| This article studies the combustion synthesis welding between Nickel aluminides and steels (45steel and stainless steel). Combustion synthesis welding is a technology in which CS (Combustion Synthesis) exothermic reaction heat serves as welding heat source and CS reaction product serves as welding agent. According to the different reaction mode, the CS reaction can be divided into two kinds of mode: the self propagating by heating ignition (SHS, Self-propagating High-temperature Synthesis) and whole explosion (TE, Thermal Explosion).In this experiment, the Ni-Al powder compact is synthesized completely to nickel aluminides. At the same time, Nickel aluminides is welded with steel dissimilar material. The metal powder which is blended in five different composition ratio of Ni and Al is pressed beforehand with the steel ring in the model in order to make full use of the quality of heat released in the reaction of the powder compact. For gaining the joint of nickel aluminides/steel in uniform ability and integrated structure, two modes (SHS by laser and TE in high temperature furnace) are exercised in the experiment. The materials of the steel ring are 45steel and 1Cr18Ni9Ti stainless steel which are used abroad in industrial engineering. To begin with, the character of the heat source is studied in this article through thermodynamics analysis, the analysis results disclose that the heat is high temperature and instantaneous, the calefactive speed is rapid. The mathematics mode for ignition time is established based on the assumed condition that the experiment processes in the adiabatic circumstance in order to the analysis the relation between the ignition delay time and the technology parameter, as well as material character. The results indicate that the adiabatic temperature Tad increases and the ignition time shortens when increasing the composition of Ni in the blended powder, those is benefit to weld the material. When the blended powder compact in Ni: Al=1:1,the reaction temperature reaches the peak value about 1500℃, and when the blended powder compact in Ni: Al=3:1,the reaction temperature reaches the peak value about 1200℃.The reaction temperature is recorded in the test of SHS reaction welding by laser ignition and TE reaction welding. The comparison between the value of test and calculated results shows that the established mode is rational. Because the heat losing in the test, the ignition time in calculation is smaller than test value. As the conductivity coefficient of 45steel is big, the ignition delay time for 45steel is longer than stainless steel in the two modes welding. SHS reaction induced by laser progresses more rapidly and completely than TE reaction, so the SHS reaction welding is ignited soon in 20 seconds but the TE reaction welding is ignited slowly in 270 seconds. Furthermore, the reaction temperature in SHS welding is higher than in TE welding 100 to 200℃.Next, the phase and microstructure in the product of nickel aluminides synthesized by powder compact and in the steel are investigated by the means of SEM (Scanning Electron microscope) and XRD (X-ray Diffraction). Eutectic and dentrite morphology are observed in the product of SHS reaction and TE reaction, the main reason for the phenomena is the different of composition in the powder compact, the different temperature in reaction and the different cooling speed for compact inside. The results of XRD analysis indicate that the main phases formed in the SHS and TE reaction are NiAl and Ni3Al. The peak value of the SHS reaction product diffraction is stronger than the value of TE reaction product because the SHS mode reacts more rapidly and completely than TE mode. The micro-hardness distribution in CS reaction welding joint indicated that the micro-hardness of joint is obviously higher than the base metal in the two sides. The micro-hardness of the specimen welded in TE reaction increases along with the heating temperature rises. The value of micro-hardness is 160~200HV0.2 near the 45steel of nickel aluminides /45steel joint, and the value of joint is between 320HV0.2 and 400HV0.2. The micro-hardness is about 290~320HV0.2 near the stainless steel in nickel aluminides /stainless steel joint, and the micro-hardness value of the joint is between 370HV0.2 and 400HV0.2.Afterwards, according to the Fick's second law and practical reaction welding condition, the concentration equation near the interface of nickel aluminides/45steel and nickel aluminides/stainless steel is established by adding initial and boundary conditions. The results of SHS reaction welding show that the calculated concentration of Ni near the interface is closed to the concentration of Ni that is measured by EDS (Energy Dispersive Spectrum), the calculated results Fe of and Al concentration has distinction with the measured values for Fe and Al concentration have strong change in the welded material. The results of Ni: Al=3:1 welding specimen concentration near the joint indicate that the element diffusion distance of Fe, Ni and Al rise along with the temperature increasing in the high temperature furnace. When the temperature reaches 1000℃, the diffusion distance of Fe in nickel aluminides and stainless steel welding is about 180μm,and the diffusion distance of Fe in nickel aluminides and 45steel welding is nearly 100μm. There are element evident diffusion in the interface and the element transform to FeNiAl solid solution between nickel aluminides/ 45steel and nickel aluminides/ stainless steel. The calculated diffusion activation energies of element Fe, Ni and Al near the interface of nickel aluminides/45steel and nickel aluminides/stainless steel are smaller than the activation energies of each element itself in the nickel aluminides, 45steel and stainless steel. At the same temperature, the each element diffusion coefficient near the interface is bigger than the diffusion coefficient in the original materials that are welded in the experiment.At the end, the combination characteristics and the shear fracture micro- morphology in the interface of nickel aluminides/45steel and nickel aluminides /stainless steel is analyzed by the means of SEM. The effect of the welding technological parameters on the combination and shear strength is researched. The experimental results of shear strength show that the joint of compact of Ni: Al=3:1 and 45seel in 1000℃high temperature furnace welding obtained the biggest shear strength whose value was 37.05MPa in all of the joint welded by the TE reaction welding. At the same technological parameters, the biggest shear strength of Ni: Al=3:1 and the stainless steel specimen is 24.85MPa. The curve of load-time during shear test measured for the specimen of Ni: Al=3:1 indicated that the fracture included plastics character. The fracture morphological observation of the joint of nickel aluminides/45 steel and nickel aluminides /stainless steel indicates that it is mainly composed of intergranular fracture. Affected by the fragile product, the fracture site centralizes in the side of nickel aluminides. The fracture causes from the cavity congregation in the cutting stress and micro-cracks produce. The micro-cracks enlarge along the inter-granular, inter-phase and impurity, so the joint ruptures.In a whole, the paper adopts CS reaction to realize the welding between the nickel aluminides and steel. The joint obtained shear strength in a certain degreed, which proved that the technology was feasible. Microstructure and diffusion mechanism in the interface of nickel aluminides and steel are first reported in the paper. The studies provide the foundation to improve the welding between the nickel aluminides and the other dissimilar material.
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