David B. Witkin

Creep Behavior of Bi-Containing Lead-Free Solder Alloys

The creep behavior of Sn-3.0Ag-0.5Cu (SAC305), Sn-3.4Ag-1.0Cu-3.3Bi (SAC-Bi), and Sn-3.4Ag-4.8Bi (SnAg-Bi, all wt.%) was studied in constant-stress creep tests from room temperature to 125°C. The alloys were tested in two microstructural conditions. As-cast alloys had a composite eutectic-primary Sn structure, while in aged alloys the eutectic regions were replaced by a continuous Sn matrix with coarsened intermetallic (Cu6Sn5 and Ag3Sn) particles. After aging, Bi in SAC-Bi and SnAg-Bi was found as precipitates at grain boundaries and grain interiors. The creep resistance of of-cast SAC305 was higher than that of as-cast Bi-containing alloys, but after aging the SAC305 had the lowest creep resistance. The creep strain rates in SAC-Bi and SnAg-Bi were much less affected by aging. The apparent activation energy for creep was also changed more for SAC305 than for the other two alloys. The creep behavior of SAC-Bi and SnAg-Bi can be understood by considering the solubility of Bi in Sn. The difference in creep behavior between as-cast and aged SAC-Bi is greatly reduced when room-temperature test results are excluded from analysis. This suggests that the strongest influence on creep in these alloys is due to Bi solute interaction with moving dislocations during deformation.

Microstructural evolution and mechanical behavior of nickel-based superalloy 625 made by selective laser melting

The mechanical properties and microstructures of Selective Laser Melted (SLM) alloy 625 procured from different suppliers were compared. The post-SLM process of hot isostatic pressing (HIP) led to a relatively coarse recrystallized gamma matrix phase that was similar in all the suppliers’ materials, resulting in nearly identical tensile properties. These similarities obscure significant differences between them with respect to the population of second phase particles, which consisted of carbides or Laves phase. During solidification, the final liquid phase is concentrated in Nb, Mo, Si and C, and leads to L → γ + carbide/Laves eutectic reactions. Secondary particles are very small prior to HIP and their composition has not been analyzed yet, but are limited to the fine-grained eutectic regions of the material prior to HIP. During HIP the gamma phase recrystallizes to remove the original as-solidified SLM microstructure, but secondary particles nucleate and grow where their elemental constituents first solidified, leading to a non-homogeneous distribution. Quasi-static tensile properties do not appear to be sensitive to these differences, but it is likely that other mechanical properties will be affected, especially fatigue and fracture behavior. Surface roughness, large grain size, and pores and voids left unhealed by the HIP cycle will also influence fatigue and fracture. Surface roughness and porosity in particular are features that could be improved by implementing novel approaches to laser processing in SLM.

Laser post-processing of Inconel 625 made by selective laser melting

The effect of laser remelting of surfaces of as-built Selective Laser Melted (SLM) Inconel 625 was evaluated for its potential to improve the surface roughness of SLM parts. Many alloys made by SLM have properties similar to their wrought counterparts, but surface roughness of SLM-made parts is much higher than found in standard machine shop operations. This has implications for mechanical properties of SLM materials, such as a large debit in fatigue properties, and in applications of SLM, where surface roughness can alter fluid flow characteristics. Because complexity and netshape fabrication are fundamental advantages of Additive Manufacturing (AM), post-processing by mechanical means to reduce surface roughness detracts from the potential utility of AM. Use of a laser to improve surface roughness by targeted remelting or annealing offers the possibility of in-situ surface polishing of AM surfaces- the same laser used to melt the powder could be amplitude modulated to smooth the part during the build. The effects of remelting the surfaces of SLM Inconel 625 were demonstrated using a CW fiber laser (IPG: 1064 nm, 2-50 W) that is amplitude modulated with a pulse profile to induce remelting without spallation or ablation. The process achieved uniform depth of melting and improved surface roughness. The results show that with an appropriate pulse profile that meters the heat-load, surface features such as partially sintered powder particles and surface connected porosity can be mitigated via a secondary remelting/annealing event.

The effect of laser focus and process parameters on microstructure and mechanical properties of SLM Inconel 718

Research on the selective laser melting (SLM) method of laser powder bed fusion additive manufacturing (AM) has shown that surface and internal quality of AM parts is directly related to machine settings such as laser energy density, scanning strategies, and atmosphere. To optimize laser parameters for improved component quality, the energy density is typically controlled via laser power, scanning rate, and scanning strategy, but can also be controlled by changing the spot size via laser focal plane shift. Present work being conducted by The Aerospace Corporation was initiated after observing inconsistent build quality of parts printed using OEM-installed settings. Initial builds of Inconel 718 witness geometries using OEM laser parameters were evaluated for surface roughness, density, and porosity while varying energy density via laser focus shift. Based on these results, hardware and laser parameter adjustments were conducted in order to improve build quality and consistency. Tensile testing was also conducted to investigate the effect of build plate location and laser settings on SLM 718. This work has provided insight into the limitations of OEM parameters compared with optimized parameters towards the goal of manufacturing aerospace-grade parts, and has led to the development of a methodology for laser parameter tuning that can be applied to other alloy systems. Additionally, evidence was found that for 718, which derives its strength from post-manufacturing heat treatment, there is a possibility that tensile testing may not be perceptive to defects which would reduce component performance. Ongoing research is being conducted towards identifying appropriate testing and analysis methods for screening and quality assurance.

Crystallographic Orientation Relationships of Grain Boundary Alpha in Additively Manufactured Ti-6Al-4V

Additive manufacturing (AM) of near net shape Ti-6Al-4V (Ti-6-4) parts is receiving increasing attention from the biomedical and aerospace communities looking to take advantage of AM’s cost and time-to-production benefits for this important and widely used alloy. Ti-6-4 AM parts built using powder bed fusion techniques such as Electron Beam Melting (EBM) or Selective Laser Melting (SLM) are prone to porosity that may not be acceptable for aerospace applications subject to high stresses or cyclic loading, so are often consolidated by Hot Isostatic Pressing (HIP). Grain boundary alpha (GBwhich typically defines elongated prior beta grain boundaries aligned in the vertical direction with respect to the AM build process, is deleterious to fatigue properties and coarsens with sub-beta transus HIP cycling. An understanding of GB’s formation and crystallographic relationships to the surrounding microstructure may assist in interpreting its effect on mechanical properties and whether mitigating heat treatments are beneficial.

Influence of low-earth orbit exposure on the mechanical properties of silicon carbide

The influence of the Low-Earth orbit (LEO) environment on the mechanical strength of silicon carbide (SiC) was evaluated on two flight experiments as part of the Materials on the International Space Station Experiment (MISSE). SiC samples for modulus of rupture (MOR) and equibiaxial flexural strength (EFS) testing were flown on the Optical and Reflector Materials experiments (ORMatE) as part of MISSE-6 (launched on STS-123, March 2008; returned on STS-128, September 2009) and MISSE-7 (launched on STS-129, November 2009; returned on STS- 134, June 2011). Two different SiC vendors provided material for each flight experiment. The goal of the experiments was to measure mechanical properties of the flight samples and compare them to an equal number of similar samples in control and traveler sample sets. Complete characterization of the strength of brittle materials typically requires many more test specimens than could be reasonably accommodated on the ORMatE sample tray and statistical models based on few samples include large uncertainties. Understanding the results of the mechanical tests of MISSE samples required comparison to results from a statistically valid number of samples. Prior testing by The Aerospace Corporation of material supplied by the same four vendors was used to evaluate the MISSE results, including flight and control samples. The results showed that exposure to LEO over the durations covered by MISSE 6 and 7 (approximately 18 and 20 months, respectively) did not alter the mechanical strength of the silicon carbide for any of the vendors’ materials.

Novel Tests for the As-Manufactured Strength of Mn-Zn Ferrite Inductor Cores

∗† ‡ § Ferrites are magnetic ceramic (oxide) materials used as inductors and power transformers. Two new tests were developed to assess the reliability of ferrite cores in a high-reliability application. The tests intended to assess the as-manufactured strength of the cores in two different locations that would be subject to relatively high tensile stress during operation. A cantilever bend test was used to evaluate a corner in the as-sintered condition, while a biaxial test was used on a machined surface. Parts from two manufacturers were tested, with two batches tested from one of the manufacturers. The results of the test showed differences in strength that could be correlated to processing-related features such as density, pore size and distribution, and surface preparation. Differences in cantilever bend test failure loads between two batches of cores from the same vendor illustrate the potential use of the cores as a check on manufacturing quality control. I. Introduction Ferrites are magnetic ceramic (oxide) materials used as inductors and power transformers. They are often used in pairs with matching machined surfaces to maintain air gaps in the path of magnetic flux. They are especially useful in high-frequency applications because their low electrical conductivity relative to other magnetic materials means they are less prone to core losses arising from eddy currents. The use of ferrites in high-reliability applications may require mechanical property data to establish safety margins and determine probability of failure under mechanical loads. There is limited information of the mechanical strength of ferrite material, however, and published reports indicate that the strength of Mn-Zn ferrites can be highly dependent on composition and processing conditions 1 . The strength of manufactured cores may be more dependent on surface flaws introduced by machining than on the intrinsic properties of the material. The finished core may include both as-sintered and machined surfaces, so the reliability of the core under stress is a function of both the applied load and the surface condition. The effect of machining and residual stress on magnetic properties has been addressed 2-4 , but the strength of manufactured cores has been reported only to a limited extent 5, 6 . Two new mechanical tests have been developed to address particular concerns about the strength of ferrite cores in an application in which they were to be epoxy bonded to a printed circuit board for a high-reliability space-based application. The ferrites selected for the design were commercial products manufactured without source control and their mechanical properties were not part of the initial design considerations. The cores featured both as-sintered and machined surfaces that would be subjected to stresses, but the relative strengths of these two types of surface condition were not known. The prevalence of shrinkage cracks at a potentially high-stress location in the cores raised concerns about the strength of the cores during testing and operation, especially due to thermal-mechanical stresses arising from mismatches in coefficient of thermal expansion (CTE). The tests were used to characterize the strength of ferrite cores acquired from two vendors. The tests demonstrated differences in strength between vendors and between batches of cores from one of the vendors. These differences had implications for margins of safety and reliability of the design.

Material testing of silicon carbide mirrors

The Aerospace Corporation is developing a space qualification method for silicon carbide optical systems that covers material verification through system development. One of the initial efforts has been to establish testing protocols for material properties. Three different tests have been performed to determine mechanical properties of SiC: modulus of rupture, equibiaxial flexural strength and fracture toughness. Testing materials and methods have been in accordance with the respective ASTM standards. Material from four vendors has been tested to date, as part of the MISSE flight program and other programs. Data analysis has focused on the types of issues that are important when building actual components- statistical modeling of test results, understanding batch-to-batch or other source material variations, and relating mechanical properties to microstructures. Mechanical properties are needed as inputs to design trade studies and development and analysis of proof tests, and to confirm or understand the results of non-destructive evaluations of the source materials. Measuring these properties using standardized tests on a statistically valid number of samples is intended to increase confidence for purchasers of SiC spacecraft components that materials and structures will perform as intended at the highest level of reliability.

Development of a systematic approach to space qualification of silicon carbide for mirror applications

Over the last few years significant progress has been made in the development of silicon carbide (SiC) for mirror applications. These improvements include lightweighting techniques, higher production yields, and larger diameter apertures. It is now necessary to evaluate and address the systems engineering challenges facing this material to ensure space qualification and integration into future space applications. This paper highlights systems engineering challenges, suggests areas of future development, and proposes a systematic path forward that will outline necessary steps to space qualify this new material.