Vacuum heat treatment of iron parts produced by selective laser melting: Microstructure, residual stress and tensile behavior

• Vacuum annealing treatment was used to ameliorate mechanical properties.
• Tensile tests were carried out for SLM-fabricated iron parts.
• The grain refinement has occurred after vacuum annealing treatment.
• Vacuum annealed iron with an ultimate tensile strength 401 MPa was achieved.
• The tensile residual stress was detected in the as-fabricated part.

Selective laser melted parts easily accumulate a large amount of residual stress due to their rapid heating and cooling, which is deleterious to their mechanical properties and limits their applications. In this work iron parts, as the basic industry material, were produced by the new emerging additive manufacturing technology, selective laser melting (SLM). SLM-fabricated iron parts were heat-treated under vacuum to eliminate the residual stress. Results have shown that the crystalline structure of iron (α-Fe) was not modified after the selective laser melting process and after the heat treatment. The broadening of XRD spectra appeared and the micro-stain decreased after the vacuum heat treatment. Columnar grains appeared in the building direction due to the temperature gradient in the molten pool during SLM process. After the vacuum annealing treatment, the grain refinement has occurred due to the residual stress as the driving force although a residual amount of the columnar microstructural architecture could be observed. Although the as-fabricated iron part possesses a higher tensile strength even than that of bulk iron material, the elastic modulus of the annealed specimens decreased to 188 ± 10 GPa and the ultimate tensile strength was much improved from 357 ± 22 MPa up to 401 ± 23 MPa. The yield strength increased from 256 ± 17 MPa up to 352 ± 21 MPa. By means of the micro-indentation method, the tensile residual stress was found in the as-fabricated iron sample.