November_December_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 1 6 4 3D PRINTSHOP A colorized electron micrograph of a nickel powder alloy used in Texas A&M’s study. Courtesy of Raiyan Seede. FINE-TUNED MICROSTRUCTURES FOR LPBF PRINTING Researchers from Texas A&M Uni- versity have used a combination of machine learning and single-track 3D printing experiments to identify the fa- vorable alloy chemistries and process parameters, like laser speed and pow- er, needed to print parts with uniform properties at the microscale. “In this study, we take deep dives into fine-tuning the microstructure of alloys so that there is more control over the properties of the final printed object at a much finer scale than before,” says Raiyan Seede, doctoral student in the department of materials science and engineering. The laser powder bed fusion (LPBF) process starts with rolling a thin layer of metal powder on a base plate and then melting the powder with a laser beam along tracks that trace the cross-sectional design of the intended part. Then, another layer of the powder is applied and the process is repeated, gradually building the final part. Alloy metal powders used for ad- ditive manufacturing can be quite di- verse, containing a mixture of metals, such as nickel, aluminum, and magne- sium at different concentrations. During printing, these powders cool rapidly after being heated by a laser beam. Because the individual met- als in the alloy powder have very different cooling prop- erties and consequently so- lidify at different rates, this mismatch can create a type of microscopic flaw called microsegregation. Seede says this defect appears as tiny pockets containing a slightly differ- ent concentration of the metal ingredients than oth- er regions of the printed part. These in- consistencies compromise the mechan- ical properties of the printed object. To rectify this microdefect, the re- search team investigated the solidifica- tion of four alloys containing nickel and one other metal ingredient. In particu- lar, for each of these alloys, they studied the physical states or phases present at different temperatures for increasing concentrations of the other metal in the nickel-based alloy. From detailed phase diagrams, they could determine the chemical composition of the alloy that would lead to minimum microsegrega- tion during additive manufacturing. Next, they melted a single track of the alloy metal powder for differ- ent laser settings and determined the process parameters that would yield porosity-free parts. Then, they com- bined the information gathered from the phase diagrams with that from the single-track experiments to get a consolidated view of the laser settings and nickel alloy com- positions that would yield a po- rosity-free printed part without microsegregation. Last, the researchers trained machine-learning models to iden- tify patterns in their single-track experiment data and phase dia- grams to develop an equation for microsegregation applicable to any other alloy. Seede said the equation is designed to predict the extent of segregation given the solidification range, material properties, laser power, and speed. The researchers add that the uniqueness of their methodology is in its simplicity, which can easily be adapt- ed to build sturdy, defect-free parts with an alloy of choice. tamu.edu . COLOR CHANGING, MICROSCOPIC GAS SENSORS A team from Trinity College Dublin and GE Research in New York are print- ing color-changing gas sensors using new materials and a high-resolution form of 3D printing. The sensors, which are responsive, printed, microscopic optical structures, can be monitored in real-time, and used for the detection of solvent vapors in air. There is great po- tential for these sensors to be used in connected, low-cost devices for homes, or integrated in wearable devices used to monitor human health. The team uses a technique known as direct laser-writing (DLW), which al- lows them to focus a laser into an ex- tremely small spot, and then use it to make tiny structures in three dimen- sions from the soft polymers developed in the lab. The tiny arrays respond to light, heat, and humidity. www.tcd.ie . Left, zoomed-in optical microscopy images showing the pixelated sensor in response to different vapors; center, photo of the glass substrate showing the 3D-printed sensor; right, scanning electron microscopy image of the pixelated sensor, showing the different heights of the periodic structure. Courtesy of Trinity College Dublin.