Metal additive manufacturing is an emerging advanced technology, it differs from conventional manufacturing methods as machining, casting and forging, which are either subtractive or forming. Firstly, the objective of this work is to elaborate a new 316L stainless steel material by Selective Laser Melting (SLM) from metallic powder according to specific operating parameters namely laser scanning speed and power. Secondly, the characterization of this developed material by 3D printing is carried out. For this purpose, metallographic observations and heat treatments at different temperatures 650, 800 and 1050°C were performed. Thus, the contribution of this study is to develop procedure and tools to enhance their mechanical properties at the level of parts obtained by conventional processes. Therefore, samples were examined by X-RF, SEM, EDS mapping, density, and hardness measurements as well. Results show that mechanical properties of additive manufactured samples can be improved in certain conditions linked to operating parameters and heat treatment. Also, this work has allowed us to confirm the resistance of the 316L stainless steel developed by SLM to high temperatures.
Research Article
Structural and Heat Treatment Analysis of 316l Elaborated by SLM Additive Manufacturing Process
https://doi.org/10.21203/rs.3.rs-2010034/v1
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316L stainless steel
additive manufacturing
heat treatment
mechanical properties
microstructural analysis
Additive manufacturing (AM) has many advantages and will be part of the manufacturing process solutions in a sustainable way. Therefore, it will have a strong impact on manufacturing industry in the near future. Many experts have seen it as an industrial revolution replacing conventional processes and accelerating gains and productivity [1–4]. Actually, wide range of materials can be used in additive manufacturing processes, including polymer, ceramic, and metallic materials. Steels, titanium (Ti) and Ti-based alloys are widely employed in aeronautics, automobiles, chemicals, defense, even biomedical because of their high specific resistance, intrinsic value of lightness, sufficient rigidity, mechanical properties and above all their remarkable resistance to corrosion [5, 6].
Up to now, obtaining high mechanical proprieties and good structural integrity are challenging steps. Often, pieces are subjected to additional post-treatment in order to improve their properties. In order to make selective choices in additive manufacturing, it is necessary to have a good understanding of the existing processes, their constraints and specificities [7, 8]. Three main inputs are required for additive manufacturing: materials, CAD model, and power source or phase transformation tool.
In the case of metallic materials, DMLS or SLS and SLM are the most used processes [9–11]. These technologies cover a variety of processes opposite to the philosophy of subtractive manufacturing where material is removed. In AM, material is joined or solidified under computer control (CAD model) to create a three-dimensional object that meets the desired shape (Fig. 1) and based on the utilized processing parameters, to create parts by SLM. The overall physical properties can be substantially varied considerably relative to the bulk properties of the same conventionally processed alloy [8, 12–15].
The physical and mechanical properties of SLM can be affected by the heating conditions, including rapid cooling (107°C/s) [12–14], that can be induce to non-equilibrium states which entrap impurities, create stress residuals or produce regions of amorphous material. Non fused powder may exist that has not properly wetted the metal underneath due to an oxide film or/and treatment power. The porosity is likewise generated by the stripping of the powder and vaporization of the metal [15, 16]. These transformations can affect directly the integrity of manufactured pieces.
Simmons et al. [17] showed that, in the case of SLM, thermal profile alters the microstructure compared to conventionally processed materials as well as locally altering the heat conductivity of complementarily constructed 316 L stainless steel. When processing conditions use suboptimal densities of energy, the overall the heat conductivities are 10% lower than the estimated theoretical prediction of the average effective thermal conductance.
The influence of different printing orientations and inclinations, with different scan times, on mechanical properties of 17-4PH stainless steel specimens produced by selective laser melting (SLM) have been studied by F.R. Andreacola et al. [18]. They have found that the highest ductility was obtained for the specimen printed horizontally with an inclination of 5° (for both as-built and heat-treated specimens) for the samples treated with scanning times of 50 and 65 s. In addition, according to Wang et al. [19], the amount of laser energy applied has a significant influence on the splash throughout the SLM process. Increased energy results in higher splash intensity, which increase and disperse around the melt and away from the molten pool.
In this paper, the purpose of this study is to develop additive manufactured material and tools that allow obtaining pieces with good integrity, flawless which lead to high mechanical properties. For this objective, samples are elaborated by using SLM process with various conditions and examined by XR-F, SEM, EDS mapping, and their density as well. The effect of heat treatment conditions on the hardness of manufactured samples is discussed. SEM and metallographic analysis are employed to verify the structural integrity of the developed samples.
2.1 Process and samples preparation
The choice of strategy has a significant impact on the process development and device design, the maturity of these processes has been significantly improved due to the search for new procedure and materials, which have led to robust and stable processes.
SLM is a typically metal powder additive manufacturing process that produces quasi-final forms of parts by the melting and assembly of powder materials in a layer-by-layer model. In this study, for elaborating samples, Nd-YAG laser was used. Once depositing every powder layer, that layer is scanned by the high power laser. Generated heat by laser melts the powder and fuses it as it cools to a solid metal. The whole operation is repeated to build the pattern layer by layer (Fig. 1). This operation is carried out under the condition of vacuum to avoid the influence of moisture absorption of powders on final forming quality [20].
The main setting parameters that may be controlled include laser power, powder layer thickness, hatch spacing and laser scan speed. In this work, the thickness of powder layer deposited is 50 µm, and the scanning strategy is Chess.
In this experimental section, samples have been fabricated from 316L stainless steel powder using AM-SLM and tested in both sides: the lateral "FS" and upper "FT" (Fig. 1-e). They were produced using different combinations of operating parameters, namely speed and power of laser. The choice of the parameters was made in accordance with the feature of the machine SLM125HL (Fig. 1-c) and the nature of the material. In order to improve the absorption rate of the laser energy by the metal powder; an Nd-YAG laser with a wavelength of 1064 nm is used. Two scanning speeds V1 = 200 mm/s and V2 = 300 mm/s and three powers P1 = 100 W, P2 = 200 W and P3 = 300 W have been selected (Table 1).
Table 1: Operating parameters (T0=90 °C) of developed samples (As-built)
The second main objective of this survey is the characterization of this new material developed by 3D printing. Therefore, metallurgical studies and mechanical measurements are performed on several samples. Scanning electron microscopy (SEM) analyses were achieved with JEOL JSM-IT500HR equipment and EDS analysis system on steel powders and also on the specimens manufactured by SLM to investigate the surface morphology and the shape of the powder particles. Then, hardness measurements are done with (F = 4.9 N, 15 s).
2.2 SEM and X-RF analysis (As-built)
The powder quality and its composition play an important role in the quality of the additive manufacturing processes as it influences not only consolidation phenomena and consequently porosities, but also microstructure and final alloy composition. Olakanmi et al. [21] demonstrate that the density of the parts is decreased when the powder particles are heterogeneous in shape. Spherical particles facilitate layer densification because they generate little inter-particle friction and have better mobility [22].
The most of the powder particles used in this SLM process have a diameter lower than 50 µm whatever the thickness of the powder bed defined. The size and particle size distribution of the particles have a strong impact on the flowability of the powder. Smaller particle sizes (diameters less than 15 µm) will tend to form clusters and thus prevent uniform deposition of the powder on the build plate [23].
A mixture of small and large powder particles is preferred for the powder bed melting process, where the small particles percolate through the larger particles to fill the inter-particle pores [24] and achieve an optimal powder bed density. The powder particle size has an effect on the final density of the parts produced by SLM. Indeed, Chen et al. [25] analyzed the influence that two differently sized powder batches had on density of final parts with fixed SLM parameters. The batch containing particles of average size lower than 16 µm led to denser parts than the batch with powder of average size 48 µm.
From the SEM image in Fig. 2, it can be seen that most of the powders are spherical, and their size ranging is between 10 and 65 µm. An addition, the chemical composition of the based material which is 316L powder before SLM manufacturing was examined by X-RF (Table 2).
|
Element |
Si |
Cr |
Mn |
Ni |
Sn |
Mo |
W |
Fe |
|---|---|---|---|---|---|---|---|---|
|
Wt.% |
0.25 |
17.74 |
1.29 |
11.69 |
0.08 |
2.94 |
0.04 |
Bal. |
It is known that the final obtained microstructure of manufactured parts is correlated to thermal behavior generated within any process. Yet, local heating and fast melting/solidification complicate the control during the multilayer deposition of the SLM process. It is generally through their microstructure and hardness that metals reveal their different mechanical properties. Hardness testing and metallographic observations are therefore essential for any study of materials elaboration. Thus, the impact of heat treatment upon the properties of a stainless steel produced by additive manufacturing is discussed for different operating parameters including power and scanning speed.
3.1 Heat treatment conditions
Heat treatment of metal alloys can be used effectively to achieve the desired mechanical properties of the alloy. Up to now, only a few number of studies have been studied the effect of the heat treatments on SLM 316L properties. Many of phase transformations that occur are controlled by the temperature and duration of the heat treatment. Hardening is one of the heat treatment processes that reinforce and toughen materials such as steel and other irons-based alloys.
Liu et al. [26] showked that the subsequent thermal cycle (STC), being a unique heat behavior within the selective laser melting (SLM) multilayer manufacturing process, has a substantial effect on the thermal behavior within the SLM process and thereby impacts the microstructure of the components manufactured by SLM. As well as, when the laser beams a layer, the nearby underlying layer can be deeply remelted, forming a perfect joint between the neighboring layers, and excellent mechanical properties can be achieved.
Table 3: Heat treatment conditions (T6, T8 and T10)
Heat treatments are used to improve the properties of the treated material. In this work, the samples are classified into three groups according to the treatment they will undergo (T6, T8 and T10). Meanwhile, the temperature and the cooling rate are adjusted, as the holding times are immutable; 10 minutes (Table 3).
In the first case T6, samples are heated up to 650°C and submitted to air cooling. In the second case T8, samples are heated up to 800°C. Then, they are annealed. Last case T10, samples are heated up to 1050°C, then objected to hyper quenching.
Microscopic observations revealed, contrary to conventional stainless steels, the transformation of the microstructure certain samples. For T6 = 650°C, the microstructures of all samples (P1V1T6, P1V2T6, P2V1T6, P2V2T6, P3V1T6, and P3V2T6) did not change significantly unless elimination of diffusion affected zone and obtaining high levels of homogeneity across the joint region. However, a dendrite structure on both sides (FS and FT) is observed in samples P3V1T8 and P3V1T10 (Figs. 4 and 5). Those samples P3V1 were subjected to the highest deposited energy before heat treatment (Table 4).
Table 4: Deposited Energy (J/mm3)
Deposited Energy indicates the amount of energy applied per volume of powder material (Table 4). The corresponding Deposited Energy ED(J/mm3) is computed according to followed equation:
$${E}_{D}=\frac{P}{{V}_{i}.h.d}$$
Where:
h: hatch space (120 µm).
d: powder bed thickness (50 µm).
3.2 Heat treatment effect
In the case of the parts obtained without heat treatment which are designated as-built, Fig. 6-a shows that when the laser power increases the hardness also increases for V2 = 300 mm/s. In addition, measured hardness in top side (FT), which is the building direction (z), is different to lateral side (Fs). This result confirmed the results obtained by tensile tests [27]. On the other hand, after the heat treatments, the hardness increases until P2 and decreases at P3 (Fig. 6-b). To explain these results, deeper analysis especially material densities are carried out.
Among the objective of selective laser melting fabrication is to obtain a good material with reliable characteristics, directly after fabrication, with respect to mechanical strength, density, material continuity, absence of voids and surface finish. However, despite the optimization of the density, porosities are inevitable defects that are always found in parts made by SLM if processing parameters are not well determined.
At fixed SLM parameters, powders of different particle sizes will lead to the formation of parts with different densities. However, powders of different particle sizes can be used on the same SLM printer, but the process parameters must be adjusted and optimized for each powder batch [28]. This is the case in the study of Spierings et al. [29], who obtained with three powder batches of different particle sizes (15, 28 and 37 µm) parts that were more than 99% dense by only adjusting the power and scanning speed of the SLM process. Similarly, Kamath et al. [30] produced 316L steel parts from two batches of powder with different particle sizes. They obtain more than 99% dense parts for both powder batches using laser power above 250 W. The energy deposited was sufficient to fuse all the powder particles and lead to a denser material.
Then, parts made by selective laser fusion on powder bed reach relative densities higher than 99% when all the parameters of the manufacturing chain are optimized [31]. With the same SLM parameters, parts made by SLM from gas atomized powders have better relative density than those consolidated from water atomized powders [32]. Based on these results, gas atomized powders are widely preferred for the selective laser melting process on powder bed [33].
In this study, density measurements are conducted with Hildebrand Densimeter H-300S. We notice that after heat treatment the power 200 W is optimal to have an adequate density notably the samples treated with the temperature of 850°C (Fig. 7). Also, the samples treated and processed with a low scanning speed of 200 mm/s (Fig. 7) are denser than the samples treated with 300 mm/s and this explains the non complete fusion of the powder and the discontinuity of the material P1V1T10 (Fig. 8).
In the following section, an energy dispersive X-ray spectroscopy (EDS) analysis was also performed thoroughly on the samples, to analyze the distribution of nano precipitates in different micro regions (Fig. 3). Moreover, based on these results (Tables 3 and 5), it can be seen that there is almost no difference between them for main chemical elements if machine uncertainty is taken into account, which confirms the performance of the SLM process in term of conservation of the initial composition of the powder. Moreover, the reported data shows that the percent of Cr decreased from 17.74 to 17.4 wt.%. This result can be explained by formation of Cr2O3 and chromium carbides in surface and grain boundaries during cooling period. These results are confirmed by EDS Spectrum analysis (Fig. 3).
Table 5: X-RF analysis of material after AM-SLM
|
Element |
Si |
Cr |
Mn |
Ni |
Nb |
Mo |
Ti |
Fe |
|---|---|---|---|---|---|---|---|---|
|
Wt.% |
1 |
17.4 |
1.2 |
11.82 |
0.02 |
2.68 |
0.02 |
Bal. |
In the case of samples where hardness and densities are low, SEM analysis revealed cavities in bulk materials. These cavities are created due to low laser power and high scanning speed or in case of rapid solidification. Their seizes are about 70 µm as shown in Fig. 8. The presence of these defects facilitates cracking path and the premature materials failure.
Selective laser melting additive manufacturing is a promising technology to produce high quality shape materials. It is becoming a very suitable process to produce high strength steel. In this paper, the effects of Laser scanning speed and power of SLM on the microstructure of the developed material have been studied. The main conclusions drawn from this work are the following:
-
Experimental results showed that the effect of power is not always linear after heat treatment. Increasing laser power may lead to low mechanical proprieties.
-
Mechanical characterization is carried out mainly the hardness and the microstructure before and after heat treatments. In the case of heat treatments (650 and 800°C), the samples did not undergo structural changes which confirms the heat resistance of fabricated SLM stainless steels. In the other hand, in the case of T10 (1050°C), the quenched samples underwent structural transformations including the appearance of dendritic structures which explains the change in hardness of the samples.
-
A structural investigation is conducted out by measuring the density of the fabricated material in order to study the effect of the scanning speed on the integrity and continuity of the material. It is summarized that the structure loses its integrity in the case of samples treated with a high scanning speed such 300 mm/s.
-
As long as the hardness of upper side of elaborated samples is higher than lateral side, the elaborated material can be considered as anisotropic. These confirmed result obtained by tensile test. However, the chemical composition of elaborated material by SLM did not change compared to initial powder composition.
| SLM | Selective Laser Melting |
| XR-F | X-ray fluorescence spectroscopy |
| SEM | Scanning Electron Microscope |
| EDS | Energy-dispersive X-ray spectroscopy |
| AM | Additive Manufacturing |
| CAD | Computer Aided Design |
| DMLS | Direct Metal Laser Sintering |
| SLS | Selective Laser Sintering |
| Pi | Laser power (W) |
| Vj | Scanning laser speed (mm/s) |
| Ti | Temperature (°C) |
| ED | Deposited Energy (J/mm3) |
| h | Hatch space (120 µm). |
| d | Powder bed thickness (50 µm). |
| HV | Vickers hardness |
ACKNOWLEDGMENTS
The authors greatly acknowledged the help and equipment support from Moulay Ismail University, and Euromed University of Fez in Morocco.
The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.
The authors have no relevant financial or non-financial interests to disclose.
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by all of them. The first draft of the manuscript was written by Kaoutar Fri (PhD student) and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
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