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Gradient Temperature Heat Treatment of LPBF 3D-Printed Inconel 718

In order to tailor and improve the performance of microstructures, it helps with many 3D-printed alloys if the post-heat treatment process is carefully designed and executed for this purpose. Researchers Yunhao Zhao, Noah Sargent, Kun Li, and Wei Xiong with the University of Pittsburgh’s Physical Metallurgy and Materials Design Laboratory published a paper, “A new high-throughput method using additive manufacturing for materials design and processing optimization,” about their work on this subject, which was supported by a NASA contract.

They explained that post-heat treatment optimization and composite design are the central parts of materials development, and that “high-throughput (HT) modeling and experimentation are critical to design efficiency.” These aspects are even more important when it comes 3D printing, because the more processing parameters are used, the more the “microstructure-property relationships of the as-fabricated materials” will be effected.

“In this work, we couple the [laser powder bed fusion (LPBF) technique with the gradient temperature heat treatment (GTHT) process as an effective HT tool to accelerate the post-heat treatment design for AM components,” they explained.

They used the Ni-based Inconel 718 superalloy, which has excellent high-temperature mechanical properties, in order to evaluate their proof of concept, as the material is often fabricated with LPBF technology.

Figure 1. (a) Inconel 718 build printed by LPBF; (b) setup of temperature record and illustration of sample cutting for microstructure characterization; (c) setup of the furnace for the high-throughput experiment; (d) experimental temperature distribution inside the bar-sample.

The researchers created a high-throughput approach by using LPBF technology to print a cuboid long-bar sample out of Inconel 718 on an EOS M290. They designed the build with 23 evenly distributed holes, which not only increase the sample’s surface area and improve convection heat transfer, but also make it more flexible “when choosing monitoring locations.” The improved heat transfer also helped lower the variation in the sample’s temperature relative to the temperature of the air.

“As a result, the air temperature calibration became more representative of the real sample temperature, which allowed the preemptive selection of the monitoring locations in the sample according to the actual needs. Using this methodology, the current work significantly reduced the total time needed for heat treatment, and the flexibility of the setup of the high-throughput experiment was increased by adopting additive manufacturing methods for sample fabrication,” they explained.

Once the long bar sample’s microsegration and AM-related grain texture had been removed, it was submerged in ice water, and then conductive high-temperature cement was used to fix eight K-type thermocouples into equidistant holes. Finally, it was time for the 15-hour aging process of the heat treatment.

“The thermocouples were connected to a computer via a data acquisition system to record the aging temperatures at each location throughout the aging process,” the researchers wrote. “The aging heat treatment was then carried out in a tube furnace with one end open to introduce gradient temperatures at different locations in the sample, as illustrated in Fig. 1(c). The furnace temperature settings and the position of the sample inside of the furnace tube had been deliberately calibrated to acquire a temperature gradient of 600~800°C, within which the δ, γ′, and γ″ phases may precipitate during the aging processes [19]. The temperature gradient during the aging process is stable without fluctuation, and the distribution of temperatures achieved at each monitored location is illustrated in Fig. 1(d). From Fig. 1(d), the experimentally obtained temperature gradient was within 605~825°C, which agreed well with our expectation.”

Figure 2. Temperature diagram of heat treatment with corresponding sample notations.

The adjacent alloy to each thermocouple was individually sectioned to characterize the microstructure, and view the effect of the various aging temperatures. After the samples were polished, they were analyzed with SEM (scanning electron microscope), so the team could identify the phases, and EBSD (electron backscatter diffraction), for grain morphology observation.

Figure 3. (a) Results of microhardness and average grain size measurements. IPFs of the aged samples with (b) HT605; (c) HT664; (d) HT716; (e) HT751; (f) HT779; (g) HT798; (h) HT816; (i) HT825.

“Within the temperature range of 716~816°C, the hardness of the aged samples are higher than that in the wrought Inconel 718 (340 HV, AMS5662) [14], indicating the AM alloys could achieve higher strengthening effects when applied suitable heat treatment,” they wrote. “The highest hardness is 477.5 HV0.1 and occurs after aging at a temperature of 716°C. It is found that the temperatures above and below 716°C result in the reduction of hardness. The lowest hardness of 248.4 HV0.1 is obtained at 605°C, which is lower than that in the as-built alloy (338 HV0.1).”

The EBSD found that coarse grains formed in all of the aged samples, and while their diameters were “plotted as a function of the corresponding aging temperatures in Fig. 3(a),” their size is independent of the temperature. This likely means that the aging temperatures did not significantly effect either the grain size or morphology, and that “the relatively large grain size achieved after heat treatment in this study has little contribution to the microhardness variation.”

To better understand structure-property relationships, the researchers chose three samples to undergo more microstructure investigation:

HT605 with the lowest microhardness of 248.4 HV0.1,
HT716 with the highest microhardness of 477.5 HV0.1, and
HT825 with the lowest microhardness of 332.2 HV0.1 in the high-temperature gradient

Other than a few NbC carbides, they did not see any other precipitates in the HT605 sample, but noted that 716°C-aging caused a little “of the δ phase to precipitate along grain boundaries” in the HT716 sample.

“However, a large number of plate-shaped γ″ particles are observed in the TEM micrographs,” the team wrote. “These γ″ particles are very fine with a mean particle length of 13.8±4.2 nm through image analysis. The typical γ′ phase with spherical shape is not found to precipitate in sample HT716. This indicates that the precipitation of γ″ preceded the formation of γ′ in the current study. Therefore, the strengthening effect is dominated by γ″ with fine particle size.”

Figure 4. Microstructures of HT605 characterized by (a) SEM-BSE; (b) bright-field TEM; (c) selected-area-electron-diffraction (SAED). Microstructures of HT716 characterized by (d) SEM-BSE; (e) bright-field TEM; (f) SAED. Microstructures of HT825 characterized by (g) SEM-BSE; (h) bright-field TEM; (i) SAED. The different γ″ variants in (f) and (i) are differently colored, and the corresponding zone axes are indicated.

Just like with the second sample, the researchers also did not observe the γ′ phase in HT825.

The team deduced that the phase transformation behaviors caused the varying microhardnesses in the aged samples, concluding that aging the 3D-printed Inconel 718 samples at 605°C for 15 hours is not ideal for precipitation-hardening.

“We developed a high-throughput approach by fabricating a long-bar sample heat-treated under a monitored gradient temperature zone for phase transformation study to accelerate the post-heat treatment design of AM alloys. This approach has been proven efficient to determine the aging temperature with peak hardness. We observed that the precipitation strengthening is predominant for the studied superalloy by laser powder bed fusion, and the grain size variation is insensitive on temperature between 605 and 825ºC.”

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Renishaw launches InfiniAM Sonic using sound to monitor additive manufacturing builds

During last month’s Formnext, British engineering firm and metal 3D printer manufacturer Renishaw launched new software and hardware package that aims to improve the quality of Laser Powder Bed Fusion (LPBF) builds through the measurement of acoustics. Named InfiniAM Sonic, the acoustic process monitoring software allows engineers to detect acoustic events within the AM build […]

Fraunhofer ILT: Making Tungsten Carbide-Cobalt Cutting Tools with LPBF 3D Printing

Obviously, the Fraunhofer Institute for Laser Technology ILT does a lot of work with lasers, and, in the same vein, with metal 3D printing processes that use lasers. Now, it’s teaming up with scientists from the Institute for Materials Applications in Mechanical Engineering IWM and the Laboratory for Machine Tools and Production Engineering WZL, both at RWTH Aachen University, to investigate laser processes for the 3D printing of cutting tools made of tungsten carbide-cobalt (WC-Co).

The new AiF project – “Additive Manufacturing of Machining Tools out of WC-Co – AM of WC-Co” – began on October 1st 2019 and will last for 30 months; funding is provided by the Otto von Guericke e.V. working group of industrial research associations.

Cutting tools made of WC-Co are very heat- and wear-resistant, which is what one generally wants in this type of application, but it’s not easy to use conventional methods of manufacturing to create them. Complex sintering processes are currently used, but it’s not ideal, as only a restricted amount of geometrical freedom is possible, and it’s expensive and difficult to introduce complex cooling structures into the tools as well.

The process development aims to generate a homogeneous, almost dense structure of the WC-Co-composite, as shown here in this SEM measurement. [Image: Institute for Materials Applications in Mechanical Engineering IWM, RWTH Aachen University]

One of the project goals is to create cutting tools with integrated complex cooling geometries in order to ensure longer tool life. That’s why the Aachen researchers are looking into Laser Powder Bed Fusion (LPBF) 3D printing for WC-Co cutting tool fabrication, which offers near-net-shape production for generation of cooling structures within these tools, and far more design freedom. This technology requires users to carefully choose their process and material parameters in order to create components with strength that’s comparable to what could be achieved with conventional manufacturing methods.

For the past few years, Fraunhofer ILT scientists have been researching a major problem in the LPBF process – temperature distribution in the part. Conventional systems slow down the cooling process with a heated base plate, but with LPBF, the metal powder is melted where the laser touches it and cools down quickly, which can cause cracks and tension.

Fraunhofer ILT has been working with adphos Innovative Technologies GmbH on this issue, and together the two created a system which uses a near-infrared (NIR) emitter to heat the component from above to over 800°C. This system is what Fraunhofer ILT and its fellow Aachen researchers are using to process tungsten carbide-cobalt material for cutting tools in the “AM of WC-Co” project.

Under the scope of the project, the researchers are investigating the process route all the way from powder formation and 3D printing to post-processing and testing the components. Together, they will qualify the materials and processes that will replace complex sintering processes in fabricating these cutting tools.

Preheating the machining plane with the NIR module significantly reduces stresses in the laser-manufactured component. [Image: Fraunhofer ILT]

3D printed WC-Co cutting tools will have a hardness comparable to those made with conventional manufacturing methods, but because of the cooling structures that the LPBF process can be used to create, they will have a longer service life. Additionally, the NIR emitter system developed by Fraunhofer ILT and adphos can lay the groundwork for processing refractory alloy systems in the future.

At formnext 2019, in Frankfurt from November 19-22, you can stop by the Fraunhofer Additive Manufacturing Alliance booth D51 in Hall 11 to learn more about the collaborative “AM of Wc-CO” project.

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[Source: Fraunhofer ILT]

The post Fraunhofer ILT: Making Tungsten Carbide-Cobalt Cutting Tools with LPBF 3D Printing appeared first on 3DPrint.com | The Voice of 3D Printing / Additive Manufacturing.

University of Pittsburgh awarded over $1 million to develop quality assurance for 3D printed turbine components

Researchers from the Swanson School of Engineering have received over $1 million in combined funding from the U.S. Department of Energy (DoE) and the University of Pittsburgh. The funding is intended to support the development of an effective quality assurance method for the additive manufacturing of new-generation gas turbine components. Lasting three years, Xiayun (Sharon) Zhao, […]

US Researchers Study Ways to Eliminate Pore Formation in Laser Powder Bed Fusion 3D Printing

In ‘Dynamics of pore formation during laser powder bed fusion additive manufacturing,’ US researchers continue to improve on 3D printing, exploring how to prevent pores from forming during laser powder bed fusion. As LPBF continues to become more popular in metal additive manufacturing processes, users seek better quality and less headaches in production, inspiring the research team to improve geometric quality of melt tracks and production overall.

Description of a laser turn point condition and experimental configuration. a–c A laser turn point is defined as the condition during laser powder bed fusion (LPBF) where the laser reaches the end of a track, decelerates, shifts a prescribed hatch spacing, changes the scan direction by 180°, and then accelerates along a new track parallel and adjacent to the previous track. The black dashed line indicates laser trajectory. d–f Time difference (t−t0), transmission X-ray images of a turn point region in Ti–6Al–4V performed at a laser power of 200 W, and scan speed of 1000 mm s−1. d The laser is scanning from the left to right with spatter and powder motion above the substrate surface and a depression in the surface of the melt pool due to vapor recoil below. The titanium–argon interface is indicated by the white dashed line. e The laser enters the turn point region and shifts by the prescribed hatch spacing. f The laser is moving right to left after the turn point forming a new adjacent track and leaving behind keyhole pores. g Simplified schematic of the experiment configuration. A white-beam X-ray source is provided by experimental station 2–2 at the Stanford Synchrotron Radiation Lightsource (SSRL). The X-ray field of view is coincident with the 1070 nm processing laser at the Ti–6Al–4V substrate surface. Images are captured using a scintillator-based high-speed optical system

As is the case with many different types of 3D printing, LPBF is a powerful technology still left highly unexplored within industry due to trepidation about quality in parts, and especially integrity of mechanical properties. Unpredictability in both thermal history and material solidification have given way to doubts and worry over potential defects and resulting instability.

Keyhole pores have been a common problem, caused by superfluous energy in the melt pool. The pores degrade mechanical properties and can have a negative impact on parts created during the LPBF process. Temperature issues were a major focus in the study.

“To improve the confidence in components built by LPBF, a greater understanding of laser–metal interaction in this extreme thermal regime and its correlation with defect generation during the LPBF process is required,” state the researchers.

The team took X-rays to examine the printing process further, attempting to get a front-row seat look at pore formation. Tests with a titanium alloy showed pores forming at laser turn points, allowing the team to begin formulating a solution to reduce defects in parts, and increase the credibility of LPBF as a technology, with X-ray imaging serving as an effective new way to explore issues during LPBF.

With the turn point being a major focus, the researchers noted that it increases due to laser power, regardless of steady-state scan speed. They also discovered that pores always form within 200 µm of the turn point. Pores closes to the turn point were also the deepest.

“Inspection of an X-ray image time series captured at each respective processing condition reveals that pores form very quickly on time scales comparable to the sampling rate of our measurement (50 µs).”

Properties of pores formed during LPBF of Ti–6Al–4V in the laser turn point region as a function of laser power and steady-state scan speed. All turn point condition scans were performed at full laser power. a Depth of pore relative to the substrate surface as a function of distance from the turn point of the laser. b Histograms of the pore initiation time, τp, after the laser completed the turn point for three different scan speeds where tturn = 0 µs. Each histogram includes pores produced with all laser powers (50–300 W) at the specified scan speed with (blue line) and without (red line) powder. No pores were formed in the turn point region prior to the laser turn in these experiments

In exploring depression depth further, the researchers found that the highest amount of vapor depression post-turn occurred due to heat buildup. This was a result of the ‘long dwell time’ of the near-stationary laser.

“When the depression exceeds a depth on the order of 100 µm the deep keyhole regime is entered and a dramatic increase in the absorption of the laser power is realized due to multiple interactions between the melt pool and reflected laser,” stated the researchers.

When surface temperatures are lower, the melt pool tension increases—causing complete collapse of the depression, with pores trapped when the material solidifies rapidly. When the laser scan is maximized, pores are created with the vapor depression transitioning into a deep keyhole regime. As the walls collapse rapidly, pores are formed. The researchers raise the question of turning off the laser at the turn point, but they decided it was not viable due to previous studies where such action ended in pore formation.

The researcher’s pore mitigation strategy was used to stop pores from forming at the turn point by ‘removing the rapid variation in depression depth inherent in the unmitigated case.’ This also refined the geometric tolerance of the tracks by eliminating problems with overhearing.

“Conceptually similar strategies should be applicable to any abrupt laser on/off points during LPBF. The successful mitigation strategy presented here illustrates the potential of in situ X-ray measurements coupled with high fidelity modeling for driving process improvements and paves the way to increasing the quality of LPBF-built components,” concluded the researchers.

This is just one of many recent studies in improving metal 3D printing processes, from finding ways to make additive manufacturing more affordable to using high entropy alloys, and even re-use powders. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

Mitigation of vapor depression depth change during LPBF of Ti–6Al–4V. a The black line (left axis) corresponds to vapor depression depth as a function of time during a 100 W peak power pore mitigation scan strategy. Error bars represent uncertainty in the distance between the base of the vapor depression and the surface caused by surface roughness. Also shown is the commanded laser power as a function of time used in the scan strategy (red line). Depression depth values were measured for the case of a bare plate experiment because depression depth measurements in bare plate were less uncertain than the powder case, but the same trend is observed in both cases. b Normalized enthalpy (ΔHhs) represented by the magenta-green color scale as a function of laser position during the turn point for the full power and mitigated cases at 1000 mm s−1 steady-state scan speed

[Source / Images: Dynamics of pore formation during laser powder bed fusion additive manufacturing]

Jabil to Enter 3D Printing Materials Market & Develop Custom Materials “Open Systems are the Future of 3D Printing”

Jabil Spools Waiting to be shipped.

Jabil is one of the largest contract manufacturing and engineering companies worldwide. They’ve been very serious about 3D Printing for a number of years as well. The company has made moves in 3D printed footwear, setting up an Additive Manufacturing Network, partnering with HP, using clustered Ultimaker 3D printers and manufacturing 3D printers for Makerbot. The $19 billion dollar group uses 3D printing extensively in-house to prototype the products that it engineers as well as manufacture things such as jigs and tools extensively. Jabil can do anything from designing or improving a product to assembling millions of examples of that product to expediting them all around the world. Little known compared to its customers there is a high probability that this 180,000 person company made some of the high tech devices that you rely on each and every day.

Today Jabil announced that it is launching Jabil Engineered Materials for 3D printing which will let customers customize, test, create and validate 3D printing materials for manufacturing applications. Additionally, Jabil will be offering FDM and SLS materials on the open market and selling them via resellers. Materials have been a major brake on 3D printing innovation. High cost of materials slows down 3D printing adoption especially in manufacturing. Low performance also inhibits 3D printing success. Closed material ecosystems do lead to good print results but do mean that there is a lock-in for the company using that 3D printer manufacturer and material. Often companies want to use the polymers they know and love or those that are standards in their applications but these may not be available for 3D printing. In the regular plastics market, thousands of specialized compounding companies optimize materials for 3D printing. For 3D printing, some specialized filament producers will make filaments to order for you in FDM (FFF, Material Extrusion). For SLS (powder bed fusion, laser sintering) only ALM and a handful of other firms can qualify and create working specialized polymer materials. In SLS and EBM metal powders lots of people say they can do this but almost no one is actually able to do this (Höganäs, GE and Sandvik are exceptions for example).

A Jabil Employee mixing materials.

In polymers, we have lots of compounders worldwide but precious few have manufacturing knowledge of 3D printing or know how 3D printing works (LehVoss being the notable exception). Through deep manufacturing knowledge and operating a lot of machines, Jabil hopes to enter and prosper in this niche that is set for high growth. Custom material formulations for manufacturing is a very high 3D printing growth segment at the moment. Clearly, Jabil is making an astute vertical integration here that will let them lower their own production costs for 3D printed goods while simultaneously making them more indispensable to customers. More margin that feeds tactical success is always nice especially if you then become more relevant to customers as well. If they pull this off this will be a brilliant long term strategically significant move by the firm that will let them outcompete once 3D printing in manufacturing becomes a reality for more industries. The key element to getting things made with 3D printing is controlling the 3D printing Octagon. Right now everyone is trying to do this through alliances between material companies and OEMs or by trying to become a platform themselves. GE and Jabil are both doing the most to themselves monetize their control over the octagon and therefore obtain a better 3D printing manufacturing future for themselves. Other firms are simply sticking to their own islands of expertise while shaking hands a lot. While polymer knowledge is being tied to settings and materials are being qualified most are forgetting the manufacturing part of this equation.

Additionally Jabil has opened Materials Innovation Center in Minnesota where the company can develop materials as well as do compounding under ISO 9001. There and at other locations the company work on developing Integrated Materials, Processes and Machines (MPM) and. “evaluate, qualify and validate materials alongside certified machines and processes as part of an integrated MPM solution that matches specific part performance with application requirements. This comprehensive approach ensures greater availability of unique materials while reducing time-to-market and cost to produce the highest quality parts.”

John Dulchinos, vice president of digital manufacturing, Jabil stated that,

“Jabil is taking advantage of its rich history in materials science innovation to advance the entire additive manufacturing market forward and produce custom materials in weeks—not months. Our ability to integrate new engineered materials into our ecosystem of 3D printers and rigorous processes will transform a new generation of additive manufacturing applications, including those for heavily regulated industries, such as aerospace, automotive, industrial and healthcare.”

The company will also work on “Value-added attributes include, but aren’t limited to, reinforced, flame retardant, conductive, lubricated, Electrostatic Dissipative (ESD) and other engineered characteristics” which would put them very close to what Clariant is doing in 3D printing. The company will focus on FDM, SLS and HSS and a “full range of services also are available, including compounding, extrusion and powder creation as well as complete system integration on standard, open source platforms supported by Fused Filament Fabrication (FFF), Selective Laser Sintering (SLS) and High-Speed Sintering (HSS) equipment.”

Some Jabil materials will also be sold through Chase Plastics and the Channel Prime Alliance (part of Ravago) and the profiles of some Jabil materials will be available in Cura.

3DPrint.com spoke to John Dulchinos, VP of Digital Manufacturing, and Matt Torosian, Director of Product Management at Jabil to find out more. They told us that the material science team will comprise of more than 40 individuals and that they would produce pellets as well as filament and powder. Their main Minnesota site has dozens of 3D printers in different technologies and they deploy hundreds of machines across Jabil. Some of the materials that they hope to make available soon include PA12 and TPU powders as well as PETG filaments. The team has rheology, mechanical engineering and all the equipment needed in house.

Tensile strength testing a coupon at Jabil.

Matt believes that, “Every industrial assembly facility should at least have 20 3D printers eventually” and that internally Jabil used 3D printing “for jigs fixtures and tooling to reduce time and cost.” Jabil’s main motivation for selling and making materials is to “better serve customers and increase adoption rates of 3D printing.” He mentions that “customers tell us that they are ready to go but that they don’t have the right quality management, regulatory or materials for 3D printing” and this is where he believes Jabil can deliver value. John mentions that “3D printing material costs have to come down” and that their purchasing power and engineering can make this happen.

He adds that Jabil is “interested in real parts, and has adopted the right quality framework for 3D printing from consumer goods to medical.” The company has defined, “how we qualify a program, how we qualify a part or machine, how we define quality and what test coupons we use, what certifications we use.” He sees “FDM technology having broad applicability” and in some cases could see desktop printers and printer farms being used for manufacturing in the future as these platforms get their “next level of refinement.” He thinks that “open systems are the future of 3D printing” and thinks that Jabil is “ahead of the curve” in this “potentially disruptive manufacturing technology.”