ABSTRACT
A major barrier for the full utilization of metal additive manufacturing (AM) technologies is quality control. Additionally, in situ real time nondestructive monitoring is desirable due to the typical high value and low volume of components manufactured with metal AM. Depending on the application, characteristics such as the geometrical accuracy, porosity, defect size and content, and material properties are quantities of interest for in situ nondestructive evaluation (NDE). In particular, functionally tailored components made with hybrid processing require quantitative NDE of their microstructure and elastic properties. Ultrasonic NDE is able to quantify these relevant characteristics. In this work, an ultrasonic measurement system is used to collect in situ real time measurements during the manufacturing of samples made with a hybrid process, which combines directed energy deposition with milling. In addition to quantifying ultrasonic properties, the measurements are used to gather insight on other geometry, material, and process effects. The results show the utility of ultrasound to evaluate relevant properties during manufacturing of a functionalized material domain, while providing perspective on additional material evolution information obtained from ultrasonic signals.
ABSTRACT
Metal hybrid additive manufacturing (AM) processes are suitable to create complex structures that advance engineering performance. Hybrid AM can be used to create functionally graded materials for which the variation in microstructure and material properties across the domain is created through a synergized combination of fully-coupled manufacturing processes and/or energy sources. This expansion in the engineering design and manufacturing spaces presents challenges for nondestructive evaluation, including the assessment of the sensitivity of nondestructive measurements to functional gradients. To address this problem, linear ultrasound measurements are used to interrogate 420 stainless steel coupons from three manufacturing methods: wrought, AM, and hybrid AM (directed energy deposition + laser peening). Wave speed, attenuation, and diffuse backscatter results are compared with microhardness measurements along the build/axial direction of the coupons, while microstructure images are used for qualitative verification. The ultrasound measurements compare well with the destructive measurements without any substantial loss in resolution. Furthermore, ultrasonic methods are shown to be effective for identification of the gradient and cyclic nature of the elastic properties and microstructure on the hybrid AM coupon. These results highlight the potential of ultrasound as an efficient and accessible nondestructive characterization method for hybrid AM samples and inform further nondestructive evaluation decisions in AM.
ABSTRACT
Magnesium alloys are a promising new class of degradable biomaterials that have a similar stiffness to bone, which minimizes the harmful effects of stress shielding. Use of biodegradable magnesium implants eliminates the need for a second surgery for repair or removal. There is a growing interest to capitalize on additive manufacturing's unique design capabilities to advance the frontiers of medicine. However, magnesium alloys are difficult to 3D print due to the high chemical reactivity that poses a combustion risk. Furthermore, the low vaporization temperature of magnesium and common biocompatible alloying elements further increases the difficulty to print fully dense structures that balance strength and corrosion requirements. The purpose of this study is to survey current techniques to 3D print magnesium constructs and provide guidance on best additive practices for these alloys.
ABSTRACT
The objective of this study is to characterize the micromechanical properties of poly-l-lactic acid (PLLA) composites reinforced by grade 420 stainless steel (SS) particles with a specific focus on the interphase properties. The specimens were manufactured using 3D printing techniques due to its many benefits, including high accuracy, cost effectiveness and customized geometry. The adopted fused filament fabrication resulted in a thin interphase layer with an average thickness of 3 µm. The mechanical properties of each phase, as well as the interphase, were characterized by nanoindentation tests. The effect of matrix degradation, i.e., imperfect bonding, on the elastic modulus of the composite was further examined by a representative volume element (RVE) model. The results showed that the interphase layer provided a smooth transition of elastic modulus from steel particles to the polymeric matrix. A 10% volume fraction of steel particles could enhance the elastic modulus of PLLA polymer by 31%. In addition, steel particles took 37% to 59% of the applied load with respect to the particle volume fraction. We found that degradation of the interphase reduced the elastic modulus of the composite by 70% and 7% under tensile and compressive loads, respectively. The shear modulus of the composite with 10% particles decreased by 36%, i.e., lower than pure PLLA, when debonding occurred.
