Controlling remelting processes for superalloys and aerospace Ti alloys

Remelting is performed to facilitate the production of clean, fully dense, homogeneous castings of superalloys and aerospace titanium alloys and is crucial to the defect-free production of these important materials. Modern electroslag remelting and vacuum arc remelting control systems are closed-loop, single input-single output systems that oversimplify the physical properties of the processes; the ever-increasing demand for cleaner, more highly engineered, chemically tuned alloys has pushed these control methodologies to their limit. A new generation of these controllers is being developed by the Specialty Metals Process Consortium and Sandia National Laboratories to answer the challenges of remelting control for the next generation of alloys; these control systems will use multiple sensor inputs and apply material-specific system and process models. D.K. Melgaard earned his Ph.D. in computer science at Kansas State University in 1976. He is currently a member of the technical staff at Sandia National Laboratories. R.L. Williamson earned his Ph.D. in chemistry at the University of Washington in 1983. He is currently a principal member of the technical staff at Sandia National Laboratories. Dr. Williamson is also a member of TMS. J.J. Beaman earned his Sc.D. in mechanical engineering at Massachusetts Institute of Technology in 1979. He is currently the Andersen professor in manufacturing systems engineering at the University of Texas.     

Filtering and fluxing processes for aluminum alloys

… two processes that have been developed for improving the quality of molten aluminum alloys before casting.     

Point defects in binary Laves phase alloys

Point defects in the binary C15 NbCr₂ and NbCo₂, and C14 NbFe₂ systems on both sides of stoichiometry were studied by bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000°C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were observed in NbCr₂ Laves phase alloys after quenching from 1400°C. However, there are essentially no thermal vacancies in NbFe₂ alloys after quenching from 1300°C. Anti-site hardening was found on both sides of stoichiometry for all the three Laves phase systems studied. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.     

A new fixture for FSW processes of titanium alloys

FSW of titanium alloys is nowadays one of the most challenging welding operations, even with a solid state process, due to the thermo-mechanical and thermo-chemical characteristics of such materials. Due to the relevant application of titanium alloys in the aeronautic and aerospace industries, in the recent years few attempts were carried out to develop FSW processes aimed to maximize the mechanical performances of the welded parts. In the paper a new fixture is presented allowing obtaining effective FSW joints of titanium blanks, which were investigated through mechanical and metallurgical tests highlighting the peculiarities of FSW of titanium alloys.     

Advanced titanium alloys and processes for minimally invasive surgery

Major advances continue to be made in enhancing patient care while at the same time attempting to slow ever-rising health costs. Among the most innovative of these advances are minimally invasive surgical techniques, which allow patients to undergo life-saving and quality-of-life enhancing surgery with minimized risk and substantially reduced hospital stays. Recently this approach was introduced for orthopedic procedures (e.g., during total hip replacement surgery). In this instance, the implantable devices will bear the same loads and will therefore be subject to higher stress. This paper provides a brief overview of several potential approaches for developing new advanced titanium alloys and processes that should provide substantial benefit for this application in minimally invasive devices. For more information, contact H.J. Rack, Clemson University, School of Materials Science and Engineering, 213 Olin Hall, Clemson, SC 29634, USA; rackh@clemson.edu.     

Oxidation at defects in scales on iron-chromium alloys

A prior investigation on the lateral spreading of oxide into defects in Wustite scales on iron was extended to study the same phenomena in Fe-Cr alloys. Included were two Fe-Cr-Mo alloys and an Fe-25Cr-6Al alloy. Three types of experiments were conducted to study flaws introduced to simulate damage to protective oxide layers caused by particle erosion. It was found that outer scales of Wustite on the Cr-Mo alloys spread into flaws in much the same way as Wustite on unalloyed iron. However, inner scales of (Fe,Cr)₃O₄ on the Cr-Mo alloys and the scale formed on the Fe-25Cr-6Al alloy had only a slight tendency to spread into flaws. These results are consistent with the known higher diffusion coefficients and higher defect concentrations in Wustite than in other oxide phases.     

Microstructure and corrosion behavior of FeCrNiMnMox high-entropy alloys fabricated by the laser surface remelting

In this study, an Nd:YAG laser was used to carry out laser surface remelting treatment on FeCrNiMnMo_x (x = 0, 0.5, 1) alloys. A study was conducted on the potential impact of Mo on the microstructure and corrosion resistance of the laser-remelted layer. According to the research results, FeCrNiMnMo_x alloys were more effective in refining the dendrites, compared with the matrix, whereas the FeCrNiMn alloys' remelted layer exhibited an almost single face-centered cubic (FCC) structure. In comparison, FeCrNiMnMo_0.5 and FeCrNiMnMo₁ alloys' remelted layer displayed the FCC and σ phase. In addition, the dendrite crystals' microstructure can be clearly refined by Mo alloying. Mo is effective in improving the corrosion resistance of the FeCrNiMnMo_x alloys' remelted layer in 3.5% NaCl solution. The pitting resistance of Mo-containing-remelted layers is significantly higher, compared with Mo-free alloy's remelted layer, and the FeCrNiMnMo_0.5-remelted layer shows the most satisfactory corrosion resistance. As revealed by X-ray photoelectron spectroscopy analyses, the addition of molybdenum promotes the generation of Cr₂O₃ and enhances the corrosion resistance of the remelted layer.     

Structure design of high-performance Cu-based shape memory alloys

The effects of various structure factors on the properties(superelasticity mainly) of Cu-based shape memory alloys(SMAs) were systematically evaluated in this review article through literatures combining with our work. It is concluded that besides the decisive role of grain orientation, the grain boundary(GB) characteristics also play important roles in the superelasticity, which include GB area, GB type, GB morphology and GB direction in descending order of the effect significance. According to the above results, the prior principles of structure design are proposed for high-performance Cu-based SMAs from most to least important:(1) obtaining grain orientation with high phase transformation strain;(2) increasing grain size or reducing GB area;(3) obtaining straight low-energy GBs, especially low-angle GBs;(4) trying to make GB direction parallel to external stress. Consistent with the main or all principles, the bamboo-like-grained and columnar-grained(CG) Cu-based SMAs show excellent comprehensive properties.     

Optimal modifying and refining processes for A356 aluminium alloys

M illions of alum inium alloy com ponents are produced using A356 alum inium alloys, which provide high fluidity, good 'castability'and m echanical properties. A356 alloys are widely used to fabricate structural castings for autom otive and other industrial applications. For α (Al) refining,A356 alum inium alloys usually apply Al-Tim aster alloys in the range 0.08-0.20% Ti, resulting in high consum ption of Al-Ti m aster alloys and higher production cost.M odification ofthe Al-Sieutectic …     

低成本高性能粉末冶金钛合金

粉末冶金是短流程制备低成本、高性能钛及钛合金的有效方法。低成本氢化脱氢（HDH）钛粉可用于制备粉末冶金钛合金制件,但由于受间隙原子含量高、烧结致密度低和微观组织粗大等因素影响,使粉末冶金钛制品的组织性能优势得不到发挥。实验采用氢化脱氢钛粉—冷等静压—真空烧结的技术路线制备了Ti-6Al-4V烧结坯,间隙原子含量低（O<0.16 wt.%, N<0.05 wt.%, H<0.015 wt.%）,具有均匀细小的近等轴?组织,良好的室温拉伸性能（UTS>930 MPa, YS>870 MPa, El>14%）。实验同时表明了HDH工艺制备低间隙原子含量钛粉的可行性,间隙原子含量的增加主要源于粉末及压坯的操作、转移和储存过程。得益于粉末冶金钛合金的细晶和近终成形特点,它无需通过开坯锻造,并且近成型的烧结坯能够提高材料利用率,减少后续热加工变形量及加工道次。因此,以粉末钛合金烧结坯替代锻坯进行后续的塑性加工能够大幅度降低钛合金构件及型材的成本。     

Investigation of surfacing processes using aluminium-based alloys

Summary

It has been recently reported that porous Ti-N sprayed coatings can be made fine-structured by laser irradiation. This paper describes an investigation of the effects of infiltrated metal species on the wear resistance of Ti-N remelted layers.

Non-ferrous metal powders were sprayed on SS400 steel plate substrates in an argon atmosphere. The coating thickness was around 200 μm. Pure titanium was also sprayed on the non-ferrous sprayed coatings in a nitrogen atmosphere. The coating thickness was around 400 μm. Coating specimens consisting of non-ferrous and Ti-N layers were remelted by laser in a nitrogen atmosphere. The remelted layers of the coatings had a fine microstructure with a hardness value above HV1000. The wear resistance of the Ti-N coatings was appreciably improved by remelting.     

Point defects and their properties in FeAl and FeSi alloys

Positron annihilation as well as differential dilatometric measurements were performed to measure the vacancy concentration in FeAl and FeSi. The highest vacancy concentration is found at the stoichiometric B2 composition with more than 3.5% at the melting point in FeAl. Additionally, diffusion and isotope effect experiments were carried out in B2 FeAl to obtain information about the diffusion mechanism. The isotope effect decreases when the material transforms from D0₃ to B2. The effective formation volume of the defects in the B2 phases is larger (1.4Ω) than one atomic volume Ω and is related to a defect involving more than one vacancy. The effective migration enthalpy of 0.5–1.8 eV as well as the effective formation enthalpy of 0.7–1.02 eV vary with the concentration of the alloys in an opposite manner. The results for the FeAl system suggest that different defect types may operate as diffusion vehicles in the different phases. The mobility of the defects dominates the thermomechanical behavior of FeAl alloys. The defect production during mechanical deformation of FeAl alloys is divided into two parts. The annealing out of mechanically produced defects occurs on the same time rate as that of thermal defects.     

Structural and phase states and irradiation-induced defects in Ni-Cr alloys

The structural and phase states in alloys of the Ni-Cr-Mo system which were induced by both heat aging and electron irradiation at elevated temperatures have been studied by the methods of measurement of residual resistivity and positron annihilation. Migration of irradiation-induced defects during irradiation at 300°C is shown to initiate processes of ordering or phase separation depending on the initial alloy microstructure and chromium content. It has been established that in the alloy with 32 wt % Cr the concentration of accumulated vacancy defects in the state of short-range ordering after irradiation with 5-MeV electrons to a dose of ～1.5 × 10^?4 dpa at 200°C is half as high as that in the state of long-range ordering with a homogeneous distribution of domains (to 10 nm in size) of the ordered Ni₂Cr phase in the matrix.     

Modeling of diffusion processes during carburization of alloys

A Model was developed for the finite-difference calculation of carburization profiles in high-temperature alloys. The method includes the ternary crossdiffusion effect due to substitutional alloying elements that are preferentially oxidized. It can be used to treat cases such as carburization of preoxidized alloys or simultaneous oxidation and carburization. Up to three distinct types of carbide precipitation reactions can be included in the calculations. The solubility product is computed for each reaction and the amount of C that reacts is removed from the diffusion process. The model can treat two sets of boundary conditions corresponding to the presence or absence of a protective oxide scale. For comparison, under protective conditions carburization profiles were obtained for preoxidized alloys using C¹⁴ radioactive tracer. The application of the model yields values for the diffusivities of C in the alloys tested. Under nonprotective conditions, the predictions of the model were compared to carburization profiles reported for pack carburization tests. The method is able to predict maximum carbide levels and penetration depths for different alloys in various conditions and can be used in the selection and design of high-temperature alloys for use in carburizing environments.