Max A. Peltz

Characterization of Metal Powders Used for Additive Manufacturing

Additive manufacturing (AM) techniques1 can produce complex, high-value metal parts, with potential applications as critical parts, such as those found in aerospace components. The production of AM parts with consistent and predictable properties requires input materials (e.g., metal powders) with known and repeatable characteristics, which in turn requires standardized measurement methods for powder properties. First, based on our previous work, we assess the applicability of current standardized methods for powder characterization for metal AM powders. Then we present the results of systematic studies carried out on two different powder materials used for additive manufacturing: stainless steel and cobalt-chrome. The characterization of these powders is important in NIST efforts to develop appropriate measurements and standards for additive materials and to document the property of powders used in a NIST-led additive manufacturing material round robin. An extensive array of characterization techniques was applied to these two powders, in both virgin and recycled states. The physical techniques included laser diffraction particle size analysis, X-ray computed tomography for size and shape analysis, and optical and scanning electron microscopy. Techniques sensitive to structure and chemistry, including X-ray diffraction, energy dispersive analytical X-ray analysis using the X-rays generated during scanning electron microscopy, and X-Ray photoelectron spectroscopy were also employed. The results of these analyses show how virgin powder changes after being exposed to and recycled from one or more Direct Metal Laser Sintering (DMLS) additive manufacturing build cycles. In addition, these findings can give insight into the actual additive manufacturing process.

Thermal properties of high-volume fly ash mortars and concretes:

As sustainability moves to the forefront of construction, the utilization of high-volume fly ash concrete mixtures to reduce CO2 emissions and cement consumption per unit volume of concrete placed is receiving renewed interest. Concrete mixtures in which the fly ash replaces 50% or more of the Portland cement are both economically and technically viable. This article focuses on a characterization of the thermal properties, namely, specific heat capacity and thermal conductivity, of such mixtures. Both the raw materials and the finished products (mortars and concretes) are evaluated using a transient plane source method. Because the specimens being examined are well hydrated, estimates of the specific heat capacity based on a law of mixtures, with a ‘bound water’ specific heat capacity value being employed for the water in the mixture, provide reasonable predictions of the measured performance. As with most materials, thermal conductivity is found to be a function of density, while also being dependent on whether the aggregate source is siliceous or limestone. The measured values should provide a useful database for evaluating the thermal performance of high-volume fly ash concrete structures.

Reducing Thermal and Autogenous Shrinkage Contributions to Early-Age Cracking

Early-age cracking continues to be a significant problem for new concrete construction. Two of the major contributors to such cracking are the heat released by cement hydration during the first few days of curing and the autogenous shrinkage that often occurs during the same time frame. In this paper, three potential alternatives for reducing these contributions by modifying the concrete mixture proportions are investigated, namely increasing the water-cement ratio (w/c), using a coarser cement, or replacing a portion of the portland cement with a coarse limestone powder. Each alternative reduces the heat generated per unit volume by either reducing the volumetric cement content or its early-age reactivity, and reduces autogenous shrinkage by increasing the interparticle spacing between grains in the three-dimensional microstructure. These reductions are quantified for paste and mortar systems by measuring their semi-adiabatic temperature rise and autogenous deformation along with measurements of compressive strength to indicate the strength trade-off that will be experienced in reducing the risk of early-age cracking. These mixtures each have the additional advantage that they should result in a cost savings in comparison with an initial (control) mixture.

Developing a More Rapid Test to Assess Sulfate Resistance of Hydraulic Cements

External sulfate attack of concrete is a major problem that can appear in regions where concrete is exposed to soil or water containing sulfates, leading to softening and cracking of the concrete. Therefore, it is important that materials selection and proportioning of concrete in susceptible regions be carefully considered to resist sulfate attack. American Society for Testing Materials (ASTM) limits the tricalcium aluminate phase in cements when sulfate exposure is of concern. The hydration products of tricalcium aluminate react with the sulfates resulting in expansion and cracking. While ASTM standard tests are available to determine the susceptibility of cements to sulfate attack, these tests require at least 6 months and often up to a year to perform; a delay that hinders development of new cements. This paper presents a new method for testing cement resistance to sulfate attack that is three to five times faster than the current ASTM tests. Development of the procedure was based upon insights on the degradation process by petrographic examination of sulfate-exposed specimens over time. Also key to the development was the use of smaller samples and tighter environmental control.

Viscosity Modifiers to Enhance Concrete Performance

The hazard rate function for concrete structures is often portrayed as a “bathtub”-shaped curve, with a finite ever-decreasing probability of early-age failures being followed by a life with a relatively low constant probability of failure that ultimately increases dramatically as the end of service is reached. Ideally, new concrete technologies should reduce the failures occurring at both ends of this service-life spectrum. VERDiCT (viscosity enhancers reducing diffusion in concrete technology) is one such strategy based on increasing the pore solution viscosity. This approach has the potential to reduce the propensity for early-age cracking while also reducing long-term transport coefficients of deleterious ions such as chlorides. In this paper, the performance of a typical VERDiCT admixture—a viscosity modifier/shrinkage-reducing admixture— is investigated in mortar and concrete, both as an addition to the mixing water and as a concentrated solution used to pre-wet fine lightweight aggregates. A reduction in early-age cracking is achieved by eliminating autogenous shrinkage stresses that typically develop in lower water-cementitious material ratio (w/cm) concrete. By substantially increasing the viscosity of the pore solution in the concrete, the resistance to ionic diffusion is proportionally increased relative to a control concrete without the VERDiCT admixture. Herein, chloride ion diffusion coefficients are evaluated for two types of concrete containing typical substitution levels of supplementary cementitious material —namely, either 25% fly ash or 40% slag by mass. For the eight concrete mixtures investigated, the effective diffusion coefficient was reduced by approximately 33% by adding the VERDiCT admixture which, in practice, may imply a 50% increase in their service life, while the autogenous shrinkage was virtually eliminated. However, these benefits in early-age cracking resistance and long-term durability are tempered by up to a 20% reduction in compressive strength that may need to be accounted for at the design stage.