Additive Manufacturing of Metals: a Holistic Approach
Additive manufacturing (AM) of metals refers to a class of AM processes where end-use parts are directly fabricated from digital data. Technologies that utilize powder metal systems hold promise to revolutionize the way complex metallic components are currently fabricated by enabling the design and production of more efficient (stronger and lighter) parts. Our approach to AM of metals is based on a holistic process, in which parts are designed, printed, post-processed, and characterized all within the Keck Center, utilizing the Center’s impressive array of capabilities and equipment. These capabilities are further described herein and illustrated in the workflow pictured below.
DIGITAL FILE PREPARATION
The Keck Center uses commercial software packages to design components for AM allowing the user to take full advantage of the design freedom provided by AM. Additionally, the array of available slicing, simulation, and topology optimization software packages provide better control over the process. Digital file preparation software has considerable influence over process structure property performance (PSPP) as inconsistencies in the scan strategy in a laser or electron beam powder bed fusion (PBF) system can lead to considerable variations in the final part. As such, the Keck Center also explores the direct customization of .cli scripts as every vector has the ability to affect the PSPP paradigm.
All powder bed AM processes are reliant on the quality of the feedstock material. The Keck Center utilizes a variety of powder characterization systems to partially describe a picture that represents overall feedstock quality. These include a Hall flowmeter to assess flowability as well as systems to assess powder size and morphology as well as chemical composition via inert gas fusion, and a scanning electron microscope (SEM) to assess powder particles at high magnifications.
Electron Beam Powder Bed Fusion (EB-PBF): The Keck Center houses three EB-PBF systems. These systems can be used to process metallic materials while maintaining elevated temperature conditions through scanning of each deposited layer with an electron beam. Layer preheating temperatures can be effectively maintained, usually in the range from 730-800°C or in excess of 1100°C. Processing in these systems is done in high vacuum or, more regularly, under controlled vacuum conditions, providing the most pristine environment to process highly reactive materials such as titanium alloys. Recently, UTEP acquired software to fully control scanning strategies, which will provide the capability to print challenging materials requiring higher energy density for processing and the ability to tailor crystallographic texture.
Laser Powder Bed Fusion (LPBF): The Keck Center also has extensive LPBF capabilities with five total systems. One of these LPBF platforms offers unique open access to all relevant LPBF parameters, many of which are not adjustable or viewable in other commercial systems. Each machine also features preheat temperatures up to 1000°C and an integrated oxygen scrubbing system for excellent atmospheric control. A dual laser LPBF system features an adjustable beam profile laser. This system can print using a standard gaussian beam or a ring beam, offering the unique ability to control the heat flow within a melt pool (only system in the USA with this feature). In addition, the dual laser system features the option to wobble the beam or scan one beam behind the other, creating further melt pool control capabilities.
Binder Jetting: A total of five binder jetting systems is present. These systems utilize a binder solution to join powder particles and require post-build sintering and curing cycles. They have been effectively employed in the processing of metal and ceramic materials that include stainless steel, Inconel, Ti6Al4V, tungsten, and ceramics.
IN SITU MONITORING
LPBF systems have integrated on-axis monitoring capabilities including an inline high-speed CMOS camera along with a laser to illuminate monitored targets. Thermal monitoring is done using two on-axis single-color pyrometers for direct monitoring of the melt-pool and the overall thermal behavior. Off- and on-axis photodiode-based sensors are utilized to measure intensity signatures from the melt-pool with. The off-axis system can image intensity from the entire powder bed including emission from the melt-pool, while the on-axis system is coupled to the laser beam path to image a small section around the melt-pool. A second system images the entire fabrication envelope in the near IR wavelength spectral range in high resolution and high-focal depth. Visible spectrum cameras are employed in these systems to image the entire field of view of the powder bed and monitor process defects, including delamination and powder layer shortage or other irregularities.
System Process Signatures: Other process signatures (including chamber temperature, oxygen levels, inert gas flow, vacuum levels, etc.) are directly captured by control systems in PBF equipment. Data can be extracted for analysis as required.
Infrared Imaging: The Keck Center houses a high-resolution infrared camera. This camera has a micron pixel detector providing accurate measurements of temperature anomalies and can be operated in high-speed mode.
Multi-Wavelength Pyrometry: UTEP houses two multi-wavelength pyrometers. The pyrometers provide measurements of temperature, tolerance, and signal strength or emissivity for every single temporal measurement made. A radically innovative approach, compared to single and two-color pyrometers, relies on the use of Planck’s ratio equation across multiple wavelength-intensity pairs to attain consensus on the temperature of the target. Also, analysis of the raw intensity-wavelength data allows calculation of the spectral emissivity from which emissive behavior (gray or non-graybody) of targets can be inferred. The Keck Center is employing this tool and to enhance thermal monitoring in PBF AM.
The laser beam in an LPBF system has tremendous impact on melt dynamics. While not much literature exists in this area, the Keck Center is going beyond by fully characterizing the laser beam’s true power and profile. The Keck Center is working closely with a laser measurement OEM to develop a custom beam profile capable of quantifying the spot size over the entire build area. Oxygen and humidity levels are monitored with additional sensors. “Black box” settings, such as laser delays, true marking speeds, and accuracy are also being evaluated.
Thermal: Thermal processes may be required post-build for LPBF or binder jetting for stress relief, infiltration, sintering, or curing. The Keck Center has a furnace for stress relief or heat treatment in an inert atmosphere with oxygen monitoring of build plates. For more extreme cooling rates and more rapid experimentation, the Keck Center has two additional high vacuum tube furnaces.
Cutting and Surface Finishing: The Keck Center has two band saws available for removing parts from a build plate or for removing support structures. Furthermore, CNC machining can be used for dimensioning or to improve surface finish on PBF-printed components.
Metallography: A suite of systems for metallography, including precision wet abrasive cutting machines, a mounting press, and grinding and polishing machines. Optical and electron microscopes provide imaging capabilities to study microstructures and grain boundaries in printed metal components. These include a scanning electron microscope, an inverted metallography microscope, and a digital microscope.
X-Ray and Computed Tomography: An X-ray and computed tomography (CT) imaging system provides temperature-controlled metrology capabilities.
Mechanical Testing: Static and dynamic mechanical testing systems to evaluate durability, fatigue crack growth, high and low cycle fatigue, fracture toughness, tension, and compression are available.
Thermal: A differential scanning calorimetry system is available to obtain information on low temperature endo- and exo-thermic events.
Dimensional Accuracy and Surface Roughness: A wide area 3D measurement system is available to study and visually inspect dimensional accuracy and surface roughness of printed components.
The active integration of multiple technologies with AM, produces unique capabilities that result in new and exiting manufacturing processes. The Keck Center's Multi3D technology combines complementary processes resulting in the realization of multi-functionality. The Foundry Multi3D System, All-In-One Multi3D System, Multi-functional BAAM System, and the Compact Multi-Tool Fabricator, combine thermoplastic material extrusion, wire and foil embedding, machining, direct-write, and robotic component placement for the fabrication of unique devices valued in industries such as aerospace, biomedical, and consumer electronics.
Large Area AM
The Cincinnati Big Area Additive Manufacturing (BAAM) System, with a print area of 140 x 65 x72 inches, allows for large-scale rapid prototyping and direct fabrication of construction and vehicle components, to name a few. The BAAM extrudes a rate of 20 lb/min with materials ranging from carbon filled ABS to polyethylene terephthalate glycol (PETG). UTEP's custom BAAM system will soon include a wire embedding tool that will allow for large-scale 3D printed parts with embedded filaments or wires for reinforcement of electrical interconnect.
The use of polymers in AM enables the production of parts with applications ranging from automobile components to biomedical implants. There exists a myriad of material options, ranging from ULTEM (a high performance thermoplastic with excellent strength-to-weight ratio) to polyethylene glycol (a biocompatible and potentially biodegradable polymer). Common polymer AM processes include material extrusion and vat photopolymerization, both technologies contained in the Keck Center's broad collection of machines.
The use of ceramics in AM is gaining popularity for their inherent mechanical, electrical, and thermal properties. Ceramics can be used in printed circuit boards, sensors, heaters, transducers, high temperature functional materials, nuclear materials, and biomedical applications such as in the construction of dental and bone implants. At the Keck Center, ceramics printing technologies such as binder jetting, vat photopolymerization, and paste extrusion have been studied as means for printing technologies such as binder jetting, vat photopolymerization, and paste extrusion have been studied as means for printig ceramic parts using materials such as AlN, BaTiO3, PZT, Al2O3, SiC, LiNbO3, and SiO2.
AM-Enabled Materials Science
By developing novel polymer matrix composites and polymer blends, we can create printable materials with tunable physical properties such as mechanical strength, hardness, flexibility, and elasticity as well as optimized electromagnetic properties such as permittivity and permeability. Materials development also allows for fabrication of components with enhanced thermal conduction or radiation shielding, as well as the creation of new biopolymer-based composites or polymers with shape memory characteristics. Similarly, for metal-based AM, nucleation agents have been selectively introduced into metal powder feedstock materials processed via powder bed fusion AM technologies to tailor microstructure. The control of the phases that develop, has also been achieved through in situ nitriding by substituting the shield gas used during laser powder bed fusion AM.
Applications Of AM
3D Printed Electronics
Over the past decade, UTEP has tuned its hybrid manufacturing capabilities for the development of 3D Printed Electronics- multi-material, heterogeneous, electronic structures exhibiting non conventional 3D component placement and conductor routing. The incorporation of copper wire/foil embedding through thermal or ultrasonic methods allows for enhanced conductivity between electronic components. These efforts are of particular importance to the aerospace industry, intelligence community,and national defense agencies.
Biomedical Printing Applications
We are capable of creating 3D anatomical models to aid surgeons and medical researchers. Individualized computer and physical models can be created from medical imaging data to simulate the anatomy of, for example, an abdominal aneurysm, a human jaw bone, or even a human brain. We also study flow characteristics in individualized cardiovascular system models, and are breaking new ground by creating bioactive tissue engineering “scaffolds” that give regenerated tissue a place to live and grow. These complex-shaped hydrogel constructs have been applied in guided angiogenesis and nerve regeneration.