Powder DED (Directed Energy Deposition) metal 3D printing is a system to efficiently supply metal powder from a nozzle to a molten part, with printing speed about 10 times faster than PBF (Powder Bed Fusion), and has features that enlargement of the equipment is easy and is suitable for printing large parts. On the other hand, the prevention of material degradation by metal oxidation during melting and solidification processes and the optimum control of printing conditions (Laser output, feed rate, etc.) according to the degree of heating and cooling which changes every moment are issues.
In order to solve these issues, a powder DED metal 3D printing machine equipped with a local shield nozzle which blocks the atmospheric inflow to the melt part by the optimum injection of inert gas, and a monitoring feedback function which can control the printing condition in real time by observing the melt pool, has been developed for the first time in the world.
Figure 1: External appearance of powder DED metal 3D printer “LAMDA”
2. Content of the technology
In the powder DED method, metal powder is irradiated and supplied from a nozzle, and is melted and solidified together with the base material by a laser (Fig. 2).
Figure 2 Principle of powder DED
2.1 Local shield nozzle
When printing metals that are susceptible to oxidation, such as titanium, the oxidation prevention is required, since the oxygen concentration in the material gives large effect on the mechanical characteristics. Therefore, in many cases, the metal 3D printing equipment has a chamber structure and the atmosphere inside is replaced with an inert gas, or it is printed in the low oxygen environment by vacuum drawing. The problem here is that productivity is low because the size of the object is limited to the size of the chamber and the replacement and vacuuming takes time. Therefore, a local shield nozzle which can shield the printing part area was developed in order to enable large printing and to improve the productivity by shortening the tact time.
In this local shield nozzle, inert gas is jetted from the outer periphery of the nozzle to form a shield against the melt pool, so that the inflow of the outside air is prevented and the chamber structure can be eliminated (FIG. 3). In order to enable the free form printing, an annular structure with no restriction in the moving direction is adopted, and parameters such as the gas blowing speed and the shape of the gas flow path are optimized to prevent the intrusion of the atmosphere and to keep the melting part and its periphery below the target oxygen concentration. As a result of printing Ti-6Al-4V test piece which is an active metal under the atmospheric environment using this function, it was confirmed that oxygen content 0.2% or less regulated by the aerospace material standard AMS 4928 was achieved.
Figure 3 Local shield nozzle
2.2 Monitoring Feedback
By installing a near infrared camera on the laser beam axis, the molten metal part can be observed from right above regardless of the printing direction. In addition, it became possible to measure the condition of the molten zone (melt pool) in real-time without receiving the effect of the disturbance by developing the technology which can accurately distinguish and measure melt pool from the observation image. By controlling the printing conditions at high speed according to the measured state change, the printing accuracy can be dramatically improved (Fig. 4).
Figure 4 Monitoring system configuration and feedback effect
The productivity and quality of large parts printing by powder DED were drastically improved by this technology. We will promote technological development in order to contribute to improving the competitiveness of the domestic manufacturing industry through the improvement of metal 3D printing technology.
Haruhiko Niitani *1
Koh Ishii *2
Hirohisa Kuramoto *2
Tomohiro Wakana *2
Hitoshi Yoshimura *2
*1 Member, Mitsubishi Heavy Industries Machine Tool Co., Ltd. (130 Rokujizo, Ritto City, Shiga Prefecture, 520 -3080)
*2 Mitsubishi Heavy Industries Machine Tool Co., Ltd. Engineering Headquarters (130 Rokujizo, Ritto City, Shiga Prefecture, 520 -3080)