Microstructural and Hardness Behavior of H13 Tool Steel Manufactured by Ultrasound-Assisted Laser-Directed Energy Deposition

Abstract

Metal additive manufacturing (AM) by Laser-Directed Energy Deposition (L-DED) usually results in the formation of textured columnar grains along the build direction, leading to anisotropic mechanical properties. This can negatively affect the intended application of the product. Anisotropy can be eliminated by modifying the material through an additional exposure to ultrasound (US-assisted) during the L-DED process. In this paper, a multi-track sample was manufactured from AISI H13 (TLS Technik, Bitterfeld-Wolfen, Germany) tool steel by a US-assisted (28 kHz) L-DED process using a specially designed cooling system. The study also included post-process annealing and quenching with the tempering heat treatment of the modified steel, resulting in the retention of the properties, as confirmed by hardness measurements. XRD analysis was used to measure the structural parameters of the unit cell, and the hardness properties were measured in two directions: longitudinally and parallel to the deposition direction. It was found that US-assisted L-DED allows us to obtain a more isotropic structure with an equal size of the coherent scattering region in two printing directions, and to reduce the residual stresses in the material. The anisotropy of the hardness was significantly reduced, with 636 and 640 HV found between the XY and XZ planes. Based on the obtained hardness data, it should be noted that some of the heat treatments studied herein can also result in a decrease in the anisotropy of the properties, similarly to the US-assisted effect.

Keywords

• additive manufacturing
• US-assisted
• laser-directed energy deposition
• anisotropy
• columnar grains
• H13 tool steel