The evaluation of mechanical properties, elemental composition and microstructure of different soldering techniques conducted on selective laser sintering and conventional cast metal substructures

Selective laser sintering (SLS) technique enables the production of metal sub-structure for the metal-ceramic restorations and creates an alternative to conventional techniques with many advantages. Errors or misfit can occur during casting procedures in the laboratory which may lead to a mismatch between prepared teeth and restorations. In this case, metal substructures must be sectioned and soldered. Either laser soldering or conventional soldering is preferred for joining metal substructures. Conventional solder can damage the prosthetic restoration due to heat. Recently, laser soldering became popular because of simplified technique and relatively short processesing time in order to avoid damaging of surrounding structures. The aim of this study was to compare mechanical and metallurgical properties of selective laser sintered (SLS) soldering vs. conventional soldering conducted on both SLS and conventionally cast 3 unit metal frameworks. The null hypothesis was: there will be no mechanical and metallurgical differences between laser soldering and conventional soldering methods. Two groups of 3 unit CoCr metal frameworks including maxiller 1st premolar, 2nd premolar and 1st molar were prepared using SLS (n=45) and conventional casting (n=45) methods. Specimens in each manufacturing method were further divided in 3 subgroups (n=15); SLS soldered, conventionally soldered and non-soldered control-groups. Specimens in soldered groups were separated between 1st and 2nd premolars. The size of the separation-gap was standardized as 2mm×3mm×1mm. Three-point-bending tests were carried out on stainless-steel molds using universal testing machine at a crosshead speed of 0,5mm/min. 2-way-ANOVA followed by Tukey-HSD test (α=0,05) was used for statistical analysis. Fracture surface of metal frameworks were evaluated using optical and scanning electron microscopes (SEM). Elemental composition of fracture surfaces were evaluated using energy dispersive x-ray spectroscopy (EDS). For SLS metal substructures, the mean fracture strength of the conventionally-soldered group (1334,3 ± 547,1 MPa) was higher than that of laser-soldered group (881,7 ± 572,8 MPa), but there was no statistically significant difference (p>0,05) between the 2 groups. For conventionally cast substructures, the mean fracture strength of the conventionally-soldered group (2899,6 ± 1801,2 MPa) was higher than that of laser-soldered group (2039,3 ± 767,8MPa) but there was no statistically significant difference (p>0,05) between the two groups. The mean fracture strength of both SLS (19330,6 ± 2405,7 MPa) and conventionally-cast (4268,5 ± 1506,1 MPa) control groups were statistically significantly higher than the soldered groups (p<0,05). A statistically significant difference was found between the control groups as well (p<0,05). Optical microscope and varying magnifications of SEM images were in accordance with the mechanical properties. Soldered surfaces were observed only on the peripheral of the fracture surface where no integration was observed on the center yielding decreased fracture strength in all soldered groups. Dendritic microstructure was observed in conventionally-soldered groups and laser-soldered groups showed a combined microstructure including both grains and dendritic microstructure in the inner layers. No matter of what soldering thechnique is used, soldering methods decrease the fracture strength of metal frameworks. Therefore, soldering of base metal frameworks should be avoided.