Papers by Author: Ding Fan

Abstract: The gradient calcium phosphate bioceramic coating was produced on titanium alloy substrate by laser cladding. The microstructure, microhardness, fracture toughness, and residual stress of the tatanium-based gradient bioceramic composite coating were investigated. The results show that the microhardness gradually decreases with further depth increasing cross-section. The highest microhardness of the coating and the transition layer is 1544HV and 1160HV, respectively. The fracture toughness K_IC is 3.72±0.03 MPa·m^1/2 of bioceramic coating and 4.55±0.02 MPa·m^1/2 of the transition layer, which is closely resembles the human compact bone. Furthermore, the residual stress gradually decreases from the coating to substrate, which is 221MPa between ceramic layer and the transition layer and 108MPa between the transition layer and substrate. This distribution is conforms to gradient composition design, which reducing harm of the specimen deformation and cracking.

Abstract: A novel and high efficient consumable double-electrode gas metal arc welding (consumable DE-GMAW) method has been introduced. Because of the coupled welding parameters, a multi-input multi-output (MIMO) control scheme was proposed and tested, which controlled the bypass arc by adjusting the bypass wire feed speed, and controlled the base metal current by adjusting the bypass current. Then, the welding experiment has been carried out. The results showed that the MIMO control scheme was effective. The base metal current was controlled at a nearly constant level and the welding process was stable. Also a good weld appearance was obtained.

Abstract: Consumable double-electrode gas metal arc welding (consumable DE-GMAW) is a novel and high efficient welding method. According to the different polarity of two electrodes in consumable DE-GMAW, the metal transfer behaviors are analyzed by using the state force balance theory. For main torch with direct current electrode positive polarity, the arc root area is enlarged by bypass arc and increased electromagnetic force promotes main metal transfer. For bypass torch with direct current electrode negative polarity, the electromagnetic force does not work on the melting area. Only gravity force is major detaching force. The volume of bypass droplet is large and not easily detache to the weld pool.

Abstract: An approach for fabricating functionally graded specimens of Co-based alloy and molybdenum disilicide (MoSi₂) compositions via Selective Laser Cladding (SLD) is presented. The focus aimed at using the functionally graded material (FGM) concept to gradually grade powdered compositions of MoSi₂ within a base material of Co-based alloy. A high power CO₂ laser was used to process the material compositions to a high density with gradual but discrete changes between layered compositions. The graded specimens initially consisted of 100% Co-based alloy with subsequent layers containing increased volume compositions of MoSi₂ (0-80wt.%). Specimens were examined for porosity, phase composition and microstructure. It was found that crack sensitivity of layer can be reduced by adding Co-based alloy into the pure MoSi₂ powder.

Abstract: Due to strong heat accumulation and low surface tension of aluminum alloy, weld width will become wider, even subsidence in pulsed MIG welding process of aluminum alloy at constant parameters. A variable double-pulse method for weld width control is proposed. Weld width control is realized by changing double-pulse duty cycle that is the ratio of high-energy pulse time in a double-pulse cycle to adjust heat input based on the vision sensing for weld width. A rapid prototyping control system is built on the basis of vision sensing and xPC Target real-time environment. Then variable double-pulse MIG welding process test is done and proves the feasibility of the control scheme. On this basis, weld width control test in pulsed MIG welding of aluminum alloy is carried out and obtains a good weld with beautiful formation and uniform weld width. The results show that weld width control can be realized well with the variable double-pulse method in pulsed MIG welding of aluminum alloy.

Abstract: Thermal deformation of welding structure is inevitable in welding process. The deformation will not only change the appearance, shape and geometry size of the welding structure, reduce assembly quality but also may reduce the bearing capacity. On this basis, a welding deformation measurement based on visual method was proposed for the problem of the thermal deformation measurement in welding process. Theoretical calculations were carried out and an image processing and measurement algorithm based on MATLAB was also put forward. Then welding deformation measurement experiment was carried out and the results show that the welding deformation obtained by actual measurement is consistent with the theoretical analysis, proving the feasibility of welding deformation measurement based on vision. The welding deformation measurement based on visual method lays a foundation for automatic measurement of welding deformation.

Abstract: Dual-bypass MIG welding (DB-GMAW) is a new kind of high speed MIG welding with three arcs. In order to monitor the weld process and control it, a high speed weld system of DB-GMAW was built. The system was run by LabVIEW programs, including getting data of system and control output signals. The test result of system showed that all equipments could be used in the same time. Beside images of weld pool and arc, the weld voltages and currents of every part had been acquired. The signals of bypass current and weld speed also had been input TIG welding sources and worktable motor successfully. Meanwhile, the high speed weld formation had a good quality, and all of these established the closed-loop control of high speed DB-GMAW.

Abstract: The calcium phosphate bioceramic coating was fabricated on titanium alloy (Ti-6Al-4V) substrate by a 5kW continuous transverse flow CO2 laser. Due to the peculiar role of rare earth oxide in laser cladding, the effect of ceria additive on the microstructure and properties of laser-cladded bioceramic coating was investigated by means of scanning electron microscope (SEM), X-ray diffraction (XRD), microhardness and corrosion resistance testing. The results indicate that the appearance of rare earth oxide ceria in the precursor powders has an impact on the microstructure and properties of the laser-cladded bioceramic coating. Calcium phosphate bioceramic such as hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) are synthesized on the top surface of laser-cladded specimens. And the addition of rare earth oxide ceria in pre-placed powders has an influence on the formation of calcium phosphate bioceramic phases. Furthermore, it reveals that the laser-cladded bioceramic coating of ceria additive in pre-placed powders has more favorable microhardness and corrosion resistance compared with the coating without rare earth oxide.

Abstract: Based on a high power CO2 laser beam passing by an integral mirror, the bioceramic coatings of gradient composition were designed and fabricated on titanium alloy substrate (Ti-6Al-4V). The relations among laser processing parameters, microstructure and thermal behavior of the gradient bioceramic coatings were investigated. The morphology of the composites was observed by scanning electron microscope (SEM). Phase composition of the coatings was analyzed by X-ray diffraction (XRD). And the thermal behavior of raw powders was evaluated through thermal gravimetry and differential scanning calorimetry (TG-DSC) test. The results demonstrated that the bioceramic coatings were metallurgically bonded to the titanium alloy substrate. The bioceramic coatings contained such bioactive phases as HA and β-TCP, which offered an advantageous condition for osseo-connection. The DSC thermograms showed the endothermic peaks at different temperature, which resulted from the different transitions process, respectively. Furthermore, the DSC results were in accordance with TG data of the powders and showed that with the increasing temperature the weight of sample accordingly decreased.

Abstract: Laser cladding technique was used to form Ni3Si intermetallic composite coating reinforced by in-situ formation NbC particles on Ni-based superalloy substrate. The process parameters were optimized to obtain cladding. The effect of Nb-C addition to the microstructure of the coating was investigated. The morphology of reinforcement particles was discussed. The experimental results showed that an excellent bonding between the coating and the substrate was ensured by a strong metallurgical interface. The clad coating was very good and free from cracks and pores. The microstructure of the coating was mainly composed of Ni(Si), Ni3(Si, Nb) and NbC. The NbC particles were formed by in-situ reaction between Nb and C during laser cladding process. NbC particles were homogeneously distributed in the composite material. Moreover, the maximum size of NbC particles was more than 4 μm.