Conventional casting processes of large nickel aluminum bronze components (NAB) used in naval applications lead to inhomogeneous microstructures, high residual stresses and long lead times. Directed energy deposition (DED), also known as wire arc additive manufacturing (WAAM) with gas metal arc pulse (GMA-P) process is an alternative processing route for manufacturing of large components for the shipbuilding industry. Although GMA-P DED offers high deposition rates, the actual duty cycle of the process can be very low. To meet interpass temperature requirements, significant build time is spent waiting for the build to cool between passes. In-situ liquid nitrogen cryogenic cooling (ILNCC) technology has the potential to address two key challenges in DED of large deposits: low productivity and microstructure control.
In this work, the effects of LN2 cooling on microstructure and properties of NAB, and cooling effectiveness for interpass and potentially cooling rate control during WAAM were evaluated. Visible light microscopy, scanning and transmission electron microscopy techniques were used to identify microstructure constituents in the weld metal deposited with and without cryogenic cooling. Microhardness measurements were performed. Different variables of the cooling source setup were assessed, including LN2 flow rate and distance behind the torch. Thermal finite element models (FEM) with and without cooling source were developed with the aim of extracting cooling rates at different locations within the weld. FEA models were calibrated based on experimental measurements of cooling rates using embedded thermocouples and resultant weld bead profile. Experimentally obtained microstructures and properties were then described as a function of cooling rates.
Results and Discussion.
For interpass temperature control, the cooling source needs to be applied behind the moving GMA-P torch in a location that is at or below stress relief temperature to mitigate risks for microstructure impact. For cooling rate control and to influence weld metal microstructure, the cooling source needs to be positioned close to the moving torch. Rapid expansion of LN2 can cause interference with the process shielding gas and induce process instabilities. An air knife mounted directly behind the torch showed potential in preventing the weld pool contamination. The use of cryogenic cooling resulted in microstructural changes affecting the size of the alpha grains and the kappa phase precipitation as compared to reference welds without cooling. These changes resulted in a slight decrease in weld metal hardness.
Cryogenic cooling for interpass temperature control was effective at removing heat and improving the duty cycle for NAB GMA-P DED applications. For cooling rate control, physical barriers needed to be employed to enable use of the cryogenic cooling source in close proximity to the GMA-P process. Results of this study demonstrated the influence of in-situ cryogenic cooling in GMA-P DED on microstructure and properties of NAB and showed the potential for its implementation in manufacturing environment.
Keywords: Nickel aluminum bronze, Cryogenic cooling, Thermal FEA, Wire Arc Additive Manufacturing, Directed Energy Deposition