Enhancing the durability of molds, jigs, and tools is crucial for the industry, and one approach to achieve this is by forming a metallic layer with high hardness on their surfaces. Metallic layers with high hardness can be formed through laser metal deposition (LMD), which is one of the additive manufacturing processes, using cemented carbide powder. However, crack initiation typically occurs inside cemented carbide layers formed by the LMD. Therefore, achieving a cladding process for cemented carbide layers without cracks is desired for practical applications. In this study, the effects of tungsten carbide (WC) ratios in WC-Co cemented carbide granulated powder on formed bead size and crack initiation during the LMD processing were investigated. The number of cracks generated during the LMD processing was evaluated using an acoustic emission (AE) technique. The number of burst-type AE signals generated was counted as the number of cracks. Seven types of WC-Co cemented carbide granulated powders with WC ratios ranging from 30.5 to 92 mass% were prepared. Beads were formed using each powder through the LMD, with AE signals being measured. In the case of a WC ratio of 42.9 mass% or less, no crack was observed. On the other hand, cracks were observed when the WC ratio was 53.9 mass% or greater, and the number of cracks increased with an increase in the WC ratio.

Topics
Acoustic emission testing, 3D printing, Laser materials, Metal deposition, Crack initiation and detection, Thermal effects, Laser beam effects, Tungsten, Carbides

Laser metal deposition (LMD) is useful for forming multimaterial structures by using metallic powders with the right material in the right place. In addition, LMD is being applied to additive manufacturing, coating hard materials, and repairing products.1–7 In addition, a multibeam-type LMD (M-LMD) method was developed for realizing precise and minimized heat affected zones. Particularly, applying cemented carbide for coating hard materials using LMD is demanded by the industry. However, when cemented carbide layers are formed by the LMD, cracks are frequently generated, leading to strength reduction.8,9 This cracking phenomenon is caused by the solidification shrinkage that occurs after laser melting and differences in material properties between the substrate and the formed cemented carbide layer. In order to suppress the cracking phenomenon caused by the differences in material properties between the substrate and the formed cemented carbide layer, one promising approach is to consider smoothing the abrupt change in material properties at the interface between the substrate and the cemented carbide layer.10 This can be achieved by implementing compositionally graded structures, in which the tungsten carbide (WC) ratio is gradually increased from the substrate to the surface layer in the formed cemented carbide layers. To realize compositionally graded structures without cracks, it is necessary to understand the effects of the WC ratios on crack initiation within the cemented carbide layer formed through LMD. In this study, WC-Co cemented carbide granulated powders with different WC ratios were prepared to form beads using the LMD process. The effects of WC ratios in the granulated powders on the number of cracks generated during the LMD were investigated through an in situ evaluation method.

For the experiments, WC-Co cemented carbide granulated powders, composed of WC particles with 1 μm in diameter and a Co binder, were prepared. Seven types of granulated powders with different WC ratios of 30.5, 42.9, 53.9, 63.7, 80.4, 88.0, and 92.0 mass% were used as shown in Figs. 1(a)–1(g). The morphology of the powders was almost spherical. The particle size distributions of the WC-Co granulated powders were in the range of approximately 25–45 μm. The median sizes of the granulated powders were about 31–32 μm, as summarized in Table I.

FIG. 1.
SEM images of particles in the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.
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SEM images of particles in the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.

FIG. 1.
SEM images of particles in the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.
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SEM images of particles in the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.

Close modal
TABLE I.

Particle size distributions of the WC-Co granulated powders.

WC ratio (mass%)Particle size distribution (μm)
30.5 D10 = 23 D50 = 31 D90 = 44 
42.9 D10 = 23 D50 = 31 D90 = 45 
53.9 D10 = 24 D50 = 32 D90 = 45 
63.7 D10 = 24 D50 = 32 D90 = 45 
80.4 D10 = 23 D50 = 31 D90 = 44 
88.0 D10 = 23 D50 = 31 D90 = 43 
92.0 D10 = 24 D50 = 32 D90 = 44 
WC ratio (mass%)Particle size distribution (μm)
30.5 D10 = 23 D50 = 31 D90 = 44 
42.9 D10 = 23 D50 = 31 D90 = 45 
53.9 D10 = 24 D50 = 32 D90 = 45 
63.7 D10 = 24 D50 = 32 D90 = 45 
80.4 D10 = 23 D50 = 31 D90 = 44 
88.0 D10 = 23 D50 = 31 D90 = 43 
92.0 D10 = 24 D50 = 32 D90 = 44 
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Figure 2 shows a schematic diagram of the experimental setup for the M-LMD. In this M-LMD system, six beams of diode laser with a wavelength of 975 nm and the maximum output power of 300 W (50W × 6) in a continuous wave can be irradiated toward the diagonal direction from the laser head. The laser focusing spot diameter was set to 280 μm. Beads with a length of 10 mm were formed on JIS SKH51 (ISO HS-6-5-2) substrates measuring 60 mm in height, 40 mm in width, and 20 mm in breadth. The bead forming conditions were set to a laser power of 120 W, a scanning speed of 20 mm/s, and a powder feeding rate of 20 mg/s, as shown in Table II. Three beads were formed for each WC-Co granulated powder with varying WC ratios.

FIG. 2.
Schematic diagram of an in situ crack evaluation method using an AE sensor in the M-LMD system.
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Schematic diagram of an in situ crack evaluation method using an AE sensor in the M-LMD system.

FIG. 2.
Schematic diagram of an in situ crack evaluation method using an AE sensor in the M-LMD system.
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Schematic diagram of an in situ crack evaluation method using an AE sensor in the M-LMD system.

Close modal
TABLE II.

Experimental conditions.

ConditionValue
Laser wavelength 975 nm 
Beam diameter 280 μm 
Laser power 120 W 
Scanning speed 20 mm/s 
Powder feed rate 20 mg/s 
ConditionValue
Laser wavelength 975 nm 
Beam diameter 280 μm 
Laser power 120 W 
Scanning speed 20 mm/s 
Powder feed rate 20 mg/s 
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A broadband sensor with a frequency response ranging from 100 kHz to 1 MHz (±10 dB) was used as an AE sensor. The AE sensor was fixed on the side of the substrate with a magnet, as shown in Fig. 2. Signals obtained from the AE sensor were amplified to 40 dB using an amplifier. The sampling rate was 2 MS/s. When a crack was generated, the AE signal exhibited a burst-type waveform. The number of these waveforms was counted as the number of cracks.11–13 

After the M-LMD processing, the cross-sectional width, height, and area of the beads were evaluated for each bead with varying WC ratios using an optical 3D shape measuring machine.

Figure 3 shows the measured cross-sectional width, height, and area of the cladding beads with varying WC ratios. The error bars in Fig. 3 represent the standard deviation. As the WC ratio increased, the cross-sectional width, height, and area of the cladding beads tended to decrease. These decreases in the bead sizes could be caused by the changes in the supplied powder volume due to the differences in the mass of the WC content within the WC-Co cemented carbide granulated powders. Since the powder feeding rate was set to 20 mg/s, the mass of the supplied powder per unit time was constant. Thus, the volumes of the supplied powders were calculated based on the mass. The particle size of the granulated powders was set to 30 μm based on the average values. Although voids exist in the granulated powders, the calculations were performed without considering voids. The mass of the particles of the granulated powders was calculated using the following equation:
(1)
where mp is the mass of a particle of the granulated powders (g), r is the radius of a particle of the granulated powders (cm), V is the volume fraction (%), and ρ is the density (g/cm3). Additionally, the subscripts m and s indicate the WC phase and Co phase, respectively. The number of particles fed per second, N ˙ p (s−1), can be calculated using the following equation:
(2)
where νp is the powder feeding rate (g/s). The cross-sectional area, As (mm2), perpendicular to the scanning direction of the bead can be obtained by dividing the volume of the number of particles fed per second by the scanning speed. Thus, As can be expressed using the following equation:
(3)
where νb is the scanning speed (mm/s). Table III summarizes the values of physical properties used for the calculation.14–17 The results of the calculations for each value are shown in Table IV. When the mass of the supplied granulated powders was kept constant as the feeding rate, the bead sizes were affected by the mass of the WC content. As the mass of the WC content increased, the cross-sectional area, As, of the beads became smaller. The cross-sectional area, As, of the bead for 30.5 mass%WC is approximately 1.4 times larger than that for 92 mass%WC. However, it should be noted that the value of As for 30.5 mass%WC, obtained by the observation, is about 2.7 times higher than that for 92 mass%WC. The laser absorptance values are 0.82 for WC and 0.38 for Co, using a laser wavelength of 1 μm.18,19 In terms of laser absorption, the amount of absorbed laser energy increases with an increase in the WC ratio. However, since the melting point of WC (3058 K) is much higher than that of Co (1766 K), it can be inferred that the difference in the melting points affects the amounts of melt powders, resulting in a decrease in the volume of cladding beads formed on the substrates.
FIG. 3.
Cross-sectional width, height, and cross-sectional area of the formed beads for each WC ratio.
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Cross-sectional width, height, and cross-sectional area of the formed beads for each WC ratio.

FIG. 3.
Cross-sectional width, height, and cross-sectional area of the formed beads for each WC ratio.
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Cross-sectional width, height, and cross-sectional area of the formed beads for each WC ratio.

Close modal
TABLE III.

Physical properties of WC, Co, and the J1S SKH51 substrate.

MaterialMelting point (K)Density (g/cm3)CM (10−6 K−1)Modulus of elasticity (GPa)
WC 3058 15.8 3.8 696 
Co 1766 8.9 13.5 182 
SKH51  8.1 12.1  
MaterialMelting point (K)Density (g/cm3)CM (10−6 K−1)Modulus of elasticity (GPa)
WC 3058 15.8 3.8 696 
Co 1766 8.9 13.5 182 
SKH51  8.1 12.1  
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TABLE IV.

Values related to powder feeding and CTE for each WC ratio.

WC ratio (mass%)WC ratio (vol. %)Calculated with a particle size of 30 μm
Mass of a particle (g)Number of particles per secondBead cross-sectional area (mm2)CTE (10−6 K−1)
30.5 19.8 1.45 × 10−7 1.38 × 105 0.097 9.2 
42.9 29.7 1.55 × 10−7 1.29 × 105 0.091 7.9 
53.9 39.7 1.65 × 10−7 1.22 × 105 0.086 6.9 
63.7 49.7 1.74 × 10−7 1.15 × 105 0.081 6.1 
80.4 69.8 1.94 × 10−7 1.03 × 105 0.073 5.0 
88.0 80.5 2.04 × 10−7 9.79 × 104 0.069 4.5 
92.0 86.6 2.10 × 10−7 9.51 × 104 0.067 4.3 
WC ratio (mass%)WC ratio (vol. %)Calculated with a particle size of 30 μm
Mass of a particle (g)Number of particles per secondBead cross-sectional area (mm2)CTE (10−6 K−1)
30.5 19.8 1.45 × 10−7 1.38 × 105 0.097 9.2 
42.9 29.7 1.55 × 10−7 1.29 × 105 0.091 7.9 
53.9 39.7 1.65 × 10−7 1.22 × 105 0.086 6.9 
63.7 49.7 1.74 × 10−7 1.15 × 105 0.081 6.1 
80.4 69.8 1.94 × 10−7 1.03 × 105 0.073 5.0 
88.0 80.5 2.04 × 10−7 9.79 × 104 0.069 4.5 
92.0 86.6 2.10 × 10−7 9.51 × 104 0.067 4.3 
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Figure 4 shows the AE signals obtained during the formation of beads for each WC ratio. The graphs show the AE signals measured for 3 s, with bead forming conducted from 0 to 0.5 s. When the WC ratio is 30.5 and 42.9 mass%, AE signals with an amplitude of higher than 0.5 V are not detected, and no burst-type signal is generated. On the other hand, many burst-type signals were observed when the WC ratio was 53.9 mass% or higher. Although the bead forming was performed from 0 to 0.5 s, burst-type signals were observed even after 0.5 s. Thus, it can be inferred that delayed cracking occurred. Figure 5 shows the results of counting the number of cracks for each WC ratio. In the range of 53.9–80.4 mass%WC, the number of cracks decreased slightly with an increase in the WC content. On the other hand, the number of cracks in the 92.0 mass%WC bead was over 1.5 times higher than that of the others. The factors that may cause cracks include solidification shrinkage and the effects of differences in material properties between the cladding beads and substrate. As for differences in material properties, variations in the coefficient of thermal expansion (CTE) may play a significant role in crack initiation. When considering CTE according to the WC ratio for WC-Co composites, the rule of mixtures that considers only the volume ratio is not appropriate because the elastic modulus of WC and Co is significantly different. Therefore, we calculated the CTE at each WC ratio using the following equation, which takes into account the elastic constants proposed by Turner:20,21
(4)
where α is the CTE (10−6 K−1), V is the volume fraction, and K is the modulus of elasticity (GPa). The subscripts of c, m, and s indicate the composite, WC phase, and Co phase, respectively. The used values of physical properties for WC and Co are summarized in Table III. The calculation results are shown in Table IV. The CTE of 30.5 mass%WC, which has the smallest WC ratio, was 9.2, while the CTE of 92 mass%WC was 4.3; the CTE of 30.5 mass%WC was more than twice than that of 92 mass%WC. The larger the difference in CTE between the cladding beads and substrate, the higher the risk for crack initiation due to the larger temperature-induced strain. The degree of CTE difference when crack initiation occurred cannot be clearly shown here because the temperature during the cladding was unknown, since the strain changes depending on the temperature during the cladding process. In future studies, we would like to measure the temperature of beads and substrate during cladding to clarify the relationship between the degree of CTE difference when crack initiation occurred and the WC content. The CTE becomes smaller as the mass of WC content increases, and the risks of crack initiation increase as the difference in the CTEs between the cladding beads and the substrates increases. An increase in the number of cracks is expected due to these factors. However, the number of cracks tended to decrease between 53.9 and 80.4 mass%WC. The reason for this phenomenon is that the powder feeding rate was kept constant for mass. Therefore, as the mass of WC content increased, the number of powder particles fed per second decreased. It can be considered that this leads to an increase in the area on the surface of substrates exposed to laser irradiation, resulting in a larger melting area of the substrates. Since the dilution of the cladding beads caused by the melt substrate can affect the microstructure and composition of the cemented carbide layers, these factors can be considered as one of the reasons for the slight decrease in the number of cracks between the 53.9 and 80.4 mass%WC beads.
FIG. 4.
AE signals detected during forming beads using the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.
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AE signals detected during forming beads using the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.

FIG. 4.
AE signals detected during forming beads using the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.
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AE signals detected during forming beads using the WC-Co granulated powders of (a) 30.5, (b) 42.9, (c) 53.9, (d) 63.7, (e) 80.4, (f) 88.0, and (g) 92.0 mass%WC.

Close modal
FIG. 5.
Number of cracks detected during forming beads for each WC ratio.
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Number of cracks detected during forming beads for each WC ratio.

FIG. 5.
Number of cracks detected during forming beads for each WC ratio.
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Number of cracks detected during forming beads for each WC ratio.

Close modal
In the 53.9–92.0 mass%WC beads, where the burst-type waveforms were obtained as shown in Fig. 4, the sum of the squares of the AE signal amplitudes, serving as an alternative AE signal energy, was obtained using the following equation:22,23
(5)
where EAE is the alternative AE signal energy (V2), ap is the AE signal amplitude (V), s is the start time of bead forming (s), and e is the AE signal sampling time of 15 s. The obtained results are shown in Fig. 6. The EAE of 92.0 mass%WC, in which the highest number of cracks were detected, was the same or smaller than that of the other WC ratios. The waveforms of the AE signals for 92.0 mass%WC showed many burst-type signals with smaller amplitudes than that for other WC ratios, resulting in smaller values of alternative AE energy. Since the signal waveform of 92.0 mass%WC is different from other WC ratios, it can be inferred that a different crack initiation mode operated. Figures 7 (a) and 7(b) show SEM images of the beads of 88.0 and 92.0 mass%WC. The 92.0 mass%WC bead possessed many fine cracks with interrupted transverse cracks. On the other hand, the cracks in the other WC ratios were only transverse cracks that crossed the bead in the width direction, as shown in the image of the 88.0 mass%WC bead. It was found that the higher WC ratio affected the crack initiation mode and its propagation path.
FIG. 6.
Sum of squared amplitudes for from 53.9 to 92 mass%WC.
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Sum of squared amplitudes for from 53.9 to 92 mass%WC.

FIG. 6.
Sum of squared amplitudes for from 53.9 to 92 mass%WC.
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Sum of squared amplitudes for from 53.9 to 92 mass%WC.

Close modal
FIG. 7.
SEM image of beads of (a) 88.0 and (b) 92.0 mass%WC.
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SEM image of beads of (a) 88.0 and (b) 92.0 mass%WC.

FIG. 7.
SEM image of beads of (a) 88.0 and (b) 92.0 mass%WC.
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SEM image of beads of (a) 88.0 and (b) 92.0 mass%WC.

Close modal

It was clarified that the content of the first layer should be 42.9 mass%WC or less when forming cemented carbide in compositionally graded structures without cracks. In our future studies, the effects of differences in CTE on crack initiation mode and its propagation path will be focused on. The appropriate WC ratios for the second and third layers will be investigated.

The LMD technique was used to form beads of cemented carbide to investigate the number of cracks and bead sizes using the WC-Co cemented carbide granulated powders with different WC ratios. The number of cracks was evaluated from the number of burst-type AE signals obtained using the AE sensor. The results obtained are as follows.

Bead sizes tended to decrease in both width and height as the WC ratio increased. Due to the mass-constant powder feeding conditions, a higher mass of WC content led to a decrease in the volume of WC and a reduction in bead size.

The crack counts for each WC ratio showed no crack initiation for 30.5 and 42.9 mass%WC. On the other hand, cracks were observed over 53.9 mass%WC, and the number of cracks decreased slightly with an increase in WC ratio from 53.9 to 80.4 mass%WC. The number of cracks tended to be higher at 92.0 mass%WC than at the other WC ratios.

This work was supported by JSPS KAKENHI (Grant Nos. JP22K03851 and JP22H05280) and the Grant-in-Aid for laser processing (No. AF-2022231-B3) from the AMADA foundation.

The authors have no conflicts to disclose

Yorihiro Yamashita: Conceptualization (lead); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (lead); Writing – original draft (lead). Mitsuki Nakamura: Data curation (equal); Formal analysis (equal). Takahiro Kunimine: Investigation (equal); Writing – review & editing (equal). Yuji Sato: Methodology (equal); Supervision (equal). Yoshinori Funada: Investigation (supporting); Supervision (supporting). Masahiro Tsukamoto: Project administration (equal).

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