| Laser Rapid Prototyping(LRP) is suitable for rapid manufacturing the fields of aeronautics andastronautics high-performance complex structure, large overall of metal parts. Laser CladdingForming(LCF) can quickly manufacture full density metal parts with complex shape, which is one ofthe prospect direct manufacturing technology in RP field. However, there may appear porosity orinsufficient mechanical properties of defects during LCF metal. It is difficult to meet the industrialpractice. But pulsed current can effectively reduce conventional some defects in the process ofsolidification. Hence, pulsed current is proposed to be introduced into laser cladding process, applyingelectromagnetic influence on the molten pool, in order to obtain fewer defects, good performance oflaser cladding formed parts. Taking the widely used in aerospace engine materials—Nickel-basedsuper-alloy as the study object, this paper has completed works as below:1. This paper studied the basic theory of laser cladding, including the interaction between laserand material, the conditions of liquid phase cladding and the process of laser cladding metal powder.In addition, this paper also studied principle of fine-grain with the pulsed current when solidification,mainly including skin effect, increase super-cooling vacation hypothesis, magneto shrinkage effectand hypothesis of broken crystal production, which provide effective theoretical support forsubsequent analysis of the simulation and experiment.2. In order to explore the alternating current flow in the rectangular cross section conductor,author used the finite element method in ANSYS environment to simulate the current distribution ofdifferent thickness width ratio under different frequency. From the simulation results, it could befound that under the condition of thick width ratio greater than1, the current distribution was moresuitable for the pulsed current assisted laser cladding experiments. In addition, when the alternatingcurrent frequency increased, the maximum current density on the surface of the substrate will alsoincreased, which would be conducive to effect on the current molten pool in laser cladding.3. The autho r set the pulsed current auxiliary device of laser cladding, and then carried out teststudy of pulsed current assisted laser cladding. On this basis, the pulse current assisted laser claddingforming parts was also completed, which was then made to test the tensile strength of tensile samples,with a contrast to no current assisted laser cladding forming. 4. Analyzed the result of pulsed current assisted laser cladding experiment, mainly focused onthree aspects: porosity, isometric distribution and size, MC carbides distribution and size. Test resultsshowed that:(1) the decrease of gap mainly due to the effect of compression of the electromagneticforce on molten pool; The increase of the intensity and frequency of the pulse current has the obviouseffect on reducing the porosity. But duty ratio has little influence.(2) The introduction of the pulsecurrent increase nucleation super-cooling degree, which can increase the nucleation rate, thus canreduce the grain size; Increase of pulse current average and frequency and decrease of duty ratio werehelpful for minishing the size of isometric; Dc current could make the grain bulky due to the effect ofpersistent heating of the molten pool.(3) The introduction of the pulse current could decrease the sizeof MC carbides, reduced the existence of the SEC or chain MC carbides, which will be conducive tothe ascension of the high temperature fatigue performance of nickel-based super-alloy. Mechanicaltensile test results showed that mechanical tensile properties of pulsed current assisted laser claddingforming increased to a certain degree, proving its grain refinement. |
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