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The Relationship Between Melt Pool Monitoring Metrics and Archimedes Density

Executive Summary

Metal additive manufacturing using laser powder bed fusion today leaves skilled practitioners with little to no insight into the process. Nor does it provide in-process feedback about part quality. Until now. Sigma Labs has developed a method to monitor and measure thermal data from the melt pool as the laser is firing, creating a thermal history that can be used to gain valuable insight into process consistency and repeatability. Ultimately, Sigma’s melt pool thermal metric data can be used to establish objective evidence of compliance to part specification requirements.

Specifically, this study reports on the relationship between melt pool monitoring metrics, global energy density and Archimedes’ density. Sigma tracked the energy output from the melt pool using a titanium alloy in different experiments. This metric data was then used to generate real-time Thermal Energy Density™ (TED™) charts.

Global energy density (GED) and Archimedes’ density were also measured during the experiments. GED is another established and accepted term to measure the output energy of a laser into the melt pool. Archimedes’ principle – that an object totally or partially immersed in a fluid (liquid or gas) is buoyed (lifted) up by a force equal to the weight of the fluid that is displaced – has numerous applications. One of these applications is the determination of density. As such, Archimedes’ density is a trusted measure of part quality in the metal additive manufacturing industry.

The resulting Sigma TED™ data gleaned from the experiments correlated strongly to the global energy density of the process. In addition, a strong correlation was found between TED™ and Archimedes’ density of the resulting part. This correlation between energy input (the laser), the radiated thermal response (TED™) and the part density is important because it demonstrates TED™’s ability to be used as a measure of post-inspection part properties. This correlation between Sigma’s melt pool thermal metric data can thus be used to establish objective evidence of compliance to part specification requirements.

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