Showing papers by "Robert F. Singer published in 2016"

Grain structure evolution in Inconel 718 during selective electron beam melting

Abstract: Selective electron beam melting (SEBM) is an additive manufacturing method where complex parts are built from metal powders in layers of typically 50 µm. An electron beam is used for heating (about 900 °C building temperature) and selective melting of the material. The grain structure evolution is a result of the complex thermal and hydrodynamic conditions in the melt pool. We show how different scanning strategies can be used to produce either a columnar grain structure with a high texture in building direction or an equiaxed fine grained structure. Numerical simulations of the selective melting process are applied to study the fundamental mechanisms responsible for differing grain structures. It is shown, that the direction of the thermal gradient during solidification can be altered by scanning strategies to acquire either epitaxial growth or stray grains. We show that it is possible to locally alter the grain structure of a part, thus allowing tailoring of the mechanical properties.

Microstructure of the Nickel-Base Superalloy CMSX-4 Fabricated by Selective Electron Beam Melting

Abstract: Powder bed-based additive manufacturing (AM) processes are characterized by very high-temperature gradients and solidification rates. These conditions lead to microstructures orders of magnitude smaller than in conventional casting processes. Especially in the field of high performance alloys, like nickel-base superalloys, this opens new opportunities for homogenization and alloy development. Nevertheless, the high susceptibility to cracking of precipitation-hardenable superalloys is a challenge for AM. In this study, electron beam-based AM is used to fabricate samples from gas-atomized pre-alloyed CMSX-4 powder. The influence of the processing strategy on crack formation is investigated. The samples are characterized by optical and SEM microscopy and analyzed by microprobe analysis. Differential scanning calorimetry is used to demonstrate the effect of the fine microstructure on characteristic temperatures. In addition, in situ heat treatment effects are investigated.

Improved creep strength of nickel-base superalloys by optimized γ/γ′ partitioning behavior of solid solution strengthening elements

Abstract: Solid solution strengthening of the γ matrix is one key factor for improving the creep strength of single crystal nickel-base superalloys at high temperatures. Therefore a strong partitioning of solid solution hardening elements to the matrix is beneficial for high temperature creep strength. Different Rhenium-free alloys which are derived from CMSX-4 are investigated. The alloys have been characterized regarding microstructure, phase compositions as well as creep strength. It is found that increasing the Titanium (Ti) as well as the Tungsten (W) content causes a stronger partitioning of the solid solution strengtheners, in particular W, to the γ phase. As a result the creep resistance is significantly improved. Based on these ideas, a Rhenium-free alloy with an optimized chemistry regarding the partitioning behavior of W is developed and validated in the present study. It shows comparable creep strength to the Rhenium containing second generation alloy CMSX-4 in the high temperature / low stress creep regime and is less prone to the formation of deleterious topologically close packed (TCP) phases. This more effective usage of solid solution strengtheners can enhance the creep properties of nickel-base superalloys while reducing the content of strategic elements like Rhenium.

Effect of B, Zr, and C on Hot Tearing of a Directionally Solidified Nickel-Based Superalloy

Abstract: The effect of the minor elements B, Zr, and C on the castability of a Nickel-based γ′-strengthened superalloy has been investigated. Tube-like specimens were prepared by directional solidification where the rigid ceramic core leads to hoop stresses and grain boundary cracking. It was found that an important improvement in castability can be achieved by adjusting the minor elemental composition. Too low C (≤0.15 pct) and too high B and Zr contents (≥0.05 pct) lead to material that is very prone to solidification cracking and should be avoided. The results cannot be rationalized on the basis of the current models for solidification cracking. Instead, pronounced hot tearing is observed to occur at high amounts of γ/γ′-eutectic and high Zr contents. The critical film stage where dendrites at the end of solidification do not touch and are separated by thin liquid films must be avoided. How Zr promotes the film stage will be discussed in the paper.

Mechanical Properties of Carbon Fiber-Reinforced Aluminum Manufactured by High-Pressure Die Casting

Abstract: Carbon fiber reinforced aluminum was produced by a specially adapted high-pressure die casting process. The MMC has a fiber volume fraction of 27%. Complete infiltration was achieved by preheating the bidirectional, PAN-based carbon fiber body with IR-emitters to temperatures of around 750 °C. The degradation of the fibers, due to attack of atmospheric oxygen at temperatures above 600 °C, was limited by heating them in argon-rich atmosphere. Additionally, the optimization of heating time and temperature prevented fiber degradation. Only the strength of the outer fibers is reduced by 40% at the most. The fibers in core of fiber body are nearly undamaged. In spite of successful manufacturing, the tensile strength of the MMC is below strength of the matrix material. Also unidirectional MMCs with a fiber volume fraction of 8% produced under the same conditions, lack of the reinforcing effect. Two main reasons for the unsatisfactory mechanical properties were identified: First, the fiber-free matrix, which covers the reinforced core, prevents effective load transfer from the matrix to the fibers. And second, the residual stresses in the fiber-free zones are as high as 100 MPa. This causes premature failure in the matrix. From this, it follows that the local reinforcement of an actual part is limited. The stress distribution caused by residual stresses and by loading needs to be known. In this way, the reinforcing phase can be placed and aligned accordingly. Otherwise delamination and premature failure might occur.

Metal injection moulding of nickel-based superalloy CM247LC*

Abstract: Metal injection moulding (MIM) is a promising new production technology for small- to medium-sized parts with high complexity for high-temperature applications like aero engines or turbochargers. This study concerns the feasibility of manufacturing parts from nickel-based superalloy CM247LC via MIM. CM247LC poses a serious challenge for MIM processing. Because of its high aluminium content, the strength potential is very high, but the sintering capability is severely restricted. Differential scanning calorimetry and dilatometry measurements as well as ThermoCalc simulations are used to optimise the sintering step of the MIM process route. Carbon, nitrogen and oxygen contents of the powder and the as-sintered specimens are measured to evaluate the pick-up of impurities during processing. The microstructure of the as-sintered specimens is characterised with respect to residual porosity, grain size, carbide content and γ′ precipitation size and morphology. Ways to further improve the microstructure and stren...

Mechanical Properties, Surface Structure, and Morphology of Carbon Fibers Pre-heated for Liquid Aluminum Infiltration

Abstract: To efficiently produce carbon fiber-reinforced aluminum on a large scale, we developed a special high-pressure die casting process. Pre-heating of the fibers is crucial for successful infiltration. In this paper, the influence of heating carried out in industrial conditions on the mechanical properties of the fibers was investigated. Therefore, polyacrylonitrile-based high-tensile carbon fiber textiles were heated by infrared emitters in an argon-rich atmosphere to temperatures between 450 and 1400 °C. Single fiber tensile tests revealed a decrease in tensile strength and strain at fracture. Young’s modulus was not affected. Scanning electron microscopy identified cavities on the fiber surface as the reason for the decrease in mechanical properties. They were caused by the attack of atmospheric oxygen. The atomic structure of the fibers did not change at any temperature, as x-ray diffraction confirmed. Based on these data, the pre-heating for the casting process can be optimized.

Wettability of Low Weight Borides by Commercial Aluminum Alloys − A Basis for Metal Matrix Composite Fabrication

Abstract: Boron compounds are promising candidates for reinforcement of metals because of their high strength and stiffness at low specific weight. For composite fabrication the wetting behavior is important. Isothermal wetting of B4C, AlB2, CaB6, MgB2, and TiB2 by liquid pure aluminum (99.5%) and the alloy AlSi10MgMn is studied at 700 °C for 15 min in vacuum using the dispensed drop method. No wetting occurs during testing for all substrates. The contact angles remain constant for CaB6, MgB2, and TiB2, while for AlB2 and B4C substrates a continuous decrease in contact angle value with time is observed.

Heat treatment process for components made of nickel-based superalloys

Abstract: Die vorliegende Erfindung betrifft ein Verfahren zur Herstellung eines Bauteils, aus einer Nickelbasis-Superlegierung, insbesondere eines Bauteils einer Stromungsmaschine, bei welchem ein Halbzeug des Bauteils einer Losungsgluhbehandlung und einer Ausscheidungsgluhbehandlung unterzogen wird, wobei die Losungsgluhbehandlung in einem Temperaturbereich von 1300°C bis 1350°C und die Ausscheidungsgluhbehandlung im Temperaturbereich von 900°C bis 1150°C durchgefuhrt wird, wobei die Losungsgluhbehandlung und/oder die Ausscheidungsgluhbehandlung zusammen mit einer weiteren Bearbeitung des Halbzeugs durchgefuhrt werden. Die Erfindung betrifft weiterhin ein Verfahren zur Aufarbeitung eines Bauteils aus einer Nickelbasis-Superlegierung, insbesondere eines Bauteils einer Stromungsmaschine, nach mehreren hundert Stunden Einsatz bei einer Einsatztemperatur von mehr als 500°C, wobei eine Wiederaufarbeitungswarmebehandlung in einem Temperaturbereich von 1100°C bis 1280°C durchgefuhrt wird.