The production of magnesium integral foam components with a dense shell and a porous core is investigated. High pressure casting methods are used where liquid magnesium mixed with a blowing agent is injected into a permanent steel mould. A compact shell develops due to fast cooling at the walls. Larger cooling times in the core allow the decomposition of the blowing agent and the evolution of a foam structure. The resulting integral foams show a high weight‐specific stiffness combined with high energy absorption capability. For the first time, foam stabilizing without additives is realized. Stabilization is by foaming during solidification with the primary α‐phase particles acting as obstacles slowing down cell wall thinning.
Integral foam moulding (IFM) is an economical way to produce castings with integrated cellular structure, i.e. a solid skin and a foamed core. IFM has been known for polymers for more than four decades and is well established in industrial production. Polymer integral foam parts are accepted as a material system with own properties which simplifies designs, reduces production costs and weight, and increases stiffness and overall strength. On the other hand, integral foam moulding for metals is a new field of research. The development of metal based integral foam moulding processes at WTM moves along analogous paths as that of polymers by transferring and adapting successful moulding technologies for polymer integral foam to metals. Two moulding techniques for metal integral foam are presented, a low and a high pressure process. In the low pressure process, the molten metal charged with blowing agent is injected into a permanent steel mould without completely filling it. In this case, the mould gets eventually filled by foam expansion.In the high pressure process foaming is initiated by expansion of the mould cavity after it has been filled completely with the mixture of the metal melt and the blowing agent. The moulded parts are characterized with respect to their cellular structure, density profile and pore size distribution. Mechanical properties such as stiffness and damping behaviour are discussed.
Rolling element bearings for aero engine applications have to withstand very challenging operating conditions because of the high thermal impact due to elevated rotational speeds and loads. The high rate of heat generation in the bearing has to be sustained by the materials, and in the absence of lubrication these will fail within seconds. For this reason, aero engine bearings have to be lubricated and cooled by a continuous oil stream. When the oil has reached the outer ring it has already been heated up, thus its capability to remove extra heat from the outer ring is considerably reduced. Increasing the mass flow of oil to the bearing is not a solution since excess oil quantity would cause high parasitic losses (churning) in the bearing chamber and also increase the demands in the oil system for oil storage, scavenging, cooling, hardware weight, etc. A method has been developed for actively cooling the outer ring of the bearing. The idea behind the outer ring cooling concept was adopted from fins that are used for cooling electronic devices. A spiral groove engraved in the outer ring material of the bearing would function as a fin body with oil instead of air as the cooling medium. The method was first evaluated in an all steel ball bearing and the results were a 50% reduction in the lubricating oil flow with an additional reduction in heat generation by more than 25%. It was then applied on a Hybrid ball bearing of the same size and the former results were reconfirmed. Hybrid bearings are a combination of steel made parts, like the outer ring, the inner ring, and the cage and of ceramic rolling elements. This paper describes the work done to-date as a follow up of the work described in, and demonstrates the potential of the outer ring cooling for a bearing. Friction loss coefficient, Nusselt number, and efficiency correlations have been developed on the basis of the test results and have been compared to correlations from other authors. Computational Fluid Dynamics (CFD) analysis with ANSYS CFX has been used to verify test results and also for parametric studies.
Integral foam molding (IFM) is an economical near net-shape technology to produce monolithic castings with solid skin, foamed core and continuous density transition between skin and core. It was developed for polymers in the 1960s. But it took about ten years to optimize the molding technique and the quality of the polymer integral foam parts to a marketable stage. Meanwhile, polymer integral foam parts are established in a lot of commercial applications. The situation is completely different for metals. The first attempts to produce metal integral foam parts were made only a few years ago. A cost effective injection molding technique for magnesium integral foam was developed at the WTM-Institute in Erlangen, Germany. The low-cost production in combination with the integral foam properties low density, high weight specific bending stiffness and remarkable damping capacity is very promising. At the beginning we produced only parts with simple shape, for example plates. But if we think about commercial applications, more complex parts with a three-dimensional shape are required. The focus of this paper is on new developments concerning the production of magnesium integral foam parts with complex shape like a casing cover or a door handle.
Bearings for aero engine applications are subjected to a high thermal impact because of the elevated speeds and loads. The high rate of heat generation in the bearing cannot be sustained by the materials used and in the absence of lubrication will fail within seconds. For this reason aero engine bearings have to be lubricated and cooled by a continuous oil stream. The heat which is generated in the bearings through friction is transferred into the oil. Oil itself has not unlimited capabilities and can only remove heat as long as its temperature does not reach critical limits. When the critical limits have been reached or even exceeded the oil will suffer chemical decomposition (coking) with loss of its properties and subsequently causing a detrimental impact on the rotating machinery. Oil is normally transferred into the bearings through holes in the inner ring thus taking advantage of the centrifugal forces due to the rotation. In its way through the bearing the oil continuously removes heat from the inner ring, the rolling elements and the bearing cage until it reaches the outer ring. Since the oil has already been heated up its capability to remove heat from the outer ring is considerably reduced. The idea to provide the bearing with an “unlimited” quantity of oil to ensure proper cooling cannot be considered since an increase in the oil quantity leads to higher parasitic losses (churning) in the bearing chamber and increased requirements on the engine’s lubrication system in terms of storage, scavenging, cooling, weight, etc, not mentioning the power needed to accomplish all these. In this sense, the authors have developed a method which would enable active cooling of the outer ring. Similar to fins which are used for cooling electronic devices, a spiral groove engraved in the outer ring material would function as a fin body with oil instead of air as the cooling medium. The number of “threads” and the size of the groove design characteristics were optimized in a way that enhanced heat transfer is achieved without excessive pressure losses. An experimental set up was created for this reason and a 167.5mm PCD (Pitch Circle Diameter) ball bearing was investigated. The bearing was tested with and without the outer ring cooling. A reduction of 50% of the lubricant flow through the inner ring associated with a 30% decrease in heat generation was achieved.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.