Parameter Optimisation
The main parameters used to control SMD are current, travel speed, step height and wire feed speed. Additionally, arc-gap, current type (AC or DC) and pulse information can also be considered. Generally speaking, once the arc-gap is set, the other parameters are specified then the wire-feed is used to fine-tune the deposition. The exact settings of these parameters are critical to allow continuous deposition to be maintained; as the parameters vary, the deposited geometry and material properties also vary in terms of part shrinkage, wall thickness, hardness, surface roughness and some metallurgical aspects of Ti-6Al-4V. To commercialise the SMD process, it was necessary to find stable parameter windows which produce known geometries with good material properties
A number of different parts were built, measured, sectioned and analysed and the results were used to narrow down the parameter range. The models produced can predict process outputs with an error of less than 10%.
Once parameter windows had been determined for parts made wholly from SMD, a similar exercise was carried out to determine critical parameters which affect deposition onto an already-existing part. Substrate temperature and thermal expansion play an important role in obtaining good results with a minimum of necking and associated stress at the join. Experiments were carried where thermocouples were embedded in a plate and pyrometers measured temperature evolution for autogenous passes and simple builds. The models developed (section 4.5) were used to determine the optimum substrate temperature and FEA analysis can then be carried out to determine the substrate expansion. These parameters can be used when creating the robot program to adjust the path dimensions and the pre-heat time.
