Improvement of Part Accuracy by Combination of Pulsed Wave (PW) and Continuous Wave (CW) Laser Powder Bed Fusion

Experimental investigations were carried out on a customized Trumpf TruPrint3000 (Trumpf Laser & Systemtechnik GmbH) LPBF machine containing a Ø = 300 mm build plate. The machine is equipped with one single-mode laser (SPI SP 500, SPI Lasers UK Ltd.) that can be operated both in cw and pw emission modes with a maximum nominal laser power P_L = 500 W, a nominal wavelength λ = 1070 nm and a spot diameter d_S ≈ 100 µm in the focal plane.

The pw emission is achieved by a modulation of the laser pump current using a gainswitch. This method of pulse generation is called “pulse-modulated emission”. For the given laser, pulse durations of t_p ≥ 50 µs can be applied. A schematic comparison of cw and pw emission mode is given in Fig. 1a, b. Due to the temporal discretization of energy deposition in the pw emission mode, expressed by the pulse duration t_p and pulse frequency f_p, additional processing parameters have to be taken into account to ensure a sufficient bonding within the melt track through the spatial pulse overlap ∆x_o. Moreover, for a given laser power PL and scanning speed v_s, the line energy input in the pw emission mode is reduced compared to the cw emission mode due to intermittent breaks of exposure during the laser off-time (compare the dashed areas in Fig. 1a, b).

For all experimental investigations, gas atomized powder of the nickel-base alloy Inconel 718 (supplied by Oerlikon Metco) with a nominal powder fraction of −15 + 45 µm was used. The chemical composition according to the supplier’s data sheet [20] is shown in Table 1. The images of the powder particles in Fig. 2a, taken by scanning electron microscopy (SEM, LEO 1455 Ep, Zeiss SMT AG), show mostly spherical particles with some satellites, while metallographic cross sections reveal minor contents of porosity. The particle size distribution, measured by Camsizer X2 (Retsch Technology GmbH) powder analyzer, shows a significant amount of fine particle fraction (diameter < 10 µm).

For this contribution, process parameters for a pw contour exposure were investigated, while the hatching region of the specimen was manufactured by conventional cw exposure. Additionally to conventional process parameters, the pw emission mode is characterized by parameters defining temporal laser emission behaviour, e.g. pulse duration t_p, pulse frequency f_p, and geometric parameters such as (relative) spatial pulse overlap ∆x_o.

To investigate the effect of the pw exposure for different contour angles in two different orientations, two different specimen geometries were manufactured within each experiment. The geometry in Fig. 3a was fabricated to vary the contour angle within the exposure plane (xy-plane), while the geometry of Fig. 3b was used to investigate different angles along the build direction (x-z-plane). Afterwards, the identified process parameters were transferred to a francis turbine as a demonstrator part (Fig. 3c). To evaluate the surface topology (see Fig. 4c and 10), cubic specimens were fabricated and the SEM images of side surfaces were captured.

In total, three experimental set ups were carried out. Within all set ups, only the process parameters for the pw contour exposure were varied, while the parameter set for the cw hatch exposure was kept constant. The cw hatch vectors with a spacing ∆y_h = 110 µm were rotated 66.7° in between consecutive layers. Laser power P_L and scanning speed v_s for hatch and contour exposure were set according to the reference parameter set provided by the machine supplier. For all experiments, a constant layer thickness D_S = 40 µm and a constant spot diameter d_S = 100 µm was applied. For the pw contour exposure, laser on-time t_on = 125 µs was set constant for all investigations. Argon was used as a shielding gas, and no preheating of the build plate, made of alloy C45, was applied.

In the first experiment (P1), a suitable process region, indicated by a sufficient metallurgical bonding between the layers was identified by a varying laser power P_L. For the pw contour exposure, the laser power P_L defines the nominal peak power within a pulse. In the following experiment (P2), the effect of increasing the scanning speed v_s on the discrete solidification behaviour of the contour melt pools was examined with the aim to identify the maximum scanning speed where discrete solidification is observed. In the final experiment (P3), the spatial relative pulse overlap ∆x_o was varied and in accordance the scanning speed v_s was adjusted to investigate possible influences of the pulse overlap on the surface topology. The process parameters investigated for each set up are summarized in Table 2. For each experiment, specimens with different contour angles ranging from 5–45°, as depicted in Fig. 3, both in the xy-plane and xz-plane, were manufactured. SEM images of the specimens were taken to evaluate the pw contour morphology and resolution of the contour corners. Metallographic cross sections to evaluate the discrete solidification mode of the contour melt pools of selected specimen were ground with silicon carbide grinding paper, polished using diamond suspension of 1 µm grain size for four minutes, and etched afterwards for 30 s. A mixture of 75 ml ethanol:25 ml hydrochloric acid:5 ml hydrogen peroxide was used as etchant.

The transferability of the results from fundamental process development studies to possible industrial applications has been investigated by manufacturing of an adapted Francis turbine demonstrator as depicted in Fig. 3c. The original .stl-file was downloaded from [21]. After adaption, the demonstrator includes features such as thin wall structured honeycomb sealings and thin walled turbine blades as critical features for the application of pulse-modulated contour exposure. The turbine blades have been fabricated support-free to investigate potential benefits of a pulse-modulated contour exposure on heat induced distortion of thin walled structures. For a comparison between pw and cw contour exposure, the same part geometry has been manufactured using the continuous reference parameter set, No preheating was applied. The geometric accuracy has been measured for both parts using an ATOS GOM (GOM GmbH) 3D-Scanning system.

For a qualitative comparison between cw and pw contour properties, the contour properties and surface topography of specimens fabricated with cw reference parameters are illustrated in Fig. 4. Significant areas of melt pool enlargement due to excessive melting and hence pronounced geometric deviations are observed for both orientations (areas marked in red in Fig. 4a, b). The surfaces of specimens fabricated with a cw contour exposure show a layered structure (melt pool boundaries are indicated with dashed lines) in build direction due to the layer wise manufacturing process (Fig. 4c). Only a few powder agglomerates are sintered to the solidified contour melt traces, which exhibit smooth and homogenous melt pool surfaces without signs of a lack of fusion or balling.

The melt pool volume and geometries are directly dependent on the laser energy input into the powder bed [5]. This dependency is also valid for the discrete energy deposition mode as applied in the pw contour exposure and illustrated in Fig. 5. Different from cw LPBF, the dimensions of the melt pool are directly dependent on the pulse energy (the energy transported within one pulse). Hence, as can be observed in Fig. 5a–c, the dimensions of the melt pool, e.g. melt pool width and depth, increase when increasing the laser power P_L. While for P_L = 92 W (Fig. 5a) lack of fusion is observed between adjacent pulses, a more stable melting is observed for an increased laser power P_L (Fig. 5b, c). Consequently, the corner tip radius is increasing for an increased laser power P_L as well. Caused by a too large contour offset ∆y_c = 100 µm, which was therefore reduced for the following experiments, pore seams occur in all investigated specimens with pw contour exposure. In contrast to the specimen manufactured with a pw contour exposure, the specimen manufactured with cw reference parameters shows a homogenously molten contour melt track with only minor porosity. Due to the significantly higher energy input compared to the specimens with a pw contour exposure, excessive melting is observed in the tip of the corner, leading to significantly enlarged melt pool dimensions reducing the geometric accuracy as shown in Fig. 4a, b and 5d.

For a given spatial pulse overlap, increasing the scanning speed v_s leads to increased pulse frequencies f_p and hence reduced time intervals between consecutive pulses. When this time interval falls below the melt pool solidification time, the previously generated melt pool is not fully solidified before the following melt pool is generated, and, thus, complete discrete solidification of the melt pools is not established. As a result, molten liquids of adjacent melt pools might merge and form a larger melt pool with increased melt pool movements. In consequence, increased geometric deviations might occur. Hence, for the given processing parameters, the knowledge of this transition area is necessary to ensure both a sufficient build part quality and a process productivity.

In Fig. 6, exemplary SEM images of pw contours manufactured with different scanning speeds are shown. The contours of specimens fabricated with a scanning speed of v_s = 100 mm/s and v_s = 125 mm/s, respectively, show discretely solidifying melt pools, while specimens fabricated with higher scanning speeds reveal (partially) merged melt pools within the contour. Due to the merging of the melt pools, these specimens also show signs of increased height differences within the contour melting track. In general, these height differences might affect the powder deposition and melting of following layers. However, within this study none of the specimens investigated showed characteristics of insufficient melting.

As can be seen from Fig. 7, the corner tip width increases from 127 to 155 µm when the scanning speed is increased from v_s = 100 mm/s to v_s = 175 mm/s. Since the spot diameter used within this study was set to d_S = 100 µm, a larger corner tip indicates a higher degree of excessive melting and therefore a decreased accuracy. Hence, along with an increased scanning speed, the geometric accuracy of the contour in the vicinity of the corner decreases. The contours manufactured with a scanning speed v_s = 100 mm/s (Fig. 6a) show pronounced and precisely shaped edges, while, with an increasing scanning speed, an increasing agglomeration of the adjacent melt pools is observed (Fig. 6c, d). This indicates a decreasing degree of discretely solidifying melt pools and, hence, an increased merging of adjacent melt pools.

By metallographic etching, the melt pool boundaries within the pw contour can be observed and, hence, the regions of discrete solidification can be identified. It can be seen from Fig. 7, that, for all investigated scanning speeds, a transition from discrete solidification to merging melt pools, indicated by vanished contour melt pool boundaries, is observed in the vicinity of the contour corner. As a consequence of the angled specimen geometry, the reduced solidified material is located below the exposed melt pool. The closer the melt pool is located to the contour corner, heat dissipation out of the fusion zone through the solidified material is increasingly impeded. As a result, solidification time of the melt pools increase so that a discrete solidification is not fulfilled anymore. With an increasing scanning speed, the afore mentioned transition tends to occur in an increasing distance from the contour corner, as indicated with the measurements marked in red in Fig. 7. With an increasing scanning speed, a higher pulse frequency is necessary for a constant pulse overlap (see parameter study P2 in Table 2). Thus, a higher amount of deposited energy per time unit must be dissipated through the solidified material underneath. Therefore, assuming a constant temporal heat dissipation rate of the solidified material, an increased heat accumulation is expected with an increased scanning speed. In consequence, an insufficient heat dissipation occurs in larger areas of the contour and increased corner tip radii are observed.

Similar effects can be observed for the second specimen geometry (see Fig. 3b) as shown in Fig. 8. Homogenous melt tracks within the last layer are only observed for the scanning speeds of v_s = 100 mm/s and v_s = 125 mm/s, respectively. Especially for an angle of 45° indications for instable melting due to merging melt pools are observed.

Exemplary SEM images of contour corners are depicted in Fig. 9 for a nominal contour angle of 15°. For all overlaps, discretely solidifying melt pools, indicated by boundaries in between adjacent pulses, are observed. With an increasing pulse overlap, the edge of the contour corner is more pronounced leading to an increased geometric deviation. This observation can be explained by the increased energy input per unit length in the contour as more pulses have to be generated with an increased pulse overlap for a sufficient bonding. Because of the discrete energy deposition in pw LPBF, the line energy input is, under the condition of a constant pulse frequency and pulse duration as applied in this experimental setup, directly connected to the number of pulses generated per unit length. Due to the increased energy input, an increased amount of process induced heat needs to be dissipated through the solidified material underneath. Hence, a melt pool enlargement in the corner edge is increasingly observed with an increased pulse overlap.

Furthermore, an increased amount of sintered powder agglomerates at the side surfaces of the edges is observed, when increasing the pulse overlap from 0.4 to 0.7. This observation is confirmed when analyzing the surfaces via SEM and assessment of the arithmetic surface roughness S_a as depicted in Fig. 10. The increased amount of sintered powder particles lead to an increased arithmetic surface roughness S_a. In general, powder particles sinter to the build part surface when the adjacent melt pool is solidified before the powder particles are (fully) molten [22]. Due to the small melt pools generated in the contour within this study, short melt pool solidification times occur and, hence, surrounding powder particles are not fully molten. Due to the increased number of melt pools per unit length with an increasing pulse overlap, an increased number of powder particles per unit length is sintered to the surface.

Based on the results of the process parameter investigation, the transferability of the results to industrial applications has been investigated by fabrication of an adapted Francis turbine demonstrator as depicted in Fig. 11. As can be seen from photographs of the as-built state in Fig. 11a, b, the blades of the turbine manufactured with the cw exposure (a) reveal a more shiny surface with an increased waviness compared to the blades manufactured with the pw contour exposure, which show a homogenous surface topology without predominant surface structures (b). The honeycomb structured sealing of demonstrator (a) is of higher wall thickness than the honeycombs fabricated with the pw emission mode in the demonstrator (b). The analysis of the geometric deviations of the turbine blades show deviations > 1 mm for some blades of the demonstrator (a), while deviations measured at the demonstrator (b) are comparatively lower (approx. < 0.6 mm). Moreover, turbine blades manufactured with the cw contour exposure reveal signs of excessive melting at the blade edge (see pronounced turbine edges in Fig. 11c). These geometric deviations might significantly affect the flow behaviour of the turbine but are, at the same time, difficult to remove.

The findings described indicate the importance of an energy input control into the powder bed at part areas with a limited heat dissipation like thin wall structures. Due to the higher energy input into the cw contour within this study, higher wall thicknesses of the honeycombs, increased thermal distortion of the turbine blades, and increased waviness of the blades’ surfaces due to larger melt pools in the build part contour are observed.