Controlling heat and mass transfer for droplet-based rapid prototyping.

摘要

An investigation is conducted on the feasibility of applying the principles of gas metal arc welding to the droplet-based layered fabrication of metallic parts. In this new process of rapid prototyping technology, known as 3D welding, a consumable electrode melts and forms small liquid droplets which are deposited onto a substrate. The transfer of the metal droplets plays a fundamental role in determining the quality of the final product. In this work, the physics of the droplet formation and deposition are studied and the roles of the different welding parameters in the metal transfer process are analyzed. Due to the strong coupling that exists among the welding parameters, the experimental work is designed in an attempt to isolate their effects so each can be studied individually.; A machine vision sensing system is designed based on a laser backlighting technique and high-speed imaging to monitor the metal transfer process. An image processing algorithm is developed to extract critical geometrical information from the electrode-droplet images, such as the droplet width, droplet length, shape and dimensions of the necking region, droplet volume, and droplet transfer frequency information.; Experimental work incorporating a wide range of process parameters and conditions is conducted to reveal which features of the metal transfer process will yield the optimal bead characteristics and penetration profile for purposes of a layering operation. Based on the obtained results, current waveforms are designed for studying the 3D welding process. It is shown that changes in the average welding current, droplet growth period, duration of the base current period, and/or the droplet transfer rate will influence the characteristics of the metal transfer process, and thereby alter the penetration profile.; A finite-element model is developed and used for simulating droplet-based 3D welding operations. Thermal analyses for the case of single-layer deposition are performed using a wide range of process parameter values, and the results are verified experimentally. Penetration results are also predicted for cases of double and triple layering operations, and experimental tests incorporating the same input parameters and conditions are performed for comparison. Microscopy results of the cross-sectional bead penetration profiles are also presented to illustrate how the microstructure responds to the repeated heating and cooling cycles that occur in a 3D welding operation.