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Laser Cladding And Laser-induced Self-propagating Synthesis Of Zr-based Amorphous Alloys

Posted on:2004-03-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:G LiFull Text:PDF
GTID:1101360095455226Subject:Material Physical Chemistry
Abstract/Summary:PDF Full Text Request
In recent years, there have constantly been breakthroughs in the research of bulk amorphous alloys. The amorphous alloys with large glass-forming ability have been found in a number of alloy systems such as Zr-based, Ln-based, Ti-based, Mg-based and Fe-based alloy systems. The discovery and development in bulk amorphous not only help to make breakthrough in the preparation of laser surface amorphous coating, that is, thick amorphous coatings with micron dimention can be produced by the newly alloying designing and laser clad processing, but also offer an opportunity to explore new preparation method of bulk amorphous alloys. Against such a background, three aspects of research work have been sequently carried out in this paperTo begin with, Zr-based amorphous alloy coatings were successfully produced by laser cladding on a Ti substrate. Microstructure of the coatings were systematically studied by using XRD, SEM, EDS and TEM. The results show that the coatings were composed of amorphous, nanocrystallines, Al2Zr3, Cu10Zr7, Ni10Zr7 phases. However, the content of the each constituent varies obviously with coating depth owing to changes of composition and crystal parameters. The result obtained by semiquantitative analysis of layer-by-layer XRD shows that the content of amorphous obviously increases in subsurface layer of the coating. In the bonding zone between coating and substrate, the phases species was completely changed due to dilution from substrate, Ti columnar crystal and a-(Ti/Zr) solid solution were formed.Microstructure and properties of the coatings are closely related to composition of doped components, thickness of preset layer, processing parameters and technical process of laser cladding. When C and (B,Si) with definitive proportion were added into clad materials, respectively, not only does the content of amorphous increase, but also do hardness and wear resistance increase; Preset layer thickness and laser scanning speed have definitive matching relation to the content of amorphous in the coatings, respectively; Increasing laser power and remelting are beneficial to increase amorphous content, hardness, and wear resistance.On the condition of dry -sliding wear, the predomination wear mechanisms of the coatings are abrasive, desquamate and adhension.Next, a new technique to fabricate amorphous alloys using Laser-induced Self-propagating Reaction Synthesis was presented as a result of the internal thermodynamic attribute of Zr-based alloy system. The LSRS for Zr-Al-Ni-Cu alloy system shows that theproduct mainly consists of amorphous, Zr3Al2 and Zr2Cu phases. The main morphology feature of amorphous in the reaction zone is cell-like shape, on which fine particles of ZrAl phase distribute. In the laser ignition zone, laser' high heat flux density and very rapid heating rate make not only amorphous purified but also its morphology diversified. Higher ignition power and shorter ignition time is beneficial to increase amorphous content in the products. Both microhardness and wear resistance increase with the increase of doped ZrC content. On the condition of SRV wear, The predomination wear mechanisms of the products translate gradually from oxidation and abrasive wears to adhesion and desquamate with the increase of doped ZrC content.Finally, crystallization behavior of Zr65Ni10Al7.5Cu17.5 bulk amorphous alloy under the action of high-energy laser beam was investigated, and numerical modeling of transient temperature field during laser crystallization was carried out. The interlayer of amorphous-dendritic crystal-amorphous was formed due to high temperature gradient in laser active region.
Keywords/Search Tags:Laser cladding, Bulk amorphous alloy, Self-propagating synthesis, Crystallization
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