Abstract Scope |
Considerable effort has been made in the recent decades to develop manufacturing tools with high level of precision, automation, and intelligence. Among the tools successfully developed, 3D printer is a game changer for manufacturing and a vital part of Industry 4.0. A recent report from Acumen Research and Consulting forecasts the global 3D printing market to grow at an astoundingly high annual rate of 20% to reach USD41 billion by 2026. 3D printer is mainly used for additive manufacturing of parts, but we used it as a convenient tool for development of ultrahigh strength steels through incorporation of ceramic particles. To introduce particles to steels for the purpose of strengthening, two basic methods can be used, namely precipitation hardening and dispersion strengthening. Precipitation hardening heavily depends on the solubility and solid-state reaction; as a result, the types of particles which can be added in the steels are very limited. Therefore, the interest in dispersion strengthening has increased, since it can provide a much better flexibility on the combinations of steel matrix, ceramic particle type, and size and shape of the particles. 3D printing via selective laser melting offers a new method for fabricating ceramic particles reinforced steels. Uniform mixing of ceramic and steel powders is the key first step. Many researchers used high energy ball milling, but we have found that it is better to use low energy ball milling. Low energy ball milling results in less plastic deformation of the spherical steel powders and thus ensure better flowability of the powders in the 3D printing process. 316L stainless steel is widely used but its applications are limited by its drawback of low yield strength (typically in the range 170-300 MPa). We managed to mix TiC and other ceramic particles (e.g., Y2O3) with 316L powders homogeneously and used the mixed powders to produce much stronger composites by selective laser melting. Strengthening of materials can be achieved quite easily in many different ways, but the persistent engineering problem is that the higher strength is usually achieved at the expense of ductility. In the current research, adding 1 and 3 wt.% TiC particles to 316L steel is shown to lead to a significantly increased yield strength (660 MPa and 832 MPa) and ultimate tensile strength (856 MPa and 1032 MPa) and maintains the good ductility (55% and 29% elongation). The significant improvement in mechanical properties is due to dispersion hardening by the added ceramic particles and the grain refinement caused by heterogeneous nucleation on the particles during the solidification process. Another key to the success lies in the optimization of 3D printing parameters. 3D printing is essentially a process of welding and joining of individual powders and adjacent layers. Therefore, common problems associated with fusion welding may occur such as sputtering, cracking, porosity, and lack of fusion. Fortunately, the capability of precision digital control of the modern 3D printer can be made use of for quick optimization of the process parameters including laser power, layer thickness, scan speed, and hitch spacing. This helps to accelerate the development of ultrahigh strength steels with good ductility.
Keywords: 3D printing; selective laser melting; low energy ball milling; 316L stainless steel; dispersion strengthening; grain refinement |