Significant work is being done in the field of additive manufacturing, also known as 3D printing. This growing field allows manufacturers to create more complicated, lighter parts with less wasted materials. Yet the industry continues to have questions about how materials that have been additively manufactured will behave in their environment. Will the materials be durable? Will corrosion be a problem? How will the part withstand stress?
With this in mind, faculty and students at Worcester Polytechnic Institute (WPI) in Massachusetts – one of the nation’s earliest technological universities and a leader in the study of material science – are working on various research projects that focus on the processing, structure, property and performance relationship in the field of additive manufacturing.
Although additive manufacturing is being done with all kinds of materials – including polymers, ceramics, glass and resins – WPI is making headlines with its additive-manufacturing research in metals and alloys.
“Our work is unique,” explained Jianyu Liang, PhD and associate professor of mechanical engineering at WPI. “For the first time, different disciplines are working together so that we can understand from a materials-science perspective how the additive-manufacturing conditions affect the property of the material.”
Some of the research that the university is doing is to assist the military, where superior performance is critical. For example, if a part on a tank, helicopter or gun breaks in a war zone, additive manufacturing allows soldiers to create a reliable replacement part in the field without waiting for repairs.
Liang’s research team works closely with Richard Sisson, a professor of mechanical engineering and technical director of the Center for Heat Treating Excellence (CHTE) at WPI (see sidebar). The team works on processing the additively manufactured parts, either through hot isostatic pressing or other heat-treating techniques, so that they have the best performance without stress fractures, corrosion, porosity or other mechanical-property failures.
The Process of Micro Cold Spray
One of the research projects the team is working on focuses on an additive process called micro cold spray, which is a solid-state deposition process similar to the better-known thermal-spray process. The major difference between the two is that in thermal spray the starting powder particles undergo melting, while in cold spray the powder stays solid throughout the process.
In Figure 1, you can see the gas inlet, which is used to accelerate the powder particles. Nitrogen or helium is typically used because both of these chemical elements have low molecular weight and high expansion ratios. The gas is split in the module and sent to both the electric heater and powder feeder. The electric heater raises the temperature of the gas to a range of 25-650˚C (77-1202˚F). The heating of the gas allows for greater acceleration in the de laval nozzle (converging-diverging nozzle).
The second gas stream is used to pick up the powder in the powder feeder and direct the solid-gas flow toward the supersonic nozzle. The two streams mix and then enter the nozzle. The first section of the nozzle compresses the mixed stream (the converging section). After this compression, the nozzle starts to diverge, which accelerates the gas and powder particles. The acceleration is so great that the powder particles reach supersonic speeds once they exit the nozzle.
This supersonic stream of powder and gas flows until it impacts a substrate set in front of the nozzle. The high impact velocities of the powder particles lead to large deformation, and, due to an underlying bonding mechanism, the powder particles begin to build a bulk solid (such as a rectangular block) layer by layer. Once the process is finished, a part has been created at the specified geometry at near-net shape.
The main obstacle to micro cold spray is that the process uses ultra-fine powders, which can exhibit poor flowability. In effect, the high cohesive characteristics of the ultra-fine particles lead to the formation of agglomerates that cause clogging.
Over the past decade, many unique feeding systems have been tested in industry, with limited success. Knowing this, a research project that WPI PhD candidate Ryan Mocadlo is working on under the supervision of Liang and Sisson aims to help design a novel feeding system that is able to consistently and accurately meter powders. This research explores the idea of using ultrasonic vibrations to provide a constant feeding force to the powders.
In these WPI research experiments, the feeding system uses a combination of surface, progressive and standing waves to accomplish this goal. Once powder flow is consistent, this system will be able to deposit lines with fine resolution and widths of less than 110 µm. It will also help researchers better understand the relationship of the microstructure of properties that undergo the process.
The Department of Defense has shown interest in using this type of additive process to create robust conformal antennas and electronic packaging with strong adhesion to withstand the extreme G forces experienced at projectile launch. In the end, this process could produce micro-circuitry with stronger bonds, without the use of harmful masks, and at a faster throughput than traditional processes for a multitude of industries.
Optimization of Processing Parameters for 304L Stainless Steel DMLS
Another additive research project being conducted at WPI explores the process of direct-metal laser sintering (DMLS) on 304L stainless steel.
DMLS is an additive-manufacturing process in which a laser – moving at a set power, speed and hatching distance – passes over a layer of powder and partially melts it to create weld-like tracks in the form of a raster pattern. This raster pattern prevents thermal stresses that can incur due to heat concentration and thermal gradients.
The energy density for the additive-manufacturing build is not high enough to incur a full melt of the powder. Instead, the powder partially melts at the shell, but the cores of the grains do not melt. This allows for the material to spread the hatching distance in order to overlap, solidify and create a layer. Once this occurs, another layer of powder is rolled over, and the process is repeated until the part is finished.
A separate research project that WPI PhD candidate Andelle Kudzal is working on under the supervision of Liang and Sisson is focused on developing experiments that will help optimize processing printing parameters for surface roughness on previously unused additive-manufacturing materials. Additionally, she is seeking to understand the DMLS process so that the final product can be more consistent. This way, the process and not each part can be qualified.
To date, research tests have been done on a ProX100 DMLS machine for 304L stainless steel. Because the machine is an open source, there are more variables to control than in commercial machines. This also means there are no constraints on what materials can be used (not reliant on commercially made powders) since the processing parameters are not preset for each material.
Future projects will examine controlling the mechanical properties of the print by regulating the final part microstructure. This will help with the ability to certify the process instead of certifying every part. This involves an understanding and characterization of initial powder bed, as well as understanding the effect of processing parameters.Initially, WPI is focusing on the initial powder characteristics, such as microstructure, morphology, packing factor, thermal properties and flowability.
Other additive-manufacturing research being done at WPI includes work with titanium alloys for biomedical applications and semisolid technology. To learn more visit http://wpi.edu/+mpi or email Richard Sisson at email@example.com.
Sidebar: About the CHTE Collaborative
The Center for Heat Treating Excellence (CHTE) is an alliance between industry and university researchers that addresses the short- and long-term needs of the heat-treating industry. Membership is unique because members have a voice in selecting quality research projects that help them solve today’s business challenges.
Member Research Process
- Research projects are member-driven. Each research project has a focus group composed of members who provide an industrial perspective.
- Members submit and vote on proposed projects.
- Three to four projects are funded yearly.
- Members have royalty-free intellectual-property rights to pre-competitive research.
- Members have the option of paying to sponsor proprietary projects.
- CHTE periodically does large-scale projects funded by the federal government or foundations. These projects keep members informed about leading-edge technology.
- Members are trained on all research technology and software updates.
Other projects that CHTE is currently working on include:
- Nondestructive testing for hardness and case depth
- Alloy life improvement
- Gas-quench steel hardenability standard
- Induction tempering
- Enhancements to CHTE software: CarbTool, CarboNitrideTool and NitrideTool
This article was originally published on January 18, 2016.