The Influence of Polymer Binder on Melting Process and Mechanical Properties in a Novel Laser Deposition Method Using Metal Paste

Additive manufacturing has been widely applied in many industrial fields including medical, automobile, and aerospace due to the capability on processing products with complex topology design. However, in metal material processing field, the two conventional additive manufacturing methods, powder bed fusion using a laser beam (PBF-LB) and directed energy deposition (DED) still have their own weakness. The lower metal powder utilization rate in PBF-LB and the requirement of support structure for overhanging parts in DED not only increase the production cost, but also cause the waste of raw materials.<br/>In order to increase the metal powder utilization rate by reducing the material waste and the economic cost in powder recycle process, a new additive manufacturing method using a laser beam as a heat source has been developed in this research. In this method, metallic paste coated on a base plate is melted by a laser beam, which not only keeps the layer-by-layer processing feature of additive manufacturing, but also reduces the waste of powder by only supplying powder on processing area. Due to the stratification phenomena of the metal powder-organic solvent mixture, the polymer is added in the paste as binder to adjust viscosity. Since polymer has much lower melting point than metal and decomposes at high temperature, it is necessary to reveal the influence of polymer on microstructure and properties of built parts.<br/>In this study, the influence of binder on melting process was investigated via single bead experiment with in-situ observation by high-speed camera. With the increase of binder content under same process condition, the size of spatters became smaller and the amount of spatters obviously increase. Lower brightness of spatter was observed in the scanning process of the sample with higher binder content.<br/>The observation of printed samples and the evaluation of the surface quality were achieved via optical microscope. Comparing with no-binder samples, with-binder samples have smoother edges of melting track and lower roughness surface. Moreover, the same conclusion with high-speed imaging can be drawn from the analyzation on the spatter size distribution: the spatter of high binder content sample has a more concentrated distribution on lower diameter. Some of the spatter generated from the sample with high binder content is hard to be distinguished from raw powder. The melting depth and the hump height were evaluated through the observation of cross section via scanning electron microscope. Lower melting depth and hump height were observed with the increase of binder content. The decomposition of binder is considered as the main reason for these changes.<br/>In the observation on the microstructure of with-binder and pure metal samples, the with-binder sample has finer grain than pure metal sample. The microhardness was evaluated by nanoindentation method to investigate the influence of polymer binder on mechanical properties. The results showed that with-binder sample has higher hardness but lower elastic modulus than pure metal samples. The EPMA analysis revealed that the with-binder samples have more carbon content than pure metal samples, which is considered as the main reason of the difference on their microhardness.