Influence of macrosegregation on solidification cracking in laser clad ultra-high strength steels

宏观偏析对激光熔覆超高强度钢凝固裂纹的影响

Surface and Coatings Technology, Volume 340, 25 April 2018, Pages 126-136

Cameron Barr, Shi Da Sun, Mark Easton, Nicholas Orchowski, Milan Brandt

摘要：Alloy selection is critical for the performance of aerospace components repaired through laser cladding, as unsuitable combinations of clad and substrate materials can lead to defects during deposition. Different combinations of laser clad ultra-high strength steels have been studied to examine the effect of material composition on clad quality. Clad/substrate combinations of Aermet®100/300M, Aermet®100/4340, and 4340/300M were trialled using a range of processing parameters. While no defects occurred in the 4340/300M samples, solidification cracking was observed in the Aermet®100 multi-track clad samples, especially on 300M substrates. The cracking originates from macrosegregation trails caused by differences in melting temperature between clad and substrate. These trails interfere with liquid feeding beneath them when the substrate has a higher liquidus temperature, with entrapped liquid leading to short solidification cracks. A second larger form of solidification cracking was found in Aermet®100/300M due to Aermet®100 solidifying faster in the late stages of solidification, as this can entrap liquid in the inter-dendritic regions leading to cracking. Both forms of cracking can be avoided by increasing the laser interaction time during cladding, as this slows the solidification process to allow for more complete mixing and liquid feeding.

Effect of Ti on the microstructure evolution and wear behavior of VN alloy/Co-based composite coatings by laser cladding

钛对激光熔覆VN合金/钴基复合涂层微观结构演变与摩擦特性的影响

Journal of Materials Processing Technology, Volume 252, February 2018, Pages 711-719

Lin Ding, Shengsun Hu, Xiumin Quan, Junqi Shen

摘要：VN alloy/Co-based composite coatings modified by Ti was prepared on a mild steel using laser cladding. The effect of Ti addition on the microstructure and wear resistance of VN alloy/Co-based coatings was investigated using optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, micro hardness tester and wear tester. The results showed that TiN and VC phases were detected in the VN alloy/Co-based composite coatings after adding Ti, in addition to γ-Co, Cr23C6 and σ-FeV phases. Massive short rod-like dendrites, equiaxed grains and fine grains appeared in the composite coatings after adding Ti, and the cross-slip and jog of the dislocation disappeared. The microhardness of the composte coatings was increased significantly with the increase of additive Ti, and was improved by 21.6%, for 9.6 wt.% Ti addition. The wear resitance of the composite coatings was firstly increased and thereafter decreased with the increase of additive Ti, and was improved by 25.9%, for 4.8 wt.% Ti addition. The abrasive mechanism of the composite coatings after adding Ti was the abrasive wear. Thus, it can be concluded that adding Ti effectively promotes the formation of TiN and VC particles to improve the wear resistance of the composite coatings.

Influence of microstructures and wear behaviors of the microalloyed coatings on TC11 alloy surface using laser cladding technique

激光熔覆微合金涂层微观结构和耐磨特性对TC11合金表面的影响

Surface and Coatings Technology, Volume 337, 15 March 2018, Pages 97-103

Chengyuan Yang, Xu Cheng, Haibo Tang, Xiangjun Tian, Dong Liu

摘要：Microalloyed coatings were fabricated on a forged TC11 titanium alloy surface by adding different amount of ZrN powders using laser cladding technique. Microstructures of coatings and heat affect zone were systematically investigated with the study of microhardness as well as wear property. Results indicated that all coatings have very uniform equiaxial grains with very fine α + β two-phase microstructure (the thickness of α lamellar is 0.8 μm), and nearly no change of microstructure for coatings was observed with the increase contents of ZrN from 3 wt% to 12 wt%. The changes of microstructure from coating to substrate are: fine α + β two-phase structure, super fine α + β two-phase structure (α lamellar of 0.2 μm), αp + αs + β structure, and α + β (forged). By increasing the contents of Zr and N, the hardness of coatings increased accordingly with no suddenly change at the interfacial region. Due to the strong solid-solution strengthening, coatings maintain high hardness even coarsening their microstructures. The enhanced hardness of coatings largely improves the wear properties of alloys to only more than twice of the TC11 matrix when addition of 12 wt% ZrN.

Microstructure and wear resistance of laser cladded Ni-Cr-Co-Ti-V high-entropy alloy coating after laser remelting processing

基于激光熔凝工艺的激光熔覆Ni-Cr-Co-Ti-V高熵合金涂层微观结构与抗磨性

Optics & Laser Technology, Volume 99, 1 February 2018, Pages 276-281

Zhaobing Cai, Xiufang Cui, Zhe Liu, Yang Li, Guo Jin

摘要：An attempt, combined with the technologies of laser cladding and laser remelting, has been made to develop a Ni-Cr-Co-Ti-V high entropy alloy coating. The phase composition, microstructure, micro-hardness and wear resistance (rolling friction) were studied in detail. The results show that after laser remelting, the phase composition remains unchanged, that is, as-cladded coating and as-remelted coatings are all composed of (Ni, Co)Ti2 intermetallic compound, Ti-rich phase and BCC solid solution phase. However, after laser remelting, the volume fraction of Ti-rich phase increases significantly. Moreover, the micro-hardness is increased, up to ∼900 HV at the laser remelting parameters: laser power of 1 kW, laser spot diameter of 3 mm, and laser speed of 10 mm/s. Compared to the as-cladded high-entropy alloy coating, the as-remelted high-entropy alloy coatings have high friction coefficient and low wear mass loss, indicating that the wear resistance of as-remelted coatings is improved and suggesting practical applications, like coatings on brake pads for wear protection. The worn surface morphologies show that the worn mechanism of as-cladded and as-remelted high-entropy alloy coatings are adhesive wear.

Formation mechanism and improved properties of Cu95Fe5 homogeneous immiscible composite coating by the combination of mechanical alloying and laser cladding

基于机械合金化与激光熔覆相结合的Cu95Fe5均质难混溶复合涂层形成机理与改进特性

Journal of Alloys and Compounds, Volume 740, 5 April 2018, Pages 194-202

Xiaoqin Dai, Min Xie, Shengfeng Zhou, Chunxia Wang, Zhengyang Li

摘要：In order to improve the homogeneity of spherical Fe-rich particles within the Cu-rich matrix, the Cu95Fe5 (wt%) immiscible composite coating has been produced by the combination of mechanical alloying and laser cladding. The results show that the milled composite powder presents a polygonal morphology and is mainly composed of face-centered-cubic (fcc)-Cu solid solution. The demixing or delimitation is eliminated in the immiscible composite coating produced by laser cladding and large amounts of spherical α-Fe particles are dispersed within the ε-Cu matrix as a result of liquid phase separation (LPS). The size of the Fe-rich particles measured is in radius of ∼1.5 μm, which is much smaller than the calculated critical radius (∼2.3 μm), confirming that the α-Fe particles containing a supersaturated Cu are driven by the dominant mechanism of Marangoni motion. Compared to Brass, the immiscible composite coating exhibits the improved homogeneous microhardness (153HV0.2) and higher corrosion resistance, as well as a saturated magnetization of ∼9.19 emu/g and low coercivity of ∼9.25 Oe.