Influence of Build Angulation and Post-Print Heat Treatment on Microstructure and Mechanical Properties of Direct Metal Laser Sintered Cobalt-Chromium Alloys for Removable Partial Denture Frameworks: An In Vitro Study

Abstract

Cobalt-chromium (Co-Cr) alloys fabricated through conventional casting are widely used in removable partial denture (RPD) frameworks; however, inherent porosity and microstructural heterogeneity may compromise their mechanical performance. Direct metal laser sintering (DMLS) offers improved microstructural control, though build angulation significantly influences material properties. To evaluate the influence of build angulation and post-print heat treatment on the microstructure and mechanical properties of DMLS-fabricated Co-Cr alloys compared to conventional casting. Thirty dumbbell-shaped specimens (ISO 6892-1:2016) were fabricated and divided into three groups (n=10): Group C (conventional cast), Group S1 (DMLS 45°), and Group S2 (DMLS 90°). All specimens were heat-treated at 1150°C for 1 hour. Mechanical properties assessed included ultimate tensile strength (UTS), 0.2% yield strength, elongation, and elastic modulus. Microstructural evaluation was performed using scanning electron microscopy and energy dispersive spectroscopy. Data were analysed using one-way ANOVA followed by Tukey’s post hoc test. Statistical significance was set at p<0.05, with highly significant intergroup differences observed at p<0.001. Group S1 demonstrated the highest UTS (960.6±3.5 MPa) and elastic modulus (225±1.83 GPa), while Group C showed the highest yield strength (751.5±3.03 MPa). Elongation increased from Group C (10.1%) to Group S2 (18.1%). DMLS specimens exhibited refined, homogeneous microstructures with minimal porosity, whereas cast specimens showed coarse dendritic structures with significant defects. Fractographic analysis revealed predominantly ductile fracture in DMLS groups. DMLS-fabricated Co-Cr alloys demonstrated significantly superior microstructural uniformity and mechanical performance compared to conventional casting (p<0.001). These findings indicate that build orientation significantly influences the clinical applicability of DMLS Co-Cr frameworks and should be carefully selected according to prosthesis design requirements.