Aerodynamic Focusing for Cold Spray 3D Printing
Northeastern University
What is Cold Spray?
Cold spray is a direct energy deposition method that was developed in the 1980s, originally for use as a coating technology on metal parts. In the cold spray process, high speed metal particles mixed with high-temperature compressed gasses are forced through a de Laval nozzle at supersonic speeds. The particles then proceed to impact a substrate at speeds greater than 300 m/s. Typical speed at impact varies based on type of material and type of gas. The most common materials used are aluminum, titanium, tantalum, and copper mixed with nitrogen or helium. Deposition of the particles occurs at the substrate where the particle goes through a plastic deformation due to the extremely high impact force. The entire process is generally performed below the melting point of the metal, hence the name cold spray.
Problem Statement
Development of auxiliary spray systems for focusing metal particles for high resolution 3D printing for cold spray additive manufacturing due to unsatisfactory print resolutions.
Myself and 3 other students aimed to solve this problem by designing a secondary spray system that would mount to the end of the cold spray nozzle and focus the particle stream using jets of nitrogen gas. The team designed the system to meet the specifications of the cold spray system used by Professor Ozan Ozdemir at Northeastern’s Burlington campus.
Computational Fluid Dynamics
To initially guide the design process, several pieces of information about the location and angle of the auxiliary jets were needed. To determine this information, a broad parameter sweep was conducted in the computational fluid dynamics software STAR CCM+ via an axisymmetric simulation.
The parameters investigated were:
Auxiliary mass flow rate as a ratio of cold spray mass flow
Auxiliary tube diameter
Auxiliary tube angle
Auxiliary tube location to both cold spray center line and substrate
Number of auxiliary tubes
Design Implementation
The design was split into two main systems: the front-end focusing array, and the back-end feed system. The front end consisted of 8x 1mm ID, 1/16th in OD stainless steel tubes that are arranged in a concentric, angled pattern around the spray nozzle. Through a series of adaptors, the 1/16th in tubes were increased to 1/4th in stainless steel tubes that bend and attach to flexible hoses and then the feed system. These tubes were held in place by 2 structural rings, supported by 4 adjustable steel arms. These arms moved the focusing assembly with respect to the nozzle allowing different flow impingement points along the output of the cold spray flow. The feed system then contains a bar manifold which split the single inlet gas stream into 8 symmetric flows.
Due to the 700°C max temperature of the spray applicator, all materials for the focusing assembly were AISI 1018 or 1020 steel. All assembly hardware was Stainless Steel 316. Components were designed for optimal machining processes.
Results
Prior to testing, the system was connected to the existing cold spray nozzle at the lab, and performed as follows:
No nitrogen leaks
Secure connections
Structural and vibrational stability
Nitrogen generation system couldn't maintain desired pressures past 740 psi
The first experiment was run with the following settings:
Varying distance from nozzle to substrate
Assembly in standard position (particle flow impinged 10 mm from nozzle)
740 psi
At 15mm to the substrate, there was a maximum resolution increase of ~7.5%.
Exit Strategy
This system will remain with Professor Ozdemir at the cold spray lab for further testing. All members of the lab are familiar with the installation and operation of the system. There is much room for future improvements to the system:
Redesign of front end – variable impingement angles, larger diameter tubing, simplification of tube connections
Upgrade NU Burlington Campus Nitrogen generation system for increased MFR to auxiliary focusing jets
System redesign for compatibility with aluminum spray nozzle