Showing papers by "Jie Sun published in 2020"

Effect of process parameters on the phase transformation behavior and tensile properties of NiTi shape memory alloys fabricated by selective laser melting

Abstract: In this work, we show that selective laser melting (SLM), apart from producing complex structures, can also act as a metallurgical method to modify the Ni/Ti ratio of NiTi shape memory alloys, and thus providing a feasible approach to tailor the transformation temperatures and to modify the mechanical performance of NiTi alloys. NiTi samples were fabricated by SLM with a large variation of process parameters, e.g. scanning speed (v) from 400 to 1200 mm s−1, hatch spacing (h) from 40 to 110 μm, and laser power (P) from 60 to 200 W. It is found that the martensite transformation temperature changes monotonously with the respective change of v, h or P. The composition analysis suggests that the different amount of Ni-loss under various SLM process conditions is the main reason for the evolution of transformation temperatures. Most importantly, good mechanical (total elongation >10 %) and functional properties under tensile mode have been obtained despite of the large variation of SLM process parameters and the presence of extensive defects. The good tensile properties and tailorable phase transformation temperatures will provide great potential to make novel NiTi smart structures.

Effect of scanning strategies on the microstructure and mechanical behavior of 316L stainless steel fabricated by selective laser melting

Abstract: Five scanning strategies were applied for relating the microstructural and crystallographic morphology to the mechanical properties of 316L stainless steel fabricated by selective laser melting (SLM). The results indicate that scanning strategies have a significant effect on the microstructure, grain growth, grain size and therefore mechanical properties of SLM-built 316L stainless steel. Applying the scanning strategy with the rotation angle between successive layers breaks the columnar growth of grains and fine equiaxial grains are formed, leading to an improvement of tensile strength and good ductility of SLM-built samples. Moreover, the microhardness is improved by applying the scanning strategy with a rectangular pattern. This work demonstrates the scanning strategy during SLM process acts as an essential factor affecting energy input, and therefore tailors the meso-/micro-scale structure and mechanical behavior of metals.

Effect of post-heat treatment cooling on microstructure and mechanical properties of selective laser melting manufactured austenitic 316L stainless steel

Abstract: This paper aims to investigate the influence of different post-annealing cooling conditions, i.e. furnace cooling (heat treatment (HT) 1 – slow cooling) and air cooling (HT 2 – fast cooling), on the microstructure and mechanical properties of selective laser melting (SLM) built austenitic 316L stainless steel (SS).,Three sets of 316L SS samples were fabricated using a machine standard scanning strategy. Each set consists of three tensile samples and a cubic sample for microstructural investigations. Two sets were subsequently subjected to annealing HT with different cooling conditions, i.e. HT 1 and HT 2, whereas one set was used in the as-built (AB) condition. The standard metallographic techniques of X-ray diffraction, scanning electron microscopy and electron back-scattered diffraction were used to investigate the microstructural variations induced by different cooling conditions. The resultant changes in mechanical properties were also investigated.,The phase change of SLM fabricated 316L was observed to be independent of the investigated cooling conditions and all samples consist of austenite phase only. Both HT 1 and HT 2 lead to dissolved characteristic melt pools of SLM. Noticeable increase in grain size of HT 1 and HT 2 samples was also observed. Compared with AB samples, the grain size of HT 1 and HT 2 was increased by 12.5% and 50%, respectively. A decreased hardness and strength, along with an increased ductility was also observed for HT 2 samples compared with HT 1 and AB samples.,From previous studies, it has been noticed that most investigations on HT of SLM fabricated 316L were mainly focused on the HT temperature or holding time. However, the post-HT cooling rate is also an equally important factor in deciding the microstructure and mechanical properties of heat-treated components. Therefore, this paper investigates the influence of different post-annealing cooling conditions on microstructure and mechanical properties of SLM fabricated 316L components. This study provides a foundation for considering the post-HT cooling rate as an influential parameter that controls the properties of heat-treated SLM components.

Tool condition monitoring in milling using a force singularity analysis approach

Abstract: Tool condition monitoring (TCM) is extremely important to ensure production efficiency and workpiece quality. It is crucial to extract and select suitable features from raw signals to improve the robustness and feasibility of TCM systems. This paper introduces a cutting force singularity analysis approach for TCM in milling, which correlates tool wear states with force waveform variations. The Holder exponents (HEs) were selected as the index of singularities. HEs are calculated by wavelet transform modulus maxima (WTMM). The raw signal is de-noised based on WTMM estimation, which can effectively preserve singularities compared with traditional filters. Fisher’s discriminant ratio (FDR) is employed to rank the discriminant capability of statistical features of HEs. It is found that means of HEs and quantities of singular points estimated from feed forces Fx show the strongest class-discriminant ability. Then, these features were chosen as training samples to propose a TCM approach based on the support vector machine (SVM). Experimental results indicate that this approach provides reliable and effective advice for tool change.

Preparation and tribological properties of MoS2-based multiple-layer structured films fabricated by electrohydrodynamic jet deposition

Abstract: As a kind of solid lubricant, molybdenum disulfide (MoS2) plays a pivotal role in reducing wear and friction. How to prepare MoS2 films with excellent tribological properties has become a research hotspot. In this paper, four kinds of slurries were prepared to deposit pure MoS2 films, MoS2/Ti composite films and Ti-MoS2/Ti multiple-layer structured films on cemented carbide surfaces by electrohydrodynamic jet deposition. The composition and microstructure of the deposited films were investigated. The adhesive strength between the film and the substrate was evaluated by scratch tests. Furthermore, the friction and wear properties were investigated by reciprocating sliding friction tests. The results show that the Ti layer can improve the film-substrate adhesive strength, and the construction of multiple-layer structure can significantly reduce the friction coefficient and prolong the friction life of the MoS2 films.

Broadband near-infrared Cr3+:β-Ga2O3 fluorescent single crystal grown by the EFG method

Abstract: Broadband near-infrared (NIR) light-emitting diodes (LEDs) have important applications in non-destructive detection across agriculture, food, chemical and medical fields. LEDs coated with IR phosphors are widely used but have some shortcomings such as low efficiency, low brightness, short service life, and so on. Here, we propose a new strategy to solve the aforementioned problems by using the Cr3+:β-Ga2O3 single crystal as a transparent and conductive substrate with NIR fluorescent for the first time. The band structure and optical properties of the Cr3+:β-Ga2O3 crystal were calculated through density functional theory, and it was successfully grown by the edge-defined film-fed growth method. The crystal had a broad band absorption peak around 423 nm, which was located at the emitting band of the GaN LED. Broadband NIR emission from 600 nm to 850 nm with a peak maximum at 690 nm was found under the blue light excitation. Furthermore, the electrical properties of Cr3+:β-Ga2O3 were also studied by the Hall test. The spectral and electrical analyses indicate that Cr3+:β-Ga2O3 is promising in broadband NIR applications as a fluorescent substrate for vertical large current GaN LEDs. Combined with the advantages of low mismatch, transparency, conductivity and broadband NIR fluorescence, Cr3+:β-Ga2O3 provides a new strategy for high-brightness near-infrared broadband light sources.

Tribological performance of AlCrN–MoS2/PTFE hard-lubricant composite coatings:

Abstract: In order to improve the tribological performance of the physical vapor-deposited AlCrN coatings, molybdenum disulfide (MoS2)/poly tetra fluoroethylene (PTFE) coatings were fabricated on the AlCrN c...

Experimental investigation on the performance of novel double cone integrated tool in one-shot drilling of metal stacks

Abstract: In order to reduce misalignment, enhance tight tolerance, and improve machining efficiency, a novel double cone integrated tool was developed and experimentally investigated during one-shot drilling of aluminum and aluminum alloy stacks in this paper. This special tool geometry can finish drilling, reaming, and countersinking processes at the same time. Its machining properties were comparatively evaluated with a standard twist drill (reference drill) by means of a series of experiments. Experimental results highlighted the influence of tool geometry on cutting force and vibration signals for drilling, reaming, and countersinking processes, respectively. The results indicated that the double cone integrated drill can induce a little larger thrust force than a reference drill due to its longer cutting edge. In addition, the chip morphologies generated by different processes and cutting variations also have been comparatively analyzed. The double cone drill under a larger feed rate was prone to produce smaller chips. Furthermore, the double cone drill can achieve smaller burrs, better surface integrity, and lighter tool wear compared with reference drill. The information obtained from this study can provide a better understanding of the one-shot drilling process and guidance for achieving higher efficiency and better hole quality.

A modified analytical cutting force prediction model under the tool crater wear effect in end milling Ti6Al4V with solid carbide tool

Abstract: This paper presents a study of the relationship between cutting force and tool crater wear of solid carbide tool during milling Ti6Al4V. The cutting force model in milling considering tool crater wear is discussed, which shows that for given the cutting conditions, tool geometries, and workpiece material, cutting force under the tool crater wear effect could be predicted easily and conveniently. The tool wear condition in milling Ti6Al4V is obtained by finite element simulation method. In addition, the experimental work of end milling Ti6Al4V with solid carbide tool is developed. Comparisons among experiment results, mathematical model results, and finite element simulation model results, including cutting force and tool wear, are discussed. The results show that the proposed mathematical model could predict cutting force under the tool crater wear effect with higher accuracy.