamino S, et al. Additive manufacturing of titanium alloys in the biomedical properties and applications. Journal of Applied Biomaterials and Functional Materials. 2018;**16**(2):57-65. DOI: 10.5301/jabfm.5000371

[24] Bartolomeua F, Sampaioa M, Carvalhoa O, Pintob E, Alvesb N, Gomesa JR, et al. Tribological behaviour of Ti6Al4V cellular structures produced by selective laser melting. Journal of the Mechanical Behavior of Biomedical Materials. 2017;**69**:128-134. DOI: 10.1016/j.jmbbm.2017.01.004

[25] Bartolomeu F, Buciumeanu M, Pinto E, Alves N, Silva FS, Carvalho O, et al. Wear behavior of Ti6Al4V biomedical alloys processed by selective laser melting, hot pressing and conventional casting. Transactions of the Nonferrous Metals Society of China. 2017;**27**:829-838. DOI: 10.1016/ S1003-6326(17)60060-8

**102**

*Friction, Lubrication and Wear*

Springer; 2010

**References**

Associates; 2017

[1] Gibson I, Rosen WD, Stucker B. Additive Manufacturing Technologies-Rapid Prototyping to Direct Digital Manufacturing. New York, USA:

[8] Brito C, Reinhart G, Nguyen-Thi H, Mangelinck-Noel N, Cheung N, Spinelli JE, et al. High cooling rate cells, dendrites, microstructural spacings and microhardness in a directionally solidified Al-Mg-Si alloy. Journal of Alloys and Compounds. 2015;**636**:145-149. DOI:

[9] Trevisan F, Calignano F, Lorusso M, Pakkanen J, Aversa A, Amborsio EP, et al. On the selective laser melting (SLM) of the ALSi10Mg alloy: Process, microstructure and mechanical properties. Materials. 2017;**10**:76. DOI:

[10] Aversa A, Lorusso M, Trevisan F, Amborsio EP, Calignano F, Manfredi D, et al. Effect of process and post-process conditions on the mechanical properties of an A357 alloy produced via laser powder bed fusion. Metals. 2017;**7**:68.

10.1016/j.jallcom.2015.02.140

10.3390/ma10010076

DOI: 10.3390/met7020068

matdes.2016.07.009

matdes.2016.02.127

jallcom.2018.06.075

[11] Rao H, Giet S, Yang K, Wu X, Davies CHJ. The influence of processing

parameters on aluminium alloy A357 manufactured by Selective Laser Melting. Materials and Design. 2016;**109**:334-346. DOI: 10.1016/j.

[12] Leary M, Mazur M, Elambasseril J, McMillan M, Chirent T, Sun Y, et al. Selective laser melting (SLM) of AlSi12Mg lattice structure. Materials & Design. 2016;**98**:344-357. DOI: 10.1016/j.

[13] Reis BP, Lopes MM, Amauri G, Dos Santos CA. The Correlation of microstructure features, dry sliding wear behavior hardness and tensile properties of AL-2wt% Mg-Zn alloys. Journal of Alloys and Compounds. 2018;**764**:267-278. DOI: 10-1016/j.

[14] Aversa A, Marchese G, Manfred D, Lorusso M, Calignano F, Biamino S, et al. Laser powder bed fusion of a high

[2] Wohlers T. Wohlers Report 2017 State of Industry, Annual Worldwide Profess Report. Fort Collins CO, USA: Wohler

[3] Cordovilla CG, Narciso N, Louis E. Abrasive wear resistance of aluminum alloy/ceramic particulate composites. Wear. 1996;**192**:170-177. DOI: 10.1016/0043-1648(95)06801-5

[4] Manfredi D, Calignano F, Krishnan M, Canali R, Ambrosio EP, Biamino S, et al. Additive Manufacturing of Al Alloys and Aluminium Matrix Composites (AMCs) in Light Metal Alloys Applications. InTech; 2014. pp. 3-34. DOI: 10.5772/58534

[5] Calignano F, Manfredi D, Ambrosio

[6] Aversa A, Marchese G, Lorusso M, Calignano F, Biamino S, Ambrosio EP, et al. Microstructural and mechanical characterization of aluminum matrix composites produced by laser powder bed fusion. Advanced Engineering Materials. 2017;**19**:11. DOI: 10.1002/

[7] Kang N, Coddet P, Chen C, Wang Y, Liao H. Coddet C; Microstructure and wear behavior of in-situ hypereutectic Al-high Si alloys produced by selective laser melting. Materials and Design. 2016;**99**:120-126. DOI: 10.1216/j.

EP, Iuliano L, Fino P. Influence of process parameters on surface roughness of aluminum parts produced by DMLS. International Journal of Advanced Manufacturing Technology. 2013;**67**:2743-2751. DOI: 10.1007/

s00170-012-4688-9

adem.201700180

matdes.2016.03.053

[26] Zhu Y, Chen X, Yang H. Sliding wear of selective laser melting processed Ti6Al4V under boundary lubrication conditions. Wear. 2016;**36**(8, 369):485- 495. DOI: 10.1016/j.wear.2016.09.020

[27] Patila AS, Hiwarkara VD, Verma PK, Khatirkarc RK. Effect of TiB2 addition on the microstructure and wear resistance of Ti-6Al-4V alloy fabricated through direct metal laser sintering (DMLS). Journal of Alloys and Compounds. 2019;**777**:165-173. DOI: 10.1016/j.jallcom.2018.10.308

[28] Liverani E, Toschi S, Ceschini Km Fortunato A. Effect of selective laser melting (SLM) process parameters on microstructure and mechanical properties of 316L austenitic stainless steel. Journal of Materials Processing Technology. 2017;**249**:255-263. DOI: 10.1016/j.jmatprotec.2017.05.042

[29] Zhu Y, Zou J, Chen X. Yang H; Tribology of selective laser melting processed parts: Stainless steel 316L under lubricated conditions. Wear. 2016;**350-351**:46-55. DOI: 10.1016/j. wear.2016.01.004

[30] Li H, Ramezani M, Li M, Ma C, Wang J. Tribological performance of selective laser melted 316L stainless steel. Tribology International. 2018;**128**:121-129. DOI: 10.1016/j. triboint.2018.07.021

[31] AlMangour B, Grzesiak D, Yang JM. Rapid fabrication of bulk-form TiB2/316L stainless steel nanocomposites with novel reinforcement architecture and improved performance by selective laser melting. Journal of Alloys and Compounds. 2016;**680**:480-493. DOI: 10.1016/j.jallcom.2016.04.156

[32] Lorusso M, Manfredi D, Calignano F, Ambrosio EP, Trevisan F, Pakkanen J, et al. Wear behavior of aluminum matrix composites by DMLS reinforced with

micro- and nano-TiB2; In Proceeding of the European Congress and Exhibition on Advanced Materials and Process (EUROMAT2015); Warsaw, Poland. 2015

**105**

**Chapter 7**

Tribological Characteristics

Rheological Fluids and

Elastomers) and Their

performances of these materials are evaluated.

smart material, friction control

**1. Introduction**

applications.

*Peng Zhang, Chenglong Lian, Kwang-Hee Lee* 

Magneto-rheological fluids (MRFs) and magneto-rheological elastomers (MREs), as smart materials, have been widely studied in various engineering fields to address vibration issues because the mechanical properties are controllable under the strength of a magnetic field. Their tribological characteristics are also important to be evaluated, as applications using MRFs and MREs contain various contact interfaces under reciprocating and rotating working conditions. The performance and durability of these materials are related to their tribological characteristics. Therefore, various working conditions and environmental conditions are taken into consideration, and their tribological characteristics are experimentally examined. In addition, applications using MRFs and MREs are introduced, and the tribological

**Keywords:** tribology, magneto-rheological fluid, magneto-rheological elastomer,

Magneto-rheological fluids (MRFs) and magneto-rheological elastomers (MREs) have been extensively studied to solve vibration problems in various engineering fields. They have one or more attributes that can be significantly changed in a controlled way by external stimuli (magnetic fields). It is essential to evaluate the tribological properties as they relate to the performance of smart material-based

MRF consists of base fluid with magnetic particles forming chain shape along the magnetic field direction. Because of its fast response speed, it has the potential to be applied to various industrial sectors such as automotive, aviation, construction, etc., [1]. Although research on MRF-based applications with control method is

For example, tribological properties of magnetic particles in MRF are examined

being conducted, tribological characteristics of MRF remain in early stage.

by Bullough [2]. It is noted that the parameters such as particle concentration,

Applications

*and Chul-Hee Lee*

**Abstract**

of Smart Materials (Magneto-

### **Chapter 7**

*Friction, Lubrication and Wear*

[26] Zhu Y, Chen X, Yang H. Sliding wear of selective laser melting processed Ti6Al4V under boundary lubrication conditions. Wear. 2016;**36**(8, 369):485- 495. DOI: 10.1016/j.wear.2016.09.020

micro- and nano-TiB2; In Proceeding of the European Congress and Exhibition on Advanced Materials and Process (EUROMAT2015); Warsaw, Poland. 2015

[27] Patila AS, Hiwarkara VD, Verma PK, Khatirkarc RK. Effect of TiB2 addition on the microstructure and wear

resistance of Ti-6Al-4V alloy fabricated through direct metal laser sintering (DMLS). Journal of Alloys and Compounds. 2019;**777**:165-173. DOI:

[28] Liverani E, Toschi S, Ceschini Km Fortunato A. Effect of selective laser melting (SLM) process parameters on microstructure and mechanical properties of 316L austenitic stainless steel. Journal of Materials Processing Technology. 2017;**249**:255-263. DOI: 10.1016/j.jmatprotec.2017.05.042

[29] Zhu Y, Zou J, Chen X. Yang H; Tribology of selective laser melting processed parts: Stainless steel 316L under lubricated conditions. Wear. 2016;**350-351**:46-55. DOI: 10.1016/j.

[30] Li H, Ramezani M, Li M, Ma C, Wang J. Tribological performance of selective laser melted 316L stainless steel. Tribology International. 2018;**128**:121-129. DOI: 10.1016/j.

wear.2016.01.004

triboint.2018.07.021

[31] AlMangour B, Grzesiak D, Yang JM. Rapid fabrication of bulk-form TiB2/316L stainless steel nanocomposites with novel reinforcement architecture and improved performance by selective laser melting. Journal of Alloys and Compounds. 2016;**680**:480-493. DOI:

10.1016/j.jallcom.2016.04.156

[32] Lorusso M, Manfredi D, Calignano F, Ambrosio EP, Trevisan F, Pakkanen J, et al. Wear behavior of aluminum matrix composites by DMLS reinforced with

10.1016/j.jallcom.2018.10.308

**104**
