Iron-based Alloy Laser Cladding Layer Quality And Performance Improvement

Cracks are easy to generate in laser-clad layers for laser cladding is a process of sharply heating and solidifying, which is one of the main problems hindering the industrial application of laser cladding. Fe-based alloy consisted of Fe-Ni-Cr-B-Si is a kind of cheapest material among the widedly-used self-melting alloys, and is of good performance under low stress abrasive condition and weak corrosive environment. But it is more sensitive to internal stress that is produced in the processing of laser cladding. The thesis systemmatically analyzes the special property and the mechanism of crack generating. According to the mechanism, we focused on the effect of processing parameters and the addition of several oxides with the function of refine the structure to cladded alloy.The laser cladding of ferrous-based alloy doped without and with various concentration of TiO2. Nb2O5, MgO on steel 45 by powder feeding were carried out under different processing parameters. A kind of commercial ferrous-based power named G312 was employed in the present work. The structural observation on the cross-section of laser coatings by an optical microscope and a scanning electron microscope (SEM) with energy-dispersive spectrobolometer (EDS) analysis was performed, and the phase analysis by X-ray diffraction (XRD) was accomplished. The cracking observation on the surface of the coatings, the hardness measurement in the cross-section of the clad layers and the tribo-testing tests were also completed. The main results and conclusions are shown as the following:1. There are two kinds of sources of cracks in the G312 coatings cladding under the processing parameters in the experiment. One kind of source is the cohesion between the basis and the coating. The cracks are formed due to the lower energy power and the bad cohesion. The solution is to increase the energy power or slow the scanning speed of laser beam. Another kind of source is the top part of cladding layers of ill-distribution phases. Cracks extend from the top to the inner part along the boundary of neighboring eutectics or across eutectics.2.The coating of G312 can be divided into four different tissues: the Y(Fe-Ni) plain crystal interface, the celluar eutectic among Y (Fe-Ni) base in the lower part, the branch-like dendritic eutectic with some blocks in the upper part, and big blocks and fine dendritic crystal among (Fe-Ni) base. EDS analysis shows that there is more amount of Cr in the blocks of the dendritic eutectic ,and except dissolved in Y (Fe-Ni) base, Si is more distributed in the big blocks among the top part.ABSTRACT3. According to properties of solidifying during laser cladding ,the conclusion can be drawn that cracks are produced due to the uniform tissue formed under sharply cooling condition and the distribution of phases (including Si and B)with lower melting point than Y (Fe-Ni) phase. The former factor causes large inner stress in coatings and the latter is the weakest part to burden the stress. Therefore, to reduce the cracking sensitivity, the tissue should be more uniform and the tenacity in the whole should be higher.4.The experimental results indicate that both concentration of TiO2 and processing parameters seriously affected the quality and properties of the coatings and a perfect coating could be produced with 3%TiO2 under 2.0-2.2 kw laser power of 3mm/s scanning speed.5. The coating is mainly made up of pseudo eutectics. The cracking sensitivity is improved at the expense of a little decrease in hardness, but the wear resistance does not reduce. The improvement is attributed to the reaction between TiO2 and Si, B, C that can be analyzed from two factors. On the one hand, the amount of phases including Si and B with lower melting points is reduced. On the other hand, the formation of TiB2 and TiC in the eutectics is benefitial to the finement of grain sizes and uniform distribution of microstructure.6. The experimental results also indicate that concentration of Nb2O5 could make the structure fine and well distributed to reduc...