In vacuum processing, metal surfaces remain very clean and free of oxides. When these near-perfect surfaces are in contact with other surfaces, certain elements have a tendency to interact between the surfaces through solid state diffusion. Therefore, a major consideration when selecting both hearth and load fixturing materials for vacuum heat treating is the possibility of solid state diffusion between different materials in contact at high temperatures. Solid state diffusion of certain elements can cause the formation of a lower melting point alloy called a eutectic. For example, solid state diffusion between carbon and nickel can begin to occur at temperatures as low as 1165ºC (2130ºF) and cause local melting, also known as eutectic melting.

The air we breathe contains a number of elements that can react with metals under the proper conditions. Moisture, oxygen, carbon dioxide and hydrogen are present in significant amounts in our atmosphere. Each can react to varying degrees with many different metals. While many of these reactions occur to only a small extent at room temperature, they are often greatly accelerated in the presence of heat. Consider the example of a piece of polished metal held over a heat source. It will eventually turn blue or black as the elements in the atmosphere react with the hot metal.

Thermal spray processes like air plasma spray and High Velocity Oxygen Fuel (HVOF) are usually thought of as being used primarily for applying protective coatings to new parts. While new part applications do indeed constitute the majority of their use, there are also a wide variety of repair techniques that employ thermal spray processes. VAC AERO has been a leader in developing repairs for aircraft structural components and gas turbine engine parts using thermal spray processes. An example of a structural component repair involves a flap track from a popular turbo-prop aircraft.  As the wing flaps of this aircraft are extended and retracted during landing and take-off, rollers run along the surfaces of a series of components known as flap tracks.

The purchase of a vacuum furnace involves a considerable capital investment. As a result, the question of buying a used furnace at a lower cost than a new furnace is a fairly common one. However, there are a number of potential issues with used equipment that should underscore the warning “buyer beware”. To begin with, good used vacuum furnaces are a rare commodity. When they do appear on the market, they don’t last long. Many of the best are purchased through industry networking and never reach the general market. Still, there are numerous dealers of used furnace equipment with inventories posted on their websites.

By J.E. Pritchard, S. Rush, A. Kiela

VAC AERO International Inc.

Abstract

Application of sacrificial coatings has long been used to reduce rotor-shroud clearances in gas turbine engines. Materials normally used for these coatings include sintered metal-powder segments, sintered metal-fibre segments, metallic honeycomb (filled and unfilled), elastomers and thermally sprayed abradable coatings. Thermally sprayed coatings offer advantages over the other materials, including direct application, easy removal and repair, variety of coating materials and good performance. New abradable thermal spray coating materials have been developed for performance in industrial turbine engines at operating temperatures up to 980ºC. Results are presented from laboratory evaluations of these coatings by burner rig and hot abradability testing.

by J.E. Pritchard & R. Laub

When joining aluminum for aerospace electronics, brazing often is the most practical choice for creating a continuous all-metal joint interface.

Because of its light weight and excellent thermal conductivity, aluminum often is the material of choice for assemblies that house or cool airborne electronics. Aluminum's properties are particularly important in combat aircraft. Weight minimization becomes a major design consideration for many components going into these aircraft. Thermal conductivity is especially important in the electronics packages because of the heat problems created by the dense packing of powerful systems in limited spaces. The complex aluminum enclosures, chassis and heat dissipators used in military avionics systems often are manufactured from numerous individual components, which must then be joined.

Building a Lean Supply Chain

Lean shouldn't stop with your company; to be truly lean, you must reach beyond your shop floor.

Companies of all sizes, shapes, and descriptions are attempting to implement lean manufacturing. As a company becomes lean, however, it inevitably runs into constraints imposed by its suppliers and customers, the adjacent nodes of the supply chain in which it operates. The logical next step to becoming lean internally is to try to spread the lean philosophy outward to those immediate trading partners. Sometimes doing so can bring significant improvements, especially when a large company is influencing one or more of its smaller suppliers. At other times it doesn't work so well, such as when a small machine shop attempts to influence a much larger raw-material supplier.