Powder metallurgy (PM) superalloys are an important class of high temperature structural materials, key to the rotating components of aero engines. In the purview of the present challenges associated with PM superalloys, two novel approaches namely, powder preparation and the innovative spray-forming technique (for making turbine disk) are proposed and studied. Subsequently, advanced technologies like electrode-induction-melting gas atomization (EIGA), and spark-plasma discharge spheroidization (SPDS) are introduced, for ceramic-free superalloy powders. Presently, new processing routes are sought after for preparing finer and cleaner raw powders for disk superalloys. The progress of research in spray-formed PM superalloys is first summarized in detail. The spray-formed superalloy disks specifically exhibit excellent mechanical properties. This paper reviews the recent progress in innovative technologies for PM superalloys, with an emphasis on new ideas and approaches, central to the innovation driving techniques like powder processing and spray forming.

https://iopscience.iop.org/article/10.1088/1674-1056/25/2/026103/pdf

Innovative technologies for powder metallurgy-based disk superalloys: Progress and proposal

Generation and forming of spray-formed flat products

Uniform droplet spray forming is a process aimed at producing near-net-shape parts directly from the liquid melt by spraying micron-sized droplets onto a movable target. In spray forming, the solidification rate of the deposited material is a key parameter that influences its microstructure formation and consequently, its mechanical properties. Ideal deposit microstructure has fine grain size and minimal segregation. The purpose of this thesis is to quantify the effects of deposit thermal history on the microstructure produced by the UDS forming process. Experiments were conducted with Zn-20wt% Sn alloy droplets using different process parameters to vary the deposit solidification rate from 60C/s to 270 C/s immediately after deposition. The temperature of the sprayed deposits was measured in situ with a fiberoptic infrared thermometer and a contact thermocouple. Furthermore, a 2-D axisymmetric finite element model was developed to predict the temperature of the deposit during and after deposition. The simulation results show that radial heat conduction cannot be neglected when the thermal diffusivity of the substrate material is comparable to the diffusivity of the deposit material. Simulation results showed good agreement with the measured temperature data. The resultant deposit microstructures were examined with scanning electron microscopy, which revealed that the deposit microstructure includes a mixture of solidified phases formed inside the droplets before impact. The characteristic cell size of the microstructure was shown to vary inversely with the square root of the deposit solidification rate. This implies that the growth of the microstructure is interfacecontrolled rather than diffusion-controlled, due to the high volume fraction of solid during deposition. Furthermore, the homogeneity of the deposit microstructure across its thickness suggests that the rapid solidification rate of the individual droplet layers upon impact played a minor role in determining the final deposit microstructure. In conclusion, this research has developed a new methodology for studying the evolution of deposit microstructure in uniform droplet spray forming. The processing regime that promotes the layer-wise growth of deposits without porosity or liquid overflow has produced fine-scale deposit microstructure. The coarsening kinetics of the microstructure are interface-controlled by the deposit solidification rate. A uniform local solidification rate after deposition resulted in homogeneous deposit microstructure.

The effects of deposit thermal history on microstructure produced by uniform droplet spray forming

Spray forming of a wide range of steels and iron based alloys have been investigated since the 1970s. These range from low-alloy carbon steels to high-carbon, high-alloy tool steels. The preform types include round billets, flat deposits, tubular preforms, clad structures, gradient deposits, and molds/dies. While the size of the deposits produced in pilot-scale plants is typically less than 100 kg, the industrial plants are in some cases capable of producing preforms with weight up to several tons. Microstructure and properties of the spray formed steels are usually far superior to those of cast material, typically resembling those of the equivalent powder metallurgy steels. The main advantage of spray forming over powder metallurgy route is the possibility to eliminate powder handling steps. This not only minimizes the risk of contamination but also results in cost savings.

Spray Forming of Steels

A three-dimensional shape model, tracing the coordinates of the moving surface of a growing flat deposit spray-formed using a scanning gas atomizer, has been developed in this study. Mass flux distributions in the spray cones generated by the scanning atomizer under a typical spray forming condition have been revealed for the shape modeling. Geometrical evolution of flat deposits in spray forming has been investigated based on analysis of the scanning mechanism of the atomizer. The influence of processing conditions on the shape and dimensions of flat deposits have been simulated and discussed. Finally, the shape modeling has been validated by experimental investigations.



In dieser Studie wurde ein dreidimensionales Form-Modell entwickelt, das die Koordinaten der bewegten Oberflache eines beim Spruhkompaktieren mittels scannendem Zerstauber erzeugten flachen Deposits verfolgt. Zur Form-Modellierung wurden die vom scannenden Zerstauber erzeugten Massenstromdichten im Spruhkegel unter typischen Spruhkompaktierbedingungen ermittelt. Basierend auf der Analyse des Scan-Mechanismus des Zerstaubers wurde die aufwachsende Form der flachen Deposits beim Spruhkompaktieren untersucht. Der Einfluss der Prozessbedingungen auf die Form und die Abmessungen der flachen Deposits wurde simuliert und diskutiert. Letztendlich wurde die Form-Modellierung durch experimentelle Untersuchungen validiert.

Modeling the shape of flat deposits during spray forming using a scanning gas atomizer

Spray forming allows the production of near-net-shaped parts characterized by a fine-grained and segregation free structure. When spray forming flat products like plates and strips a porous boundary layer within the deposit occurs, which necessitates further processing by forming. This paper deals with first results concerning the forming of spray-formed steel parts by rolling as well as concerning the spraying of plates as near as possible to the ideal shape. Especially the impacts of the spray forming and the rolling processes on the structure are illustrated in detail. Furthermore the paper demonstrates the measured deformation resistance of spray-formed and casted steels in rolling.

M. Schünemann

Papers

Generation and forming of spray-formed flat products

Sprühkompaktieren und Walzen von Flachprodukten