Effect of SiC Addition on Microhardness and Relative Density during Selective Laser Melting of 316L Stainless Steel
The utilization of 316L stainless steel has been very common in marine, automotive, architectural, and biomedical applications due to its adequate corrosion resistance to cracks after the completion of welding process. However, there has been ongoing attempts to investigate the potential enhancement in the strength and durability of 316L stainless steel by reinforcing it with silicon carbide (SiC). The present work adopts the selective laser melting (SLM) technique to fabricate SiC-reinforced 316L steel to boost its microhardness and strength properties. The methodology involved the addition of 1% wt. silicon carbide with particle sizes <40 μm to reinforce the stainless steel matrix. An SLM metal printing machine equipped with a continuous wave of 300 W fiber laser is employed to form the specimens. To measure the properties of the final product, EDX, XRD, FESEM, and universal tensile test machines have been used. The maximum value of 296 HV was obtained for a 1% volume of SiC compared to the 285 HV microhardness of pure stainless steel 316L. FESEM examination showed that the SiC microparticles were dissolved completely and they were randomly distributed in the melting basin. The samples were dissolved entirely, and the best porosity was obtained at 0.4% with influential parameters of 200 W laser power, 70 µm hatching distance, 30 µm layer thickness, and 700 mm/s velocities. The results also revealed that the microhardness at these parameters is the best compared to the samples produced with different values. The volumetric energy density was also considered. The findings can be informative to the researchers and manufacturers interested in 316L steel industry.
Lasers are utilized in a variety of industries including welding, drilling, cleaning, ablation, and so on [1–5]. Additive manufacturing is an emerging field for lasers in industrial applications, and it has a promising future. The additive manufacturing (AM) technique is defined by ASTM International as the process of joining materials to build parts or objects from 3D model data, usually layer upon layer, in contrast to subtractive manufacturing methodologies . Metal matrix composites (MMCs) are typically fabricated by continuous spreading for a reinforcing into a monolithic metallic material matrix . A metallic matrix is typically combined with strong ceramic reinforcements to create metal matrix composites (MMCs). Traditional examples of hard ceramic materials that are used as reinforcements are SiC, TiC, B4C, Al2O3, TiB2, and ZrO2. The main reason for using silicon carbide (SiC) as a reinforcement material is because of certain distinct advantages that are usually non-existent in other reinforcements, such as low cost, adequate hardness, and excellent corrosion resistance in addition to its resistance to oxidation at high temperatures [8, 9]. Additive manufacturing (AM) (also called 3D printing technique) allows for the construction of very complex geometries in metals, ceramics, composites, and polymers that are impossible to be fabricated using traditional subtractive processing methods [10–12]. The laser powder bed fusion (LPBF) is an effective laser-based AM technology that has been lately employed for fabricating complex metal components. This technology is also known as selective laser melting (SLM) [13, 14]. The SLM has sparked broad interest due to its advantages concerning design flexibility and efficient production, which overcome the limits of traditional processing technologies [15, 16]. In the SLM process, a beam scans over a powder bed and selectively melts powder particles to form 3D objects layer by layer according to the layout in a computer-aided design file [17–21]. The process of SLM has been found appropriate for several materials, especially for the metal alloys used in engineering [22–25]. It is agreed that the SLM technology has significant popularity as a speedy prototyping technique with a delicate microstructure because of the high cooling rate which could lead to yielding a solidification process that is non-equilibrium. As a consequence, the process can produce special characteristics regarding aspects such as microstructures, chemical composition, phase, and mechanical properties . Nevertheless, one of the recognized disadvantages of this process is the limitation of the sample size, which is related to the issue of elaborating the dimensions of chamber [27, 28]. According to the literature, in aerospace, marine, and biomedical applications, the widely used alloy is the austenitic stainless steel, mainly due to two of its outstanding properties—corrosion resistance and ductility . Furthermore, it was reported that the specific recognized austenitic stainless steel’s properties, such as adequate formability and excellent resistance to oxidation and corrosion, have popularized its use in several modern industries as an engineering material .
Asif Ur Rehman et al. investigated the influence of SiC particles on Al2O3 in powder bed selective laser process (PBSLP). It was reported that the use of silicon carbide as an additive can aid in averting the appearance of cracks; this is usually accomplished by two mechanisms: crack deflection and crack pinning. During both the classical and additive manufacturing techniques, Al2O3 and SiC have several applications when Al2O3 is adopted as a matrix or when it is used as an additive in the silicon carbide . When SiC is used to boost 316L metallic matrix composites, it influenced the densification effectiveness, microstructure development, crystallographic orientation, and tribology-related properties . The potential influence of the processing factors of direct laser deposition (DLD) additive manufacturing of 316L/SiC metal matrix composites on the microhardness and structure of the resulting samples was examined . Due to the combined properties of the Al matrix and reinforcements such as SiC/Al, it was demonstrated that particulate-reinforced aluminum matrix composites (PAMCs) are significant materials for many applications . The density, microhardness, and parts produced with varied particle sizes and mass fractions of TiC with 316L stainless steel were also studied . The effects of the Gr content on dry sliding wear and hardness were explored, and according to the findings, a composite with good tribological performance can be efficiently manufactured via SLM .
This study aims to manufacture a product from AISI 316L that is reinforced with only 1% silicon carbide to improve the mechanical properties using the selective laser melting technique. The current study used laser parameters and particle sizes for steel and silicon carbide that mostly differ from those adopted in the studies previously mentioned.
2. Materials and Methods
The experimental process involved the use of commercial 316L stainless steel powder obtained by gas atomization with particle sizes <65 μm. Figure 1 illustrates that the SS 316L powder is spherical after being examined by a scanning electron microscope (SEM). Figure 2 demonstrates the energy-dispersive X-ray spectrometry (EDX) for stainless steel sample products where the proportions of elements in stainless steel can be observed.
The silicon carbide with particle sizes <40 μm was used as an additive to reinforce the stainless steel matrix. A certain percentage of silicon carbide grade (1% wt) has been added to investigate their potential effects on the microhardness and relative density of stainless steel 316L manufactured by SLM. The composite powder was created utilizing mechanical mixing in a ball mill with a rotation speed of 100 rpm for 45 minutes. The average chemical composition of the stainless steel is illustrated in Table 1. Cubic specimens with dimensions of (10 × 10 × 10) mm3, as depicted in Figure 3, have been fabricated from stainless steel 316L with 1% silicon carbide.
An SLM metal printing machine (M100) of laboratory scale that is equipped with a continuous wave of 300 W fiber laser and an approximate laser beam of 80 µm was employed to form the specimens. All the considered specimens were of 30 µm thickness and their process was performed under an argon atmosphere. The objective is to as much as possible avoid the oxidation phenomenon that might occur during laser melting. For pure stainless steel substrate, the process was performed under 80°C, and no heat-specialized action was considered to treat the specimens after fabrication. The volumetric energy density can be computed using equation (1). The volumetric energy density (VED) represents the total input energy to the material per unit volume during the SLM process, usually expressed in J/mm3 .where VED is the volumetric energy density of the powder bed (J/mm3), is the laser power (W), is the laser scanning speed (mm/s), h is the hatch distance (mm), and d is the powder bed layer thickness (mm) [22, 23]. The measurement of porosity for the additively manufactured parts is essential in this technique. According to equation (2), the porosity can be measured by the Archimedes method. The manufactured body is firstly weighed at dry condition and then it is weighed while being suspended in water, and finally, the body is weighed at saturated condition after leaving it for 24 hours in the water .
3. Results and Discussion
Following layer deposition, it is challenging to prevent process-induced flaws such as pores generated by the SLM process, non-optimal process parameters, powder contamination, and local voids. FESEM images of SiC-reinforced SS 316L samples were observed during the SLM preparation using a FESEM machine (Carl Zeiss, Germany) as shown in Figure 4(a).
The samples’ images in Figure 4 reveal complete fusion at different magnifications. When ceramic particles are fused, an oxide layer forms on the surface of the particles, as shown in Figure 2. The SiC particles dissolve in the 316L matrix during manufacturing, leading to an increase in the proportion of reinforcement and a change in the matrix’s composition. Oxides are formed (Fe2O3, Cr2O3, and Fe3O2) when silicon combines with Fe3Si, as can be seen in Figure 5. Good wettability between SiC and SS 316L appears as there is no gap between the reinforcement interface and matrix, providing a solid metallic bond of Sic mixed with stainless steel. There were no small cracks when enlarging the image, despite the high heat which results in heat stress in the material due to the best parameters chosen, as shown in Figures 4(b)–4(d). The relation between the hardness values and the samples produced according to the results obtained for the hardness of the surface was between 277 HV and 296 HV as shown in Figure 6. It was found that the differences in hardness could be attributed to the scanning speed. At each scanning speed, the changes in the hardness value correspond to a certain scanning speed; by increasing the mass density of the samples, the hardness value also increases. The rational reason is that the developed defects can weaken the material strength, and the indenter can penetrate the Vickers hardness tester more effortlessly.
The maximal hardness achieved was 296 HV for S3 sample, as shown in Table 2, which has the highest density of 99.6% among the manufactured samples (Figure 7). The effect of scanning speed seems to be very important for the hardness and relative density alike, as the hardness is low at low speeds and high laser power. The residual heat in the material can cause undesirable consequences like agglomeration, keyhole and with pelleting phenomenon due to the high cooling rate and high laser power. This causes a change in the mechanical properties of the material.
At high velocity, there will be incomplete melting of the material, and therefore the hardness will also be lower due to the voids that occur. Therefore, choosing the appropriate velocity with other parameters is very important to obtain accurate and suitable hardness for the product without these defects. The apparent increase in the rate of hardness means that the particles knit more strongly. The volumetric energy density was calculated from the parameters of the samples. After examining the porosity of the models and microhardness, the best VED chosen was 136 J/mm3, as shown in Figures 8 and 9.
According to Figure 7, the relative density increases as the speed increases to a specific limit until the relative density of the body reaches its highest value, indicating the complete melting for the body with the stability of the laser energy density, which leads to filling the spaces between the molecules. Since the SLM technique is characterized as a procedure that has a high cooling rate, at high velocities, the fusion is incomplete; therefore, there will be gaps in the melting basin and the relative density will decrease.
The tensile test quantified the samples’ tensile strength (Figure 10(a)). The tensile samples were prepared and tested at a junction velocity of 1 mm/min according to ASTM-E8. Forthe samples with 1% SiC, the rise in the load was found to reach about 60 kN (570MPa) to break the sample with only 8.1 distortion; whereas for 316L samples without SiC, it was 565 MPa and with 8.95 mm distortion (Figure 10(c)). Figure 11 depicts the fracture that occurred during the tensile test. For compression tests, in contrast, the samples were prepared and tested at transverse velocities of 1 mm/min according to ASTM- E9, as shown in Figure 12. For all samples, the load was fixed at 120 kN (1569 MPa), and the deformation in the model was calculated at this reading as in Figure 13. The distortion rate of the sample was less than that of stainless steel without silicon carbide. The maximum distortion of the sample was 9.2 mm, while the distortion of stainless steel was 9.85 mm; note that the same load has been applied to both samples—120 kN. The major contributing factor in increasing the strength of the samples is the presence of C rather than Si . Increasing the SiC percentage in 316L MMC stainless steel leads to increased hardness but decreased wear resistance . In the case of SLM 316L-SiC composites, migration of C (and Si) into the austenitic matrix could have occurred, introducing solid solution strengthening effects. Furthermore, particles at the austenite grain boundaries may be responsible for dislocation pinning, which contributes to the strength of the 316L-SiC composites .
Based on the selective laser melting method, the continuous wave fiber laser was used to successfully produce SS 316L MMCs that are enhanced with silicon carbide particles. The gaps at the interfaces between SiC and SS 316L phase were trivial, signifying the proper compatibility between reinforcement and matrix in the argon atmosphere. With different parameter settings, the fine SiC particles enhanced the microhardness of the 316L matrix. It was found that adding 1% of SiC improved the hardness and resulted in a high product density. A relative density of 99.6 was achieved in MMC samples. The scanning speed and the increased laser power were very crucial after setting other parameters such as hole spacing, layer thickness, and spot size in additive manufacturing utilizing a highly selective laser melting process. In addition, the tensile and compression strengths of the samples were higher than those in stainless steel without silicon carbide.
No data were used in this study.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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