Research On Microstructural Evolution Of Uniform Molten Aluminum Droplets During Controlled Deposition Fabrication

Uniform micro-droplet deposition manufacture is a novel branch of rapid manufacturing technology based the dispersion-accumulation principle. In this new technique, uniform micro-droplets as the basic building blocks are deposited onto the substrate to fabricate the desired parts layer by layer directly according to STL model. Due to its advantages such as short cycle, high flexibility, broad applied materials and no need for special tools, this technique is considered as an effective and low-cost method for rapidly fabricating miniature metallic parts. However, the mentioned deposition manufacture process is significantly influenced by various factors and coupling fields, which involves the impact behavior of single droplet and the interaction between successive deposition droplets. Therefore, the microstructural evolution of as-deposited metal parts should be very different from in the traditional solidification processing technology, and there is still a lack of systematic study about the key issue till now.In this paper, a series of researches were conducted on metal micro-droplet deposition experimental platform designed by our group. High-strength 7075 aluminum alloy was chosen as the experimental material. Some important problems, including the deposition manufacture process with uniform micro-droplets, microstructural evolution of as-deposited metal alloy, formation mechanism of droplet interfacial bonding, and various common internal defects, were studied by theoretical, numerical and experimental methods. The above works is helpful to promote the actual application of this novel technique in the rapid manufacturing field of miniature metal parts.The main contents are listed as follow:(1) The pneumatic DOD ejection process of aluminum droplets was simulated with the improved level set method. The variations of thermodynamic and kinetic parameters of single droplet during the inflight stage were obtained through numerical calculation and experimental observation. Further, the size, velocity and temperature of the impacting droplet were obtained based on the appropriate deposition distance. Based on above, the transient behaviors during the non-isothermal deposition including the spread, retraction, oscillation and ensuing solidification were studied by high-speed photograph technology and theoretical analysis. And the solidified surface characteristics(ripples, etc.) and internal microstructure(coarse grains, fine directional dendrite and coarse non-directional dendrite, etc.) of solidified droplet were observed and studied in order to determinate their formation mechanism during the controlled deposition process.(2) The two typical successive deposition models were analyzed with high-speed photograph technology. And the external appearance and internal microstructures of the corresponding one-dimensional structures(column and line) were studied respectively. The researches focused on the thermal cycling, thermal accumulation and competitive growth during the mentioned deposition process, which could lay a good foundation for understanding the microstructural evolution of as-deposited 3D parts.(3) The manufacture process of 3D aluminum blank with uniform micro-droplets was analyzed at first. And then the solidification microstructure, and the composition and distribution of common phases in the as-deposited blank were compared with those in as-casted ingot in order to determinate the strengthening mechanism of this novel technique.(4) The main types of interface bonding between the contacted metal micro-droplets were first studied. Further, the numerical model based local remelting theory was established for predicting the interfacial bonding condition. The effect of these critical parameters including the droplet temperature, substrate temperature, and interfacial thermal resistance was also determinated. The above numerical calculation results were conducted to analyze the differences of microstructure, mechanical properties and fracture mechanism between the as-deposited aluminum blanks under different processing parameters. These researches are very helpful for the parameter selection, microstructure control and property improvement.(5) The morphology, formation mechanism and influencing factors of common defects(pore and crack) in aluminum parts fabricated by this technique were also studied. And then the methods for eliminating those defects were proposed with the aim of promoting the actual application of as-deposited miniature metal parts.