*3.3. Comparison of the SLM Titanium and Copper-Coated Diamond/Copper Composites*

Interfacial bonding holds the key to determining the thermal and mechanical performance of composites [53]. 1 vol.% copper-coated diamond/copper composite sample showed relatively better interface bonding of the copper matrix and diamond reinforcement (Figure 7c), no remarkable defects like flaws or cracks were seen at the interface. The pull-out of diamond particle was merely discovered in the polished surface, implying strong interfacial bonding between the copper matrix and diamond particles. However, 1 vol.% titanium-coated diamond/copper combined material sample showed the pull-out of diamond particle that could be discovered in the polished surface (Figure 7a). For the 3 vol.% titanium-coated diamond/copper combined material sample, the copper matrix and diamond reinforcement bonding were extremely poor and showed obvious cracking (Figure 7b). Resulting from poor interface bonding between diamond and copper, 3 vol.% titanium-coated diamond/copper composite showed a low TC of 57 W/mK at

an energy density of 280 J/mm<sup>3</sup> (140 W, 200 mm/s). There were small cracks in the edge area between copper matrix and diamond reinforcement for the 3 vol.% copper-coated diamond/copper composite sample (Figure 7d). Good interfacial bonding was exhibited in the 3 vol.% copper-coated diamond/copper composite sample for comparison with the 3 vol.% titanium-coated diamond/copper composite sample. The reason why the interfacial bonding of titanium-coated diamond/copper composite was worse than that of copper-coated diamond/copper composite was that introducing electroless copper plating process could avoid essentially the particles gathering and it could improve the relative density, the interfacial bonding and the TC of the diamond/copper composites [28]. stress [50]. The printing parameters were optimized to form the dense copper/diamond composites. Scanning speeds ranging from 50 to 300 mm/s and laser power levels ranging between 130 and 180 W caused numerous printing defects (e.g., balling and pores). To obtain parts with minimal printing defects, the narrow processing window was determined, with a scanning rate of 100 mm/s and a high laser power (160 W). With a high laser power (170– 180 W) and a low scanning rate (50 mm/s), the printed parts exhibited superfusion (Figure 5c,e). These parameters increased the molten pool size, thereby increasing the height and width of the powder tracks.

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 10 of 16

proximately 517 μm occurred, and micro-cracks appeared due to the accumulation of excess heat related to the high power and low scanning rate caused by the high residual

**Figure 5.** 1, 3 vol.% copper-coated diamond/copper combined materials and pure copper featuring various process parameters within the XY plane and surface morphologies: (**a**) 1 vol.% copper-coated diamond/copper composites; (**b**) Pure copper; (**c**,**d**) 3 vol.% copper-coated diamond composites and process window of laser power and scanning rate; (**e**) Typical track types of zones A, B, C, and D.

*Micromachines* **2022**, *13*, x FOR PEER REVIEW 11 of 16

roughness of the composite. The lifetime of the molten pool is the key parameter that influence the flatness and surface roughness, which will be discussed in the future [52].

**Figure 6.** 1 vol.% copper-coated diamond/copper combined materials relationship between the surface roughness and (**a**) laser power and (**b**) scanning rate. **Figure 6.** 1 vol.% copper-coated diamond/copper combined materials relationship between the surface roughness and (**a**) laser power and (**b**) scanning rate. *Micromachines* **2022**, *13*, x FOR PEER REVIEW 13 of 16

*3.3. Comparison of the SLM Titanium and Copper-Coated Diamond/Copper Composites* 

**Commented [M1]:** The picture is replaced

**Figure 7.** SEM images of the copper matrix and diamond bonding of (**a**) 1 vol.% and (**b**) 3 vol.% titanium-coated diamond/copper combined materials; (**c**) 1 vol.% and (**d**) 3 vol.% copper-coated diamond/copper combined materials; (**e**) The bending stress and (**f**) CTE values of titanium and copper-coated diamond/copper combined materials and pure copper. **Figure 7.** SEM images of the copper matrix and diamond bonding of (**a**) 1 vol.% and (**b**) 3 vol.% titanium-coated diamond/copper combined materials; (**c**) 1 vol.% and (**d**) 3 vol.% copper-coated diamond/copper combined materials; (**e**) The bending stress and (**f**) CTE values of titanium and copper-coated diamond/copper combined materials and pure copper.

SLM technology was used to form titanium and copper-coated diamond/copper

(1) The values of roughness gradually decreased with increasing laser power. When the laser power reached the maximum value of 180 W, the surface roughness (*Sa*) reached a minimum value of 5.751 μm. The surface roughness *Sa* reached a minimum at the scan-

(2) 1 vol.% copper-coated diamond/copper composite sample showed relatively best interface bonding of the copper matrix and diamond reinforcement, corresponding the

lowest CTE and the strongest bending strength.

**4. Conclusions** 

performance were studied.

ning rate of 200 mm/s.

The bending strength of 1 and 3 vol.% copper-coated diamond/copper composites significantly exceeded that of corresponding titanium-coated diamond/copper composites. The maximum bending strength of 3 vol.% copper-coated combined materials was 108 MPa, while that of the 3 vol.% titanium-coated combined materials was only 36 MPa. The maximum bending strength of the 1 vol.% copper-coated combined materials was 150 MPa, that of the 1 vol.% titanium-coated composites was 148 MPa. And the printed composites with 1 vol.% copper-coated diamond were approximately three times stronger than the printed copper (≤58 MPa) for the bending strength. When the diamond concentration rose from 1 to 3 vol.%, the bending strength decreased as the relative density decreased (Figure 7e), the viscosity of melt increases obviously as the diamond particle content increases at a constant size, resulting in the decrease in the fluidity and deterioration in the sample surface quality, leading to the decrease in relative density [54]. These experiments indicated that a moderate TC (174 W/mK) was produced by printing 1 vol.% titanium-coated diamond/copper mixed powders at an energy density of 360 J/mm<sup>3</sup> (180 W, 200 mm/s), a maximum TC (336 W/mK) was produced by printing 1 vol.% copper-coated diamond/copper mixed powders at an energy density of 300 J/mm<sup>3</sup> (180 W, 200 mm/s) and a moderate TC (162 W/mK) was produced by printing 3 vol.% copper-coated diamond/copper mixed powders at an energy density of 533 J/mm<sup>3</sup> (160 W, 100 mm/s). Additionally, the printed composites with 3 vol.% copper-coated diamond were approximately three times larger than those with 3 vol.% titanium-coated diamond for the TC. A moderate TC (183 W/mK) was produced by printing the copper powders at an energy density of 171 J/mm<sup>3</sup> (180 W, 350 mm/s) as shown in Figure 5b. This difference was likely caused by the dissimilarity of those interfacial bonding between the copper matrix and diamond reinforcement. The higher energy density required to print diamond/copper composites compared to pure copper was caused by solid coated diamond particles into the molten copper pool. The solid coated diamond particles improved the molten metal viscosity and restricted its ability to flow and fuse. And as the coated diamond particle content further increased, the porosity and the laser absorptivity of the mixed powder further increased, the powder fluidity and the thermal conductivity further decreased [6].

Figure 7f shows the CTE curves of the 1 and 3 vol.% titanium and copper-coated diamond/copper composites, as well as copper upon raising the temperature from 30 to 400 ◦C. The CTE of 1 and 3 vol.% copper-coated diamond/copper composites was significantly below that of corresponding titanium-coated diamond/copper composites, the minimum CTE of 1 vol.% copper-coated diamond/copper combined materials was very close to that of the copper. The shear stress had little effect on the interior of the coppercoated diamond/copper composite during heating, demonstrating the interfacial bonding of the copper matrix and diamond was better in the 1 vol.% copper-coated diamond/copper composites. During the heating process, the shear stress along the interface of the copper matrix and diamond had little effect on the CTE and led to a better thermal stability. This highlighted the advantages of copper plating on diamond particle surfaces, while the copper-coated diamond/copper composite properties reached high levels in comparison.

#### **4. Conclusions**

SLM technology was used to form titanium and copper-coated diamond/copper composites. The microstructure, roughness, interface bonding, thermal and mechanical performance were studied.

(1) The values of roughness gradually decreased with increasing laser power. When the laser power reached the maximum value of 180 W, the surface roughness (*Sa*) reached a minimum value of 5.751 µm. The surface roughness *Sa* reached a minimum at the scanning rate of 200 mm/s.

(2) 1 vol.% copper-coated diamond/copper composite sample showed relatively best interface bonding of the copper matrix and diamond reinforcement, corresponding the lowest CTE and the strongest bending strength.

(3) 1 vol.% copper-coated diamond/copper composites had the highest TC (336 W/mK) at an energy density of 300 J/mm<sup>3</sup> (180 W, 200 mm/s). 3 vol.% copper-coated diamond/copper composites had the moderate TC (162 W/mK, 533 J/mm<sup>3</sup> , 160 W, 100 mm/s). 1 vol.% titanium-coated diamond/copper composites had the moderate TC (174 W/mK, 360 J/mm<sup>3</sup> , 180 W, 200 mm/s). 3 vol.% titanium-coated diamond/copper composites had the lowest TC (57 W/mK, 280 J/mm<sup>3</sup> , 140 W, 200 mm/s). The copper powders had the moderate TC (183 W/mK, 171 J/mm<sup>3</sup> , 180 W, 350 mm/s).

The article offered electroless plating and evaporation methods for SLM to coat copper and titanium on the diamond particle surface for modifying and improving the wettability of diamond/copper interface, which opened up a new way for laser 3D printing technology to print a broad range of diamond-particle-reinforced MMCs. Thereby, unleashing their full potential for electronic package and thermal management applications.

**Author Contributions:** L.Z. and Q.S. designed the experiments and wrote manuscript; L.H. provided the initial idea of this paper and financial support; Y.L. revised the manuscript; M.S. and H.G. performed the experiments; S.L. improved the language; Q.C. and P.G. analyzed the data. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is funded by Wuhan Applied Foundational Frontier Project from Wuhan Science and Technology Bureau Project, China (No. 2020010601012172), the National Natural Science Foundation of China (No. 61805095, No. 51675496, No. 51902295), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG2021234).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Xiao Yang 1,\*, Shuo Wang <sup>1</sup> , Hengpei Pan <sup>1</sup> , Congyi Zhang <sup>1</sup> , Jieming Chen <sup>1</sup> , Xinyao Zhang 1,2 and Lingqing Gao 1,2**


**Abstract:** For NiTi alloys, different additive manufacturing processes may have different compressive recovery capabilities. In particular, there are relatively few studies on the compressive recovery ability of NiTi alloys by the laser-directed energy deposition (LDED) process. In this paper, the compression recovery properties of NiTi alloys with the LDED process were investigated quasi-in-situ by means of transmission electron microscopy, an electron backscatter diffractometer, and focused ion beam– fixed-point sample preparation. The results showed that the material can be completely recovered under 4% deformation and the B19' martensite phase content and dislocation density are basically unchanged. However, the recovery rate was only 90% and the unrecoverable strain was 0.86% at 8% deformation. Meanwhile, the B19' martensite phase content and dislocation density of the material increased. Furthermore, with the increase in deformation, the relative dislocation pinning effect of the Ti2Ni precipitated phase in the alloy was enhanced, which reduced the compressive strain recovery to a certain extent.

**Keywords:** laser additive manufacturing; compression; microstructure transformation; dislocation pinning; recovery ability
