Effect of heat treatment on tribological behavior of direct metal laser sintered alloy 718

Summary

The main aim of this work is to enhance the wear performance of the direct metal laser sintered (DMLS) alloy 718 by solution treatment aging (STA) method at room temperature (RT) (28°C) and 400°C in dry sliding conditions. The effect of microstructure, phase analysis, and microhardness on the wear behavior and the influence of STA on the specimen at elevated temperatures were studied. The microstructure revealed the presence of melt pool boundary (MPB) in untreated DMLS alloy while recrystallized grains were observed in the STA-treated alloy. The wear results elucidated that STA-treated alloy exhibited better wear resistance than as-built alloy due to high hardness at both conditions. Severe wear loss occurred at high temperatures caused by the delamination of the brittle oxide glazing layer, while oxidation and adhesive wear were the predominant wear mechanisms. Results also portrayed that the test temperature and STA treatment equally influenced the wear behavior of alloy 718.

Graphical abstract

Highlights

• •Fabrication of alloy 718 through direct melt laser sintering (DMLS)
• •Solution treatment aging (STA) was used to enhance the properties of the alloy
• •Precipitation strengthening phases increase the STA-treated alloy hardness by 40.9%
• •STA-treated alloy 718 reduces the wear rate by 46.09% at high temperature (400°C)

Materials science; Materials processing; Heat treatment

Introduction

Ni-based superalloy 718 is a precipitation-hardening alloy widely used in critical applications such as energy and aero-engine sectors owing to its high mechanical properties, corrosion, and oxidation resistance.^1,2 Alloy 718 is extensively prepared for high-temperature conditions such as turbochargers, motor shafts, heat exchangers, steam engines, and nuclear systems. The strength of the alloy 718 could be enhanced by the formation of γ′(Ni₃(Al, Ti)) and γ″(Ni₃Nb) precipitates. It is significant to fabricate the complex and near net shape alloy 718 parts in critical applications, which is very challenging by conventional methods (casting and forming), and machining is also challenging due to precipitation hardening (work hardening nature) and high hardness.^3,4

Additive manufacturing (AM) is the most suitable method to fabricate complex and net-shaped profiles in a single step. Among AM techniques, direct metal laser sintering (DMLS) is a very popular and the most promising method to fabricate metallic components using a high-power laser to melt and fuse the metal powders and deposit them in a layer-by-layer manner.^5,6,7,8 The microstructure of the DMLS-built alloy has distinct microstructure as compared to the wrought alloy. Several researchers studied the microstructure and mechanical properties of the additively manufactured alloy 718. Amato et al.⁹ studied the microstructural, tensile and hardness behavior of the DMLS-built alloy 718. Sumanth et al.¹⁰ studied the tensile and microstructural characteristics of the 718 using the laser scanning strategy. The authors observed the columnar microstructure along the build direction and segregation at the grain boundaries.

The components used in the aerospace sectors are subjected to thermal fatigue, wear, creep, and erosion at elevated temperatures. Wear is a serious issue at high temperatures owing to the loss of mechanical strength and hardness of materials and the development of a brittle glaze layer. In critical applications such as thrust propulsion systems, aircraft thrust bearings, steam generator components, and control rod drive mechanisms of nuclear reactors, the alloy 718 components are prone to wear at high temperatures due to friction. This may alter the surface properties of the components and further reduce performance. For this reason, the wear test was conducted at a higher temperature.^11,12 Samuel et al.¹³ evaluated the effect of temperature on the tribological performance of alloy 718 fabricated using laser powder bed fusion (LPBF) method. The authors found that the abrasive wear mechanism at room temperature (RT) and oxidative wear mechanism at elevated temperature. In addition, wear loss and debris size increase with an increase in temperature. Sun et al.¹⁴ reported that wear rate of the alloy 718 was high at 100°C and 200°C. It was also reported that the wear rate decreased with the increase in temperature to 500°C and 600°C owing to the formation of glazing oxide layer. Krishnan et al.¹⁵ investigated the wear and friction behavior of the DMLS fabricated alloy 718 at different loads (10, 15, and 20 N) and speeds (2 and 4 m/s). The authors reported that the wear rate increased with an increase in applied loads and speed. However, the co-efficient of friction (COF) rate decreased at higher loads and speeds. In addition, delamination and an adhesive wear mechanism were noticed from the abrasive wear mode.

Several studies have been significantly focused on improving the life of industrial components under service conditions. Heat treatment (HT) is a widely used and effective post-processing technique that could enhance the strength by grain structure modification, relieving the residual stress, increase the hardness and wear resistance due to precipitation hardening observed by many researchers.^16,17 The effect of HT on the wear properties of alloy 690 was investigated by Hong et al.,¹⁷ who reported plastic deformation under an abrasive wear mechanism. Shanmugam et al.¹⁸ compared the elevated temperature wear behavior of DMLS-built alloy Ti-6Al-4V in STA condition and untreated condition. It was stated that STA treatment significantly enhanced the wear resistance of the alloy. Nedim Sunay et al.¹⁹ performed the HT process on the LPBF-built alloy 718 with different process parameters and further studied the alloy’s mechanical and wear performance. The authors concluded that the HT significantly increased the microhardness and wear performance due to the microstructural changes.

From the literature, it is evident that there are not enough investigations on the influence of STA on the wear behavior of the alloy 718 at high temperature fabricated through DMLS technique. The microstructural variation between the as-built DMLS alloy and STA-treated alloy affects the wear behavior of the alloy. Therefore, it is essential to study the influence of fabrication method, microstructure, and HT on the wear performance of the alloy 718. To understand the wear mechanism, the worn samples were undergone scanning electron microscope (SEM) analysis and debris was also analyzed.

Results and discussion

Characterization of STA alloys

Figure 1 depicts the spherical shape morphology of the alloy 718 powders (compositions are given in Table 1) used for DMLS fabrication (print parameters are given in Table 2) in the range of 20–40 μm. X-ray diffraction (XRD) result of powder (Figure 1B) illustrates the γ-Ni-Cr (FCC). Optical images (Figure 2A) depict the MPB on the as-built alloy 718, which exhibits small columnar along with cellular dendrites because of rapid solidification, reheating and remelting. Dendrites growth direction is orientated with heat flux direction, i.e., building or deposition direction. During the solidification, the heat flow direction is perpendicular to the pre-deposited layer or the substrate surface.

Figure 1.

SEM and XRD analysis of Alloy 718 powders

(A and B) Alloy 718 powder’s (A) SEM morphology (100 μm scale bar) and (B) XRD analysis.

Table 1.

Compositions of alloy 718

Elemental constituents (wt. %)	C	Cr	Mo	Mn	Al	S	Ti	Nb	Fe	Ni
Alloy 718	0.05	18.2	3.06	0.06	0.65	0.03	0.85	4.98	17.4	54.72

Table 2.

Process parameters

Parameters	EOS:M280 (DMLS)
Laser power (W)	320
Hatch distance (mm)	0.2
Scan speed (mm/s)	900
Beam diameter (mm)	0.08
Layer thickness (μm)	20–30

Figure 2.

Microstructure and XRD analysis of as-built and STA alloy 718

(A and B) As-built and STA alloy 718 (A) and (B) microstructure (100 μm scale bar).

(C) and (D) XRD analysis.

STA processed alloy (Figures 2A and 2B) depicts coarse recrystallized grains when compared with the untreated alloy. This indicates the grains recrystallization after STA, causing coarse grains to be found. In addition, grains and grain boundaries were replaced with melt pool boundaries. Thereby, columnar and cellular dendrites did not appear.^20,21 XRD results (Figures 2C and 2D) reveal the presence of precipitation strengthened γ(Ni-Cr-Fe), γ′(Ni₃(Al, Ti) and γ″(Ni₃Nb) phases on the as-built and STA alloy.

Effect of treatment on tribological properties

Figure 3A represents the COF vs. sliding time curve of the as-built and STA alloy 718 at RT and 400°C. During the starting stage of the wear test, COF was raised rapidly because of the static to kinetic friction transformation and rupture of material from the samples by the dry friction process.¹³ As-built sample at RT, more fluctuation with subsequent raise in COF (0.74) up to a certain time (360 s) was noticed at the beginning of the test due to the abrasion wear mechanism. Then, it gets stabilized (0.6), which is attributed to the asperity junction formation between the sample and the disc. High COF (0.68) was noticed on the untreated sample during the run-in period at 400°C. The main reason for the unstable run-in period is the growth of asperity junction and its rupture because of excessive friction.

Figure 3.

COF and specific wear rate of DMLS-built STA alloy

(A and B) DMLS-built STA processed alloy 718 (A) COF and (B) specific wear rate.

STA sample at RT shows a low COF value in the running-in period (till 350 s) and raises rapidly (0.64), then stabilizes after a sliding time of 600 s at 0.55. STA sample at 400°C does not showcase any significant change in running-in-period (0.5). Then, COF was raised suddenly (0.74) due to the rough contact surface being attributed to the delamination of the glaze layer, and the wear debris (third body) was cold welded with wear track and further stabilized (0.64) till the end of the test. The COF plot describes that testing temperature and time play a significant role in determining the COF. Furthermore, high COF was found in both conditions at 400°C owing to the development of the glaze oxide layer, which enhances the asperity junction interaction with the rough surface.^13,22 The plastic deformation asperities are more severe at 400°C, resulting in an unstable friction coefficient (COF) during the initial sliding stage, which is called as running in time. As the temperature increases, the running-in distance decreases due to reduced yield strength, allowing faster rupture of the surface asperities. In addition, the alloy 718 has higher oxidation resistance at elevated temperatures. However, the oxide glaze layer was developed on the alloy at 400°C. This may be attributed to the increase in specimen temperature during the wear test due to the friction between the substrate and disc. Similarly, Samuel et al.¹³ found the formation of an oxide glaze layer on the alloy 718 during the wear test at 400°C. The authors reported that the developed oxide glaze layer significantly played a major role in affecting the wear and COF rate. In addition, the existence of wear debris between the substrate and disc also further increase the COF rate during the wear test.

The wear rate plot (Figure 3B) demonstrated that high wear loss was found at elevated temperatures in both samples. Due to friction, the oxide glaze layer became weaken and delamination, resulting in more wear loss. Similarly, Samuel et al.¹³ observed the increase in wear rate of the alloy 718 during the wear test at elevated temperatures (400°C and 600°C). The hardness of the alloy was investigated by the Vickers microhardness testing method, and the average hardness of the as-built and STA alloys was reported as 322 ± 8.15 and 454 ± 10.58 HV, respectively (Table S1). The hardness of the alloy was significantly improved by STA (40.9%) due to the occurrence of precipitation-strengthening phases. STA samples significantly reduced the wear loss than as-built alloy due to high hardness.²⁰

Worn surface morphology of STA alloys

SEM worn surface of the as-built and STA alloy tested at RT (Figures 4A and 4C) shows deep grooves and a delaminated area, which indicates the abrasive mechanism and some fine debris was found. A similar wear mechanism was observed on both samples tested at RT. Hertzian contact stress is responsible for increasing the asperity contact region, causing spalling pits to form, which are in the same direction and oriented toward the sliding direction. Loose wear debris and disc asperities are responsible for the groove’s formation. A glaze layer was not found on both the wear-tested samples at RT, indicating that heat produced due to friction is not enough to produce the layer.^12,17

Figure 4.

SEM analysis of wear track and debris of as-built and STA alloy at RT and 400°C

(A–D) SEM wear track and debris analysis (A) as-built RT (30 μm scale bar); (B) as-built at 400°C (50 μm scale bar); (C) STA at RT (30 μm scale bar); and (D) STA at 400°C (40 μm scale bar).

The worn surface of the wear-tested samples at 400°C (Figures 4B and 4D) exhibits the presence of loose scales, cracks, and delamination, which intensifies the adhesive, abrasive, and oxidative wear mechanism. Frictionally induced temperature and test temperature are the main reasons for oxidation, causing the glazing layer to be developed.^23,24 The produced glaze layer acts as a lubricant, causing low COF for some time. Then, COF was increased suddenly due to the failure of the glaze layer, causing spallation (delamination). This leads to debris which also could damage the layer and further accelerate the wear and friction rate.²⁴ This point wear loss was dominated by both oxidation wear and delamination. Crack nucleation leads to initiating delamination, and crack propagation was obtained due to the oxide glaze layer, which is brittle in nature.^13,25,26 Worn surfaces at elevated temperatures become rougher, causing high COF. Wear debris morphology (Figure 5) illustrates that their size linearly increased with the test temperature, suggesting the occurrence of continuous delamination. The large size of wear debris particles indicates higher wear and friction rates at high temperatures. The presence of a large amount of wear debris on the worn surface may be due to the brittle nature imparted onto the alloy 718, resulting in brittle fractures at the interface and less plastic deformation.

Figure 5.

Debris SEM analysis of as-built and STA alloy at RT and 400°C

(A–D) SEM debris analysis (A) as-built RT (5 μm scale bar); (B) as-built at 400°C (5 μm scale bar); (C) STA at RT (5 μm scale bar); and (D) STA at 400°C (5 μm scale bar).

The schematic diagram (Figure 6) shows the wear mechanisms of the alloy, which clearly explains the development of the oxide layer during the initial stage of the wear test. Oxidative wear is the primary mechanism due to the formation of an oxide glaze layer during the wear test on alloy 718 at 400°C. The wear debris was produced due to the delamination of the oxide glaze layer. The debris can adhere due to the substrate surface frictional heating during sliding, which leads to scratching or rolling motion. When increasing the test time, oxide delamination occurred due to the abrasive wear mechanism at RT and adhesive and oxidative wear mechanisms at high temperatures. It could be understood from the experimental results that a high wear rate was found at elevated temperatures.

Figure 6.

Wear mechanism of the alloy at room temperature and 400°C

Limitations of the study

Alloy 718 fabricated via DMLS can be significantly improved through HT processes such as STA. In all cases, the STA method provides a microstructure with recrystallized grains that increase hardness and result in improved wear resistance at RT and high temperatures. Results also revealed that the influence of temperature on the wear behavior of alloy 718 was quite considerable. This is especially the case at RT where the major mode of wear experienced by the alloy is abrasive contact wear. But for a temperature of 400°C, the wear processes transform to adhesive and oxidative wear due to the formative and delaminating phase of a fragile oxide glazing layer. These increasing temperatures lead to greater wear, a higher COF, and a greater size of wear debris. The microstructural characterization shows the presence of MPB in the as-built alloy (DMLS) and the recrystallized grains in the alloy subjected to STA. STA process changes the crystal structure and as a consequence, improves hardness and resistance to wear. The STA treatment significantly reduced the abrasion and delamination wear of alloy 718. In addition, STA reduced the coefficient of friction due to the high elastic strain limit, resulting in less plastic deformation. Therefore, the wear and friction rates are very low in the heat-treated specimens (reduces the wear rate by 46.09% at 400°C). High-temperature stability of alloy 718 is further enhanced by STA treatment; making it ideal for demanding applications such as heavy-duty turbine blades, aerospace components, automotive applications, and more that are prone to high-temperature mechanical and thermal stresses.

Meanwhile, there are certain limitations observed during current research. The deterioration of the material in this work is investigated under two different temperatures; the RT and the elevated temperature of 400°C. Regarding the wear properties of alloy 718, there is no research done on this material at a temperature higher or lower than the researched range. This limits the understanding of the capability of the alloy in a diverse range of operation conditions. This work is specifically mainly focused on the assessment of short-term wear behavior. This paper does not consider long-term behavior in the form of reference creep-fatigue properties and in terms of the results of repetitive temperature cycling. This factor is very critical for those who require reliability for extended periods. The analysis only concentrates on dry sliding conditions. The wear characteristics of the alloy under these conditions may therefore be different and should be investigated to understand its performance under these conditions of lubrication or other circumstances that alter the tribological conditions.

Conclusions

The direct metal laser sintering technique was used to fabricate alloy 718 followed by a STA process. The wear behavior of the untreated and treated alloy 718 was compared at room and high temperatures. The microstructure of the treated alloy showcased that the solution aging resulted in the formation of recrystallized grains. STA significantly enhanced the wear resistance of the alloy and reduced the plastic deformation due to high hardness. The wear rate and the coefficient of friction were high at elevated temperatures when compared with wear rates at RT. High wear loss was obtained at 400°C at both conditions, causing failure of the brittle glaze layer due to friction. The wear-tested alloys exhibited abrasive wear at RT and high-temperature adhesive and oxidative wear were observed due to the formation of an oxide glazing layer, Loose oxides mainly occurred in the alloy tested at elevated temperatures. The particle size of the wear debris was found to be directly proportional to the test temperature.

Due to its superior high-temperature strength retention, STA heat-treated alloy 718 is a good option for more demanding applications such as turbine blades, aerospace components, and select high-performance automotive parts subjected to both thermal and mechanical loading. For further research work on the long-term mechanical properties of alloy 718 at elevated temperatures under creep-fatigue loading conditions, other temperature ranges, and different corrosive environments could also be considered. The intention is to broaden the fields of application for alloy 718 through these investigations.

Resource availability

Lead contact

Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Sathishkumar M (m_sathishkumar@ch.amrita.edu).

Materials availability

This study did not generate new unique reagents.

Data and code availability

• •All data needed to evaluate the conclusions in the paper are present in the article. The datasets generated and/or analyzed during the current study are available from the lead contact upon reasonable request.
• •This article does not report the original code.
• •Any additional information required to reanalyze the data reported in this article is available from the lead contact upon request.

Acknowledgments

The authors would like to thank Chennai Institute of Technology, India and Amrita Vishwa Vidyapeetham, India for providing support in carrying out this research work.

Author contributions

Conceptualization and methodology M.S.M. and V.A.; resources, V.A. and D.V.; data curation, V.K.L.; writing – original draft, M.S.M. and V.K.L.; writing – review and editing, S.M., V.M., and L.R.; visualization, D.V., V.M., and L.R.; supervision, S.M.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Alloy 718 powders (20–40 μm)	EOS GmbH	https://www.eos.info/en
Kelling etchant (4 g CuCl₂ + 80 mL HCl +80 mL C₂H₅OH)	Laboratory preparation	N/A
Argon gas	Praxair	https://www.praxair.co.in
Other
Direct metal laser sintering (DMLS) Machine	EOS M280	https://www.njitmakerspace.com/equipment/eos-m280-dmls-metal-3d-printer
Optical emission spectroscopy (OES)	Thermo Fisher Scientific	https://www.thermofisher.com/in/en/home/industrial/spectroscopy-elemental-isotope-analysis/oes-xrd-xrf-analysis/optical-emission-spectrometry.html
Muffle furnace	Nabertherm	https://nabertherm.com/en/products/laboratory/muffle-furnaces?gad_source=1&gclid=Cj0KCQjw1Yy5BhD-ARIsAI0RbXb_H36RJL8gLZ9lOgtnhm_dBHzDCGfjtTvlgxLPLWTNcy48k4npPOUaAqQpEALw_wcB
Pin-on disc wear tester	DUCOM-TR-20-PHM600	https://www.ducom.com
X-ray diffraction (XRD)	BRUKER USA, D8 Advance, Davinci	https://www.bruker.com/en/products-and-solutions/diffractometers-and-x-ray-microscopes/x-ray-diffractometers/d8-advance-family/d8-advance.html
Optical microscope (OM)	Leica microsystems, Leica DM2700 M	https://www.leica-microsystems.com/products/light-microscopes/p/leica-dm2700-m/
Microhardness Tester	Matsuzawa MMT-x	http://www.matsuzawa-ht.com/us/item/m_vick.htm
Calculator.net	Calculator.net	https://www.calculator.net/math-calculator.html
Scanning electron microscope (SEM)	Thermo Fisher Scientific, Apreo 2 SEM	https://www.thermofisher.com/in/en/home/electron-microscopy/products/scanning-electron-microscopes/apreo-sem.html

Method details

DMLS process and alloy 718 preparation

Alloy 718 is used as a substrate material in this study and is extensively used in high-temperature applications such as aerospace, turbines, bolts and fasteners. Alloy 718 samples were produced in cylindrical form (4 mm diameter and 40 mm height) using direct metal laser sintering (DMLS) in an EOS M280 machine. The gas-atomized spherical shape alloy 718 powders of 20–40 μm were used to prepare the specimens for the DMLS process. The particle size of the powder is found to have a significant influence on the properties of the fabricated specimens as well as wear characteristics. Finer powder particles have a positive effect on the packing density of the applied powders and, as a result, better part density and lower porosity in the DMLS process. The composition of the DMLS-built alloy 718 was determined using Optical emission spectroscopy. The chemical composition of the fabricated alloy and the parameters used for the DMLS process are listed in Tables 1 and 2 respectively. Many trials with different parameters were performed to obtain the optimum parameters for the DMLS process to fabricate the alloy 718. To minimize the thermal residual stress and defects, the substrate is maintained between 70 and 100°C and continued until the finished product is obtained. Such a controlled temperature became essential to come up with quite a good product through minimization of the warping action while at the same time ensuring that there was uniformity in the various material properties. The process was performed under an argon gas atmosphere to reduce oxidation.

Heat treatment process

STA was done on the DMLS-built alloy to alter the properties according to AMS 5664. The HT was performed on alloy 718 in an argon-inert gas atmosphere (to prohibit foreign particle intervention) using a muffle furnace. Samples were exposed to a temperature of 1040°C for 2 h over a certain period. During this solutionizing, the alloying components were expected to dissolve into a solid solution and after they had been raised to this temperature they were air-cooled until they reached RT. To accomplish the desired precipitation hardness, the solutionized samples were put through a two-step aging procedure, which included the following steps: The samples were heated to 720°C and held at that temperature for 8 h after which the samples were cooled in air. After the first stage of aging, the samples were heated to 620°C and held at the temperature for a further 8 h. Further, samples were left to cool in the air, until they reached RT. After heat-treatment, the specimens were polished with SiC sheets to remove the oxides and other impurities.

High-temperature wear test

Pin-on disc wear testing was carried out on the DMLS and STA-treated alloy 718 at different temperatures (room and high temperature) according to ASTM G99 standard. Here, alloy 718 was used as a cylindrical form pin with a dimensions of 35 mm length and 6 mm diameter, and EN31 steel (Hardness: 64HRC) was used as a disc material. The parameters used for the wear test, such as sliding speed, distance, and load, are 1 m/s, 1000 m and 20 N, respectively. A data acquisition system interfaced with the tribometer was used to record the coefficient of friction values. The temperature sensor is connected to specimens during the test to ensure the test temperature. The wear debris particles were collected for further analysis. The effect of STA-treatment on the wear damage mechanism was analyzed at RT (28°C) and 400°C in detail.

Characterization

The phases and microstructure of the as built and heat-treated samples were investigated by XRD (Make: BRUKER USA, Model: D8 Advance, Davinci) and Optical microscope (Make: Leica microsystems, Model: Leica DM2700 M). For the microstructural analysis, the alloy 718 samples were polished mechanically with SiC sheets of different grit sizes followed by polishing in an alumina disc to remove the scratches. Then, the mirror-finished samples were etched with Kelling etchants (4 g CuCl₂ + 80 mL HCl +80 mL C₂H₅OH). The worn surface morphology and wear debris morphology were characterized using a scanning electron microscope (SEM) (Make: Thermo Fisher Scientific, Model: Apreo 2 SEM) to understand the wear mechanism.

Microhardness measurements

The microhardness analysis (as per ASTM E384) was performed on the DMLS-built and STA-treated alloy 718 at the cross-section using Matsuzawa MMT-x Vickers microhardness tester. For the hardness measurement, a load of 100 gf was applied using a diamond-tipped indenter for the dwell time of 10 s with a regular interval of 0.5 mm. The microhardness readings were taken at 10 different locations for as-built and STA-treated alloy 718 (Table S1).

Quantification and statistical analysis

Data of microhardness are given as mean ± standard deviation. The Table S1 contains the actual mean, standard deviation, variance and estimated margin of error for the sample size of 10. As-built alloy 718 sample has a mean of 322 HV, a standard deviation of 8.15 and a variance of 66.4. The STA-processed alloy 718 sample has a mean of 454 HV, a standard deviation of 10.58, and a variance of 112. In each of the datasets, the margin of error (confidence interval) is calculated using standard error of the mean (SEM) with a 95% confidence level. Statistical analysis was conducted using calculator.net (https://www.calculator.net/math-calculator.html) for obtaining the precision of the mean at the 95% confidence level.

Additional resources

No additional resources were generated or used.

Published: November 18, 2024