Influence of Process Parameters on Porosity and Hot Cracking of AISI H13 Fabricated by Laser Powder Bed Fusion

Abstract

Laser powder bed fusion is an attractive manufacturing technology promising novel components for the aircraft, automobile, and medical industries. However, depending on the material, some defects in the parts, especially pores or microcracks, cannot be avoided in the LPBF process. To achieve a part with low defect density, the optimal parameter sets must be determined. Many investigations have focused on how laser speed and laser power influence the melting process and the relative density of as-built parts. In this study, we considered laser and heated powder beds as two energy input sources, represented by volume energy density and preheating temperature, respectively. The interaction of these two energy inputs for the fabrication of AISI H13 was investigated. It was found that high preheating temperatures shifted the optimal parameter sets from the low energy density area to the high energy density area. In addition, high preheating also led to hot cracking, which was confirmed with Scheil solidification simulations.

1. Introduction

Laser powder bed fusion (LPBF), also known as selective laser melting, is one of the additive manufacturing processes specified in ASTM F2792-12a. An outstanding feature of LPBF is its high manufacturing precision, making it suitable for complex structures in various industry applications [1], such as electric mobility [2,3], fuel cell [4], and aircraft applications [5]. However, this layer-by-layer process brings naturally process-related defects, such as pores and cracks, into the components, which are deleterious to the lifetime of the components [6,7].

Three types of porosity are observed in LPBF: gas porosity, keyhole porosity, and lack-of-fusion porosity [6]. To achieve high densities, process parameter optimization, in particular laser power (P) and laser speed (v), has been intensively investigated [6,8,9,10,11,12,13,14]. In these studies, the process window described in P vs. v maps are typically plotted. With high laser speed and low laser power, the low energy density area leads to insufficient melting and lack-of-fusion defects. In detail, lack-of-fusion appears if the overlap between melt pools is not sufficient to ensure the melting of all particles [15]. In contrast, the high energy density area causes balling and keyhole pores. Therefore, the medium energy area in the laser processing map is selected as the process window to produce parts with high relative density and without the above defects. Overall, a stable melting pool is critical to minimize defects, and highly dense components require relatively high energy input while avoiding keyholes [16]. As another high energy input source, a preheated powder bed can improve the printability of alloys [17,18,19,20,21]. However, only the optimal parameters were considered within these studies. The interaction of preheating temperatures with laser parameters has rarely been illustrated.

Hot cracks are another type of defect that have been reported among different alloys, such as magnesium alloys [22], aluminum alloys [23,24], high-entropy alloys [25], and steels [26] produced by LPBF. Hot cracking during solidification results from the fracture of the semisolid films at the boundaries of dendrite grains [27]. The thin layers of the melting pool rapidly solidify during the LPBF process, which leads to dendritic microstructure mainly along the building direction. These long directional dendrite boundaries limit liquid feeding and act as very sharp notches which is conducive to solidification cracking [10]. Even though the hot crack susceptibility is determined by the chemical composition of alloys, the LPBF process parameters can also affect the formation of hot cracks [22]. For instance, the size of the melting pool is also relative to the formation of hot cracks [28].

The martensitic hot work tool steel AISI H13 has been widely investigated in the field of LPBF since it combines good printability with comparably high hardness [29]. The melt pool geometry of single tracks was categorized with various laser powers and laser speeds [9]. As for 3D parts, a relative density higher than 98% can be achieved with the optimal P and v sets [26,30,31,32,33]. Cracks were also observed in most of these studies [26,31,32,33]. Normally, stress-induced cracks can be avoided by heating the powder bed [32] and by reducing stresses from thermal gradients and martensite formation. However, hot cracks are mainly related to the solidification process [9] and might respond differently to powder bed heating.

In the presented work, instead of studying the effects of laser power and laser speed, we considered all laser parameters together as one energy input source, which is represented by the volume energy density. The interaction between the volume energy density and the preheating temperature as the second energy input source was investigated with a focus on porosity and hot crack density. In addition, the Scheil solidification simulation was applied to estimate the hot crack susceptibility. The obtained results broaden the knowledge about how preheated powder bed interacts with process parameters and influences porosity and hot crack density of H13 produced by LPBF.

2. Materials and Methods

Hot-work tool steel AISI H13 (DIN No. 1.2344, DIN X40CrMoV5-1) powder was atomized in argon atmosphere. The corresponding chemical composition of the powder according to the supplier (Steel Service Krefeld GmbH, Germany) is within the specification in DIN EN ISO 4957 [34], as shown in

Table 1. Chemical composition of AISI H13 powder and standard range of chemical composition according to DIN EN ISO 4957 [34].

Chemical Composition [wt%]
	Fe	C	Cr	Mo	V	Si	Mn
LPBF powder	bal.	0.4	5.4	1.29	1.14	0.91	0.32
Standard	bal.	0.35–0.42	4.8–5.5	1.2–1.5	0.85–1.15	0.8–1.2	0.25–0.5

. The particle size distribution of AISI H13 powder was determined by laser diffraction (Partica LA-950, Horiba, The Netherlands). As presented in Figure 1, the powder particles are mainly spherical and distributed with D₁₀ = 28 µm, D₅₀ = 42 µm, and D₉₀ = 63 µm.

The ReaLizer SLM 100 machine (ReaLizer GmbH Company, Germany), equipped with an Ytterbium fiber laser with a wavelength of 1070 nm and maximum output power of 200 W, was used for sample fabrication. Cubic samples of 10 × 10 × 10 mm³ were fabricated under an argon atmosphere with an oxygen content below 300 ppm. An overpressure of about 30 mbar prevailed in the chamber in comparison to the ambient atmosphere. Samples were built without preheating or with 100 °C, 200 °C, or 400 °C preheating. The layer thickness of 50 μm was constant. Different process parameters were studied by varying laser power, laser speed (equals to d_point/t_expose), hatch distance, preheating temperature, and laser rotation angle, and 90 samples were built randomly positioned on the baseplate in seven building jobs.

The defect density was determined using image analysis. The cross sections parallel to the building direction were prepared by standard metallographic procedures including mounting, grinding, and polishing, and 16 optical images of each sample were taken with a magnification of 250 times to combine an overview of the cross section. Under this magnification, all defects can be clearly observed. These overviews were used for the density determination with the software ImageJ [35]. In this study, defects were separated into pores and hot cracks according to their circularity above or below 0.35, respectively. Cold cracks which typically appear at the surface of large components were not observed. Figure 2 presents an example of the defects in an optical overview separated into pores and hot cracks.

Samples were etched by Nital 5% at room temperature for 10 to 20 s to reveal the microstructure. Microstructure images were taken by an optical microscope and the scanning electron Helios Nanolab G3 CX (FEI, Hillsboro, OR, USA).

The Scheil solidification simulation with back diffusion was carried out in Thermo-Calc software (2021b, Thermo-Calc Software, Sweden) with the TCFE9 and MOBFE3 database for the AISI H13 powder chemical composition.

3. Results and Discussion

The LPBF process parameters in this study can be divided into three groups, volume energy density (VED), preheating temperature, and scan strategy, as illustrated in Figure 3. All of these parameters influence the relative density of as-built parts.

The volume energy density can be calculated by the equation:

$$VED=\frac{P}{v·d·t}$$

(1)

where P, v, d, and t are laser power, laser speed, hatch distance, and layer thickness, respectively. VED describes the volumetric energy input from the laser and it is a sufficient parameter when we consider the part as a whole, for instance, the relative density analysis. However, the single-track melting behavior depends on the laser power and the laser speed, which determine the width and the depth of a melting pool [9].

The powder bed preheating system was developed for the LPBF process to avoid large thermal stresses in the material, especially for tool steels. Many investigations have confirmed that the preheating temperature influences microstructure and residual stresses of as-built parts [17,18,20,21,32]. In the present study, the heated powder bed is considered as another energy input source and, together with VED, determines the porosity and hot crack density.

The scanning strategy is the spatial moving route followed by the energy beam, which is of great importance to the local thermal profile and part quality. For a single layer, the scan strategy varies by different laser patterns, whereas, between two layers the scan strategy can be differentiated by laser rotation angle. Since the pores and hot cracks are mainly influenced by remelting process between the layers rather than the scan pattern in the single layer, only the rotation angle is discussed in the present study.

3.1. Influence of Process Parameters on Porosity

As reviewed in the introduction, laser power–laser speed sets have been investigated comprehensively, from single-laser tracks to the whole as-built parts. Here, we move one step further, to illustrate how the two kinds of input energy source, namely the laser and the heated powder bed, influence the porosity. Figure 4 presents the effects of VED and preheating temperature on porosity. Without preheating or with 100 °C preheating, the lowest porosity is obtained in the VED range from 65 to 75 J/mm³. For the higher preheating temperatures, 200 °C and 400 °C, samples built with the VED from 55 to 60 J/mm³ show the lowest porosity. In other words, a high preheating temperature shifts the optimal parameter set to lower VEDs.

To visualize the effects of the preheating temperature on porosity, exemplary cross sections of samples built with 73 J/mm³ VED and various preheating temperatures are shown in Figure 5. With increasing temperature, more spherical pores are observed. In the overmelting area, it is common to observe spherical pores produced by entrapped gas or vapor [36]. This phenomenon indicates that the heated substrate, as another energy source to the powder, can effectively contribute energy by heat conduction to the laser melting process.

The laser scan rotation angle between the layers resulted in the rotation of heat flux and thus the alternation of the melting pool. Theoretically, a scan rotation of 67° leads to a more even temperature gradient and better fusion of the particles. Therefore, relatively homogeneous microstructure and mechanical properties have been reported with this kind of scan rotation [37,38,39]. As for porosity, a scan rotation of 67° can enhance densification during LPBF [38]. However, in our study, as shown in Figure 6, these two scan rotations result in similar average porosity values but with slightly less scatter with 67° laser rotation.

3.2. Influence of Process Parameters on Hot Crack Density

Solidification and liquation cracking are well known from welding processes. Solidification (hot) cracks form in the melting pool and often reveal the dendritic morphology, whereas liquation cracks normally form in the heat-affected zone [40]. In the as-built parts, cracks can be detected from the cross-section, as shown in Figure 7. The melting tracks and dendritic solidification microstructure are revealed after etching. These cracks are located in the middle of the melting pool parallel to the building direction, which confirms their formation during solidification. Solidification cracking occurs at the final stage of solidification, where dendrites have grown almost fully into grains that are separated by a grain boundary liquid film, as illustrated in the back scattered electron (BSE) image.

The crack density of samples with different VEDs and preheating temperatures are shown in Figure 8a. The hot crack density increased significantly at high preheating temperature, which is more precisely presented in Figure 8b. Up to 200 °C preheating, average hot crack densities are below 2%. This value surges to 3.8% at the 400 °C preheating, which indicates that when the preheating temperature is high enough to slow down the solidification, more hot cracks form in the melting pool.

The Scheil solidification simulation with back diffusion was performed to analyze how preheating temperatures influence the hot crack density. The index for the crack susceptibility during solidification was proposed by Kou [27], which is

$$\left|dT/d\left(f_s^{1/2}\right)\right| near f_s^{1/2}=1,$$

(2)

where $T$ is the temperature and $f_s$ is the fraction of solid. This index reflects the length of the grain-boundary liquid channel between two neighboring grains, as illustrated in Figure 9a. The higher this index, the higher the susceptibility to hot cracking. Calculated curves of $T$ vs. $f_s^{1/2}$ for AISI H13 are shown in Figure 9b; $\left|\Delta T/\Delta \left(f_s^{1/2}\right)\right|$ was used as an approximation to compare the crack susceptibility. When the cooling rate increases from 1 K/s to 100 K/s, the temperate difference $\Delta T$ between f_s^1/2 = 0.98 and f_s^1/2 = 0.99 drops clearly (solid fraction f_s = 0.96 and f_s = 0.98, respectively), indicating lower susceptibility to hot cracking. As the cooling rate is higher than 100 K/s, hot crack susceptibility remains almost the same. This Scheil solidification simulation generally illustrates why more hot cracks formed at 400 °C preheating, whereas there is no significant difference with the preheating lower than 200 °C. However, we only assume that high preheating temperature leads to a lower cooling rate due to the smaller thermal gradient. Further sophisticated experiments and simulations are necessary to determine the cooling rate of the melting pool during the LPBF process.

The statistical analysis of the hot crack density of parts built with 67° or 90° laser rotation angles is presented in Figure 10a, where samples built with the same process parameters but different laser rotation angles are compared. Considering all of the experimental data, there is no significant difference in hot crack density between the two rotation angles. However, for the optimized parameter sets with low porosity, the laser rotation angle of 67° leads to fewer hot cracks, which can be observed in the cross-sections in Figure 10b,c. With a scan rotation of 90°, hot cracks align with a distance corresponding to the hatch distance because the laser melted at the exact same position every two layers, and hot cracks always formed in the center of the melting pool. The scan rotation of 67° can avoid this repetition. Therefore, the parts built with the 67° scan rotation present fewer hot cracks.

4. Conclusions

This study investigated the influence of volume energy density, preheating temperature, and scan rotation angle on porosity and hot cracking of as-built AISI H13 parts. The heated powder bed can help the laser to melt metallic particles. With increasing preheating temperature, the optimal parameter sets shift to low volume energy densities. For AISI H13, high preheating temperature results in more hot cracks, which can be explained by slower solidification. The scan rotation of 67° can to some extent alleviate hot cracking by avoiding the repeated solidification at same position. The Scheil solidification simulation with back diffusion can qualitatively predict the hot crack susceptibility. This can be particularly useful when comparing hot crack susceptibility for different alloy compositions. However, further investigations are necessary for more accurate calculations.