Dr. Mudda Nirish, Dr. Buschaiah Karolla and Dr. R Rajendra

Department of Mechanical Engineering, University College of Engineering (A), Osmania University, India

This study aim of investigates the obtained process parameters required to improved fatigue strength with highest dense parts manufactured by selective laser melting (SLM) of AlSi10Mg alloy and thermal simulation also important role for conducted before printing process to observe the thermal gradient and displacement, other stress developed on specimen i.e., save time, cost and material. The specimens were manufactured by SLM for cyclic bending fatigue performance. The metallurgical pores, fractures, and isolated areas with overlap of thermal gradient created at high power with low scan speed such as keyhole, are presented in this work, and their effects on fatigue strength, density, and hardness of AlSi10Mg alloy are discussed. The overlap and isolated samples have a similar microstructure with little difference grain size and slight difference in strengths. The laser density as main influence on the mechanical properties and defects are formed when the laser energy density is too high or too low (such as defects, porosity, and micro crack). To avoid the defects (pores, cracks) and achieve full dense part 98% with laser power as 250 W, scan speed as 500 mm/s, hatch spacing as 80µm, and with layer thickness of 30 µm.

Keywords: Additive Manufacturing; Laser Powder Bed Fusion; Selective Laser Melting; AlSi10Mg Alloy; Thermal Simulation; Fatigue Strength; Microstructure; Porosity Defects; Density and Hardness

Present situation, modern industry also requires manufacturing geometrically complex shape structures with reducing cost, time and light weight [1]. These results mainly possible in metal additive manufacturing (AM) using powder with a laser power source as selective laser melting (SLM) or direct metal laser melting (DMLM) to build layer by layer [2]. In this SLM, specimens are building plate with a given process parameters i.e., laser power, scan speed traversing every layer as x-y plane [3]. After each layer, the piston is lowered to allow for the deposition of the next layer of powder and this operation is repeated to several times until the part is finished [4]. When it is compared with other SLM produced materials, AlSi10Mg aluminium alloy powders have a low density, poor flowability, high reflectivity, and high thermal conductivity [5]. One of the main biggest challenges in manufacturing AlSi10Mg alloy parts by SLM is to minimize porosity and maximum researches have looked into how processing parameters affect porosity [6]. Although SLM is produce high density components near the nominal density, due to oxides, gas bubbles and particles may become stuck due to process instabilities [7]. The pores are unavoidable and can be act as nuclei in the cracks, which can lead to reduced of mechanical characteristics [8]. The unmolten power particles, on the other hand, are indicated by irregular elongated pores, which are often caused by a lack of energy (such as hatch pattern defects) [9].The main impartment process parameters considered in SLM printing process are layer thickness, scanning direction, build platform temperature, laser spot, laser power, scanning speed and hatch distance [10,11].

In this paper, thermal simulation is presented and affected the part by using a different laser power and scanning speeds [12]. Optimized the suitable process parameter on fatigue strength, hardness and density without defects (porosity pores and cracks), which is capable of predicting the thermal warping of parts manufactured by SLM AM process [13]. Some of the researcher are evaluated by the mainly focus on mechanical properties (UTS, YS, and E %,), microstructural characterization and defects due to building orientation [14].

Material

The used as AlSi10Mg alloy for SLM printing process of chemical composition as shown in Table 1 [15], it was supplied by SLM solution group AG Germany and in SLM printing process distribution of powder particle size ranges from 20 to 63 µm [16].

Table 1: Chemical composition of AlSi10Mg alloy.

L-PBF of SLM Process

The AlSi10Mg alloy printed samples used for in this investigation were produced by an SLM solution M280 2.0 L- PBF system (Germany) [17]. The laser power of 400 Watts continuous Yb-fiber laser in argon gas atmosphere and build platform volume of 280×280×365 with various important key process parameters involved in printing process as shown in Figure 1 [18]. The considered of SLM input process parameters as laser power, scan speed and hatch distance mentioned in Table 2 [20], other parameter kept constant like build platform temperature of 1500°C, laser spot diameter of 75 µm, layer thickness of 30 µm, scanning direction and build orientation was horizontal [21].

Table 2: Consider the process parameter for SLM printing.

Figure 1: a) SLM printing process diagram and b) process parameter [19].

After given the individual process  parameter to specimens (such as; T1, T2, T3, T4, T5 and T6) in SLM inbuilt software then generate the printing process to start machine as shown in Figure 2 [22]. The printing process is given two levels i.e., considered first level as 250 watts and second level as 300 watts with same scan speed range of (400,500, 600 mm/s) and hatching distance (60, 80 and 100µm) [23].

Figure 2: a) specimens on SLM build platform and b) after SLM printed parts.

The machine was automatic generated for two levels of printing time with aluminium alloy powder uses; the total estimated print time was 12 hours and total AlSi10Mg material consumed was 6.36 kg (169.52 grams part material, 15.02 15.02    grams support material and 636.98 grams loss material like spatter) [24]. The T4 was not printed properly as show in figure 9(a) due to recoater blade heating to part with high laser power and low scans speed [25]. The SLM parts manufactured with a high density in between range of 95.6 to 98.0% based on the given process parameter [26]. The SLM manufactured part dimensions (total length of 130 mm, diameter of 11 mm, and gauge diameter with radius of 7.5 mm) as per ASTM standard E2948 for cyclic bending fatigue test [27].

After completed printing process then post processing used the bosch cutting machine to removed the support structure from build plate and I epco pneumatic machine used for surface roughness on specimen [28]. The laser energy density was calculated by the equation of

[29], used this for above process parameter and calculated by the energy density was 347.2, 208.3, 138.8, 416.6, 250, 166.66 j/mm3 as plotted in Figure 3 [30].

Figure 3: a) Specimen for fatigue test and b) Energy density values i.e., process parameter.

In this work determined the effect of porosity influence on fatigue behaviour with horizontal building orientation conducted by bending fatigue test and also observed microhardness and density parts of AlSi10Mg alloy produced by SLM [31]. The experimental procedure conducted by following steps [32].

• Identified the SLM printing process of key parameters.
• Developed the thermal simulation before printing process.
• Considered the laser power upper 300 and lower 250 watts.
• The output performance as considered as fatigue strength, microhardness and density.
• Developing the Design of experiment for optimal process parameter.
• Conducting the experiments as per given process parameter on fatigue test.

Thermal Behavior for Optimal Process Parameter

Before SLM printing process to conducted the thermal analysis simulation as per given process parameter means saving the material, cost and time [33]. In that thermal simulation stresses was developed such as displacement, temperature, plastic strain, von mises stress and other stress [34]. The thermal analysis simulations were conducted at individual specimens with different process parameters as laser power (250 and 300 watts), scan speed (400, 500 and 600 mm/s) and hatch distance (60, 80 and 100 μm) as shown in Figure 4 [35]. The predicted of maximum temperature operating is 246.6°C at T4 and maximum displacement at T3 due to increasing the laser power with scan speed [36]. The highest von mises stress operated of 95.73 MPa at T5 (when increasing the scan speed) and the lowest plastic strain was developed of 0.33 at T3 (when increasing the hatch distance) [37]. During the optimization in this object purpose was reduce the porosity and improve the strength, finally the simulation results obtained of optimal process parameter with less displacement and minimum operating temperature at T2 (such as the laser power of 250 W, scanning speed of 500 mm/s and hatching distance 80 µm) as shown in Figure 5 [38].

Figure 4: Thermal simulation as per given process parameter a) Von mises stress and b) Temperature.

Figure 5: Thermal simulation results as per given process parameter a) Temperature and b) Displacement.

Artec 3D scanner for GD & T

The used for Artec 3D scanner to find out the geometric dimension and tolerance (GD&T) of SLM-AM manufactured geometry parts dimensions according to given design dimensions as shown in Figure 6 [39]. This technology mainly used for 3D printing to scan the object geometry and convert into cad files is also called reverse engineering (RE) [40]. After scanning object get lot of described points is called cloud data from these cloud data work on it and improved to cad file with according to objective dimensions [41]. The scanned after manufactured geometry part and compared with design dimensions as shown in Figure 7 and after measure the all values when increasing laser power from 250 to 300 watts at scan speed 400 mm/s with thermal distortion is more compare to other scan speed at 500 and 600 mm/s [42].

Figure 6: a) 3D Artec scanning and b) After scanning measured results for thermal deviation.

Figure 7: a & b) Compared dimension design diameter and manufactured diameter.

Fatigue Performance

The fatigue test as indicated as shown in Figure 8, is used and found of horizontal building orientation specimen fatigue strength of SLM manufactured by AlSi10Mg alloy [43]. The SLM-AM part building directions is the most important role in fatigue strength. Even though there was a certain scatter in the fatigue data, it can be seen that when used high laser power with low scan rate [44]. The specimen’s defects, pores and porosity were observed that after test crack on part surfaces and defects have a give negative impact on the sample's fatigue strength [45].

Figure 8: a) Fatigue testing machine and b) After test results plotted at each specimen.

The samples were tested at a constant load was applied to determine the fatigue strength, and the obtained of optimal process parameters [46]. The motor speed was calculated by the time at sample was broken and the number of cycles in time, as shown in Table 3 [47].

Table 3: Fatigue experimental test results.

The fatigue strength calculation:

• The motor rpm is 2880, 1 minute is = 2880/60 rev (i.e., 1 sec = 2880 = 48 rps),
• The number of cycles =  time taken (s) × 48 rps and
• Length of the shaft is 190 mm,
• Diameter of the shaft (D) is 15 mm,
• Weight (W) = 10 × 9.81 = 98.1 N, (applied load for fatigue test is 10 kg),
• Bending moment (M) = L × W…. (2) = 48069 N-mm and
• Bending stress (σb) = 32M … (3) [48,49] = 145.14 MPa.

Microstructure, hardness and density

From the Figure 9 shows that scanning electron microscopy (SEM) image of AlSi10Mg alloy powder particle distribution micrograph and it can be seen that powder particles are not spherical [50]. The particles' morphology is very irregular shapes, with many little irregular satellite particles connected to the large powder particles and other places have seen these irregular structures with little satellite particles [51]. The powder flowability in SLM process, as well as their melting behaviour from laser power and average particle size from 20 to 63 m, is influenced by the particle size distribution [52].

Figure 9: Powder particle size distribution.

All samples are produced in a melt pool with good metallurgical bonds. As shown in Figure 10, we can see that melt pools in the overlap area not the same as those in the isolated area [53]. Used the high energy (E) input then created large pores with low scan speed. Overlapping between molten pools is expected to affect defect formation, in the cross section of the as-built samples, irregularly shaped pores were apparent with more thermal distortion by 300 W of high laser power and 400 mm/s of lower scan speed [54]. The majority of the burring layer was present in the cross section at a high laser with scan speed of 600 mm/s [55]. From this, it can be completed that the burring phenomenon is mainly typical metallurgical defects on specimen at high laser with fast scan speeds [56]. The SLM samples manufactured at T2 is showed a better microstructure without any visible pores, the laser energy density defined as the optimal parameter for obtaining minimum defects, porosity, pores, cracks for a range of 250 W, 500mm/s, 80 µm, and 30 µm of layer thickness for SLM of AlSi10Mg alloy to achieve defect-free component [57].

Figure 10: Melt Pools in The Overlap Area.

The following parameters selected for hardness and density with tested as build condition: laser power is 250, 300 W, scan speed is 400, 500 and 1800 mm/s with hatch distance 60, 80 and 100 μm and layer thickness is 30 μm [58]. For as built condition were tested samples, the microhardness higher at T2 is 108.3 HV, lower at T6 watts is 82 HV due to high laser with overlap samples [59]. Finally the hardness and density values are plotted as shown in Figure 11 with achieved the highest hardness and density is 108 HV, 98 % [60].

Figure 11: The Hardness and Density Values.

The metallurgical pores, fractures, and isolated areas with overlap of thermal gradient created at high laser power and low scan speed, i.e., keyhole, are presented in this work, and their effects on fatigue strength, density, and hardness of AlSi10Mg alloy are discussed [61,62].
•    The pores in overlap boundary are expected to cause early fracture in the samples during fatigue testing [63].
•    The pores overlap and defects was achieved at increasing the hatch distance from 80 to 100 µm (i.e., heat buildup with cool slowly) [64].
•    The overlap and isolated samples has a similar microstructure with little difference of grain sizes. The energy density (E) has an important influence factor on mechanical properties and defects are formed when the laser energy density is too high or too low (such as defects, porosity, and micro crack) [65]. When balling is occurs at scan speed is increase, promotes the capture of powder that hasn't entirely melted by the laser beam scanning the next layer, resulting in the formation of a keyhole pore [66].
•    To avoid the defects (pores, cracks) and achieve full dense part should be selected low laser power and scan speed, for that best process parameter was obtained as scan speed is 500 mm/s, hatch spacing is 80µm, and lower power is 250 W with layer thickness is 30 µm at fatigue strength is 1.46 × 103, hardness is 108.3 HV and density is 98% with laser energy density is 208.3 j/mm3 [67,68].
The future work will be mainly focus on how to eliminate pores, crack without thermal distortion. The optimization technique will be use to determine the suitable process parameter for SLM to improve fatigue strength with full dense parts.

The authors would like to thank for Research was supported by Centre for Product and Development and Additive Manufacturing (CPDDAM) under Rashtriya Uchchatar Shiksha Abhiyan (RUSA 2.0) at University College of Engineering (A), Osmania University, Telangana-India.

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Corresponding author: Mudda Nirish, Department of Mechanical Engineering, University College of Engineering (A), Osmania University, India.

Copyright: © 2023 All copyrights are reserved by Mudda Nirish, published by Coalesce Research Group. This work is licensed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.