As being able to convert digital models directly into 3D objects, 3D printing has long been recognized as a technology which can potentially change our way of manufacturing. The technology makes it possible to rapidly build up objects with complex and customized geometries. With the accelerating development of the technology in recent years, 3D printing, especially metal 3D printing, is quickly progressing toward widespread industrial application. Despite the bright future of this technology, the quality of the products from metal 3D printing has been prone to skepticism. In most metal 3D printing processes, products are directly built up from metal powders, which makes it susceptible to defects, thus causing deterioration of mechanical properties.
Recently this problem has been addressed by a joint research team from Stockholm University, Sweden; Zhejiang University, China and Birmingham University, UK.  Professor Zhijian James Shen led the Stockholm University team with Leifeng Liu, Yuan Zhong, Ji Zou, Kamran Saeid, Jon Olsen and Mirva Eriksson as major researchers. They discovered that by optimizing the process parameters during 3D printing, a widely used stainless steel can achieve exceptional combinations of strength and ductility compared to the counterparts from conventional processes. This work shows that contrary to the skeptical view, 3D printing is actually a technique to make the metals stronger while at the same time make it more ductile. This discovery is crucial to move the technology one step forward to manufacturing heavy duty parts where a high added value can be found.
Strength and ductility are natural enemies of one another. Most methods developed to strengthen metals consequently reduce ductility. The 3D printing technique is well known to produce objects with previously inaccessible shapes. The newly published work shows that it also provides the possibility to produce the next generation structural alloys with significant improvements in both strength and ductility. This is thanks largely to the ultrafast cooling rate estimated to range from a thousand °C per second to a hundred million °C per second. This was never possible in bulk metal production process until the emergence of 3D printing. Metals cooled down so quickly result in a so-called non-equilibrium state, hence some amazing microstructures like the sub-micro-sized dislocation network which was revealed as the main reason of the improved mechanical properties. This work gives researchers a brand new tool to design new alloy systems with ultra-mechanical properties. It also helps metal 3D printing to gain access into the field where high mechanical properties are required like structural parts in aerospace and automotive industry.  
The results are just published in Materials Today.
Publication detail: L. Liu et al., Dislocation network in additive manufactured steel breaks strength–ductility trade-off, Materials Today, 2017,  https://doi.org/10.1016/j.mattod.2017.11.004