In situ measurement and closed-loop control for powder supply processes

The powder mass flow rate is one of the main parameters regarding the geometrical precision of built components in the additive manufacturing process of laser metal deposition. However, its accuracy, constancy, and repeatability over the course of the running process is not given. Reasons among others are the performance of the powder conveyors, the complex nature of the powder behavior, and the resulting issues with existing closed-loop control approaches. Additionally, a direct in situ measurement of the powder mass flow rate is only possible with intrusive methods. This publication introduces a novel approach to measure the current powder mass flow rate at a frequency of 125 Hz. The volumetric powder flow evaluation given by a simple optical sensor concept was transferred to a mass flow rate through mathematical dependencies. They were found experimentally for a nickel-based powder (Inconel 625) and are valid for a wide range of mass flow rates. With this, the dynamic behavior of a vibration powder feeder was investigated and a memory effect dependent on previous powder feeder speeds was discovered. Next, a closed-loop control with the received sensor signal was implemented. The concept as a whole gives a repeatable and accurate powder mass flow rate while being universally retrofittable and applicable. In a final step, the improved dynamic and steady performance of the powder mass flow rate with closed-loop control was validated. It showed a reduction of mean relative errors for step responses of up to 81% compared to the uncontrolled cases.

Microstructural and Mechanical Study of Inconel 625 – Tungsten Carbide Composite Coatings Obtained by Powder Laser Cladding

This study focuses on the investigation of fine (~0.54 μm) tungsten carbide particles effect on structural and mechanical properties of laser cladded Inconel 625-WC composite. Three powder mixtures with different Inconel 625 – WC weight ratio (10, 20 and 30 weight % of WC) were prepared. Coatings were made using following process parameters: laser beam diameter ø ≈ 500 μm, powder feeder rotation speed – 7 m/min, scanning velocity – 10 m/min, laser power – 220 W changed to 320 W, distance between tracks – 1 mm changed to 0.8 mm. Microstructure and hardness were investigated. Coatings produced by laser cladding were crack and pore free, chemically and structurally homogenous. High cooling rate during cladding process resulted in fine microstructure of material. Hardness improved with addition of WC from 396.3 ±10.5 HV for pure Inconel 625, to 469.9 ±24.9 HV for 30 weight % of WC. Tungsten carbide dissolved in Inconel 625 which allowed formation of intergranular eutectic that contains TCP phases.

A New High Precise Powder Feeder for Fabricating Functionally Gradient Metal Component

In this paper, a new high precise powder feeder designed for fabricating functionally gradient metal component was presented. The mechanism, control principle and computer control program of the powder feeder were introduced and the powder feeding precise test experiments were carried out with Ni-based alloy powders, Fe-based alloy powders and ceramic powders in the particle average diameter of 15-350 Microns. Experimental results showed that the powders mixtures obtained from the high precise powder feeder are continuous and stable. The precision of delivering powders mixture can be weighed within 2% error scope comparing to the desired mixture quality and the flexible proportion of different powders mixture can be easy changed in the computer control program by synchronously controlling the powder feeder motor’s rotation speed.

Effect of Cold Spray Processes on Friction Stir Welded Microstructures of AZ31 Magnesium Alloys

In order to improve corrosion resistance of friction stir welds of Mg alloy, Al coatings were fabricated by cold spraying on Mg alloy substrate. The microstructure and microhardness of friction stir weld and coatings are investigated with optical microscope and hardness machine. The results indicate that Al coatings present a smooth surface, low porosity and well bonding with substrate. The grains size of friction stir welds increase as substrate speed decrease or powder feeder wheel speed increase, while the microhardness decrease since residual stress in coatings and substrate reduces. The experimental results demonstrate that Al coatings fabricated by cold spraying can provide potential protection for as cast friction stir welds of AZ31 Mg alloy.

Surface Properties of Different Travel Speeds and Powder-Feeder Rates for Plasma-Sprayed Coatings on Aluminum Substrates

The surface properties of plasma-sprayed partial Y2O3 stabilized ZrO2 coatings with different travel speed and powder-feeder rate of sprayed gun on aluminum substrates have been reported. Several coatings analytical parameters were studied and these factors affecting coating properties were carried out in the plasma-sprayed experiments. Experimental results have shown that the structure of the sprayed coatings that makes relatively diverse in texture for most of the coatings was examined by a SEM. The hardened strength with a low level of a slant of powder-feeder rate became significantly weaker as the travel speed was decreased. Interestingly, the structure of the specimens for the travel speed of 30mm/s is more homogenous than that of the others, indicating a coating with good homogenous structures, while the interfaced structure of the specimens for the powder-feeder rate of 25 mm/s is more homogenous than that of the others, indicating a coating with good adhesive structures. The tendency of the effect of travel speed of sprayed gun on the hardened surface of coatings at powder feeder rate of sprayed gun varied to 30g/min from 20g/min seems to be apparent.