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Why EB Welding Is Still Cutting Edge Technology
By John DeLalio
The first practical electron beam welding machine was put into use in 1958. Since that time, many other welding methods—such as tungsten inert gas (TIG), metal inert gas (MIG), and laser welding—have evolved technically. However, electron beam (EB) welding is still the absolute best welding method for many critical, high-tech applications. Whether it’s creating high temperature exhaust nozzles for rockets or jet aircraft, or joining cutting-edge 3D printed parts, EB welding has advantages that other welding processes simply don’t.
Following are the top five reasons why electron beam welding is still as cutting edge as it was almost 60 years ago.
EB Welding Has the Deepest Penetration with the Smallest Heat Affected Zone
At the top of the list of EB welding’s admirable qualities is unsurpassable weld penetration. A typical 150kV electron beam welding machine can create a weld spike in steel over 2 inches deep with a heat affected zone less than 0.2 inch wide. Simply put, there is no other welding process that can penetrate that deeply and precisely.
That EB welding can accomplish such a feat is due to the physics of the electron beam welding process. Basically, EB welding works by shooting a high velocity beam of electrons into a part being welded. The electrons penetrate the material at the atomic level, imparting their kinetic energy as they strike molecules. The excited molecules heat up, resulting in a significant amount of energy in a very small area. The part is heated very quickly and very locally to the beam.
Other welding processes rely mainly on heat conduction to transfer energy from the welding device into the part at the point where the welding device touches the surface of the material. Conduction, however, leads to the energy spreading out across the part as it heats, which limits penetration and increases the chances of melting or deforming the workpiece. Recently, there has been some work done to develop lasers that can approach the penetration of an electron beam. However, these lasers require exceptionally high power (close to 100 kW), which makes them both exceptionally expensive and exceptionally dangerous to work with. Electron beam welding technology is proven safe and incredibly effective.
Welding in Vacuum is Ideal for Eliminating Weld Impurities
A beam of accelerated electrons cannot be created or maintained in air because the electrons strike gas molecules and are deflected and scattered. Hence, electron beam welding must occur in a vacuum, and often this is viewed as a criticism: The welding chamber has to be pumped down, and this takes time. Although this requirement is a complication, it is outweighed by the benefits that welding in a vacuum creates.
One of the biggest challenges in welding involves minimizing the impact of the molten metal’s interaction with ambient gases. These gases can react with the metal, creating oxides and other compounds that change the metallurgy of the weld pool and lead to impure welds. Often a cover gas is used to minimize these effects. However, nothing can compare to the cleanliness of welding in a vacuum. In addition to being void of atmospheric gases, some impurities actually burn away during welding, and the result is the purest, cleanest weld there is.
Consider welding titanium: When heated, titanium becomes extremely reactive to the gases in air, resulting in carbides, nitrides, and oxides, which cause brittleness and can reduce fatigue resistance and notch toughness in the heat affected zone of the weld. The backside of the weld is also a problem because it is as prone to these problems as the front. But in the vacuum chamber of an EB welder, pumped down to 10 -4 Torr, these problems simply disappear. The elimination of ambient gases, combined with the energy density of the electron beam, easily creates very strong and aesthetically pleasing welds. As one of our welders once put it, “Titanium welds like butter in an EB machine.”
Aerospace Control and Quality Standards
The EB welding process has, since its inception, been closely tied to the high tech military and civilian aviation industry, as well as the manned space programs of the 1960’s. All of these technologies grew up together. Electron Beam welding was particularly applicable to aerospace applications not only because of the strength of the welds, but because the EB process lends itself to high quality machine controlled welds.
Because of the required vacuum, EB welding cannot be performed by hand. This means that controlling the power of the beam and the motion of the part beneath that beam has to occur with some form of automation. In the early days, this was accomplished by electro-mechanical fixtures and manipulators. However, with the advent of computers, EB machines quickly evolved into full CNC control. For a design engineer, this meant that a very precise weld could be applied in a highly repeatable way.
Because of EB welding’s precision and automated repeatability, the aerospace industry developed quality standards to make sure the human elements of the welding process were tightly controlled. At first, these specifications were created by NASA, Grumman, Lockheed Martin, and other leading aerospace companies. Eventually, industry wide standards were developed, such as Aerospace Material Specification AMS 2680 and AMS2681. These specifications govern all aspects of the welding process, including joint design, material preparation, cleaning, testing, operator training, and process certification. Electron beam welding has a precision, repeatability, and a “built in” culture of mil-spec high quality.
Superior Welding of Materials with High Thermal Conductivity or Unique Properties
The energy and thermodynamic characteristics of an electron beam are very unique. This ability to apply exceptionally high levels of heat energy to a very small area makes it the preferred welding method for many hard-to-weld materials.
Copper is one such material. Copper has superior thermal conductivity, which, for some applications, is a great attribute. But that high thermal conductivity also makes copper notoriously difficult to weld. High thermal conductivity creates challenges for heat conduction-based welding methods, such as MIG and TIG. These methods tend to melt the material on the surface of the weld area while not achieving significant weld penetration. Basically, the heat disperses quickly, either not heating the weld area enough, or overheating the entire part and causing it to melt and warp.
Laser welding is perhaps an option, but weld penetration is limited by not only thermal conductivity, but also reflectivity. The amount of power a laser can apply to a work piece is limited by the reflectivity of molten metal. Essentially, the weld pool becomes a mirror reflecting energy away, again resulting in poor penetration or the over application of power, which can result in melting and distortion of the part. For copper, EB welding is often the most feasible option.
As mentioned earlier, a typical high voltage EB machine can obtain a weld penetration of about 2 inches in steel. This same machine can weld about 0.75 inch deep in aluminum and 0.5 inch in copper. As in steel, the welds will again be very narrow, with a small heat affected zone.
Electron beam welding’s unique ability to throw a lot of energy into a very small area also means that it is a great option for welding dissimilar material combinations where different melting points or conductivity might be a problem, or for welding alloys that are crack sensitive or prone to porosity. 3D printed materials are particularly well suited to EB welding. Typically, metal additive manufacturing relies on melting a powdered material into a solid. This method tends to create voids within the material lattice of the part. When welded, these voids combine, causing significant porosity in the weld. With careful control, an electron beam welder can join parts with minimal porosity issues.
EB Welding is Affordable
EB welding can also be a very cost effective joining technology. It is true that for very large parts or complicated weld paths, EB may not be the best option. Parts have to fit in a vacuum chamber, and the welding beam has to be able to follow the path of the joint. A trained and certified stick welder is very hard to beat from a versatility perspective. However, for smaller parts and high volume, repeatable welds, EB can be amazingly efficient.
As an example, the welding of precision gears for the commercial aviation or medical device industries is an excellent application for EB welding. Gears for these industries require exceptional quality in high volume and at a low cost. In a typical gear assembly, the gear itself is made from a hardened alloy, while the shaft or base is made from a less expensive and lighter alloy. Electron beam welding’s excellence at joining dissimilar materials comes into play at this point. Making the weld strong and pure isn’t an issue, and, fortunately, with a bit of well-engineered tooling and a degree of automation, these high quality welds can be achieved with very short cycle times and low cost.
For certain applications, the quality to cost provided by automated electron beam welding is impossible to beat.
Almost 60 Years Old and Still Going Strong
Electron Beam welding was developed in the late 1950’s, came of age during the 1960’s, and today is a tried and true technology that remains unsurpassed for weld penetration, weld purity, and precision repeatability. The process is highly standardized, with a tradition of high quality baked in. However, the EB welding process also has proven flexibility, adapting with the times such that it is an important part of even the most modern of manufacturing technologies.
Overcoming Porosity in Welding Aircraft Grade Aluminum
The Problem: Welding T2219 Aluminum per AMS2680
A customer recently came to EB Industries with a particular problem: They needed a precision weld on a gas bottle that would be used on helicopters to extend the landing gear in an emergency situation. The bottle had to be light, yet maintain a fairly high gas pressure.
The ideal alloy for the application was T2219 aluminum because of its light weight and high strength. But given the highly varying fatigue loads that the part would undergo, the only acceptable joining process was electron beam welding, as per AMS2680. The problem was that T2219 aluminum is porosity prone when welded, and porosity is not allowed under AMS2680.
Porosity is the tendency for a material to trap gas bubbles in it as it melts and cools. This results in a weld that is weak and unreliable — exactly what you never want in a life critical aerospace application.
Electron beam welding typically utilizes a keyhole type weld. The electrons from the EB welder superheat the material, causing it to vaporize and forming a hole, which is called a “keyhole.” The power and focus of the beam essentially melt this keyhole to a certain depth. The welding process moves the keyhole across the joint, melting fresh material on its leading edge while the trailing edge cools, thus forming the weld.
As the keyhole moves, it can collapse in on itself, trapping gases, which results in gas bubbles in the material — porosity. The width of the keyhole can also cause material to fall in more readily. Usually, controlling the welding feed rate, the focus of the beam, and the amount of power involved solve porosity problems.
But T2219 aluminum has three material properties that make it very challenging to weld. First, the material has a relatively high thermal conductivity, so heat applied to the area of the joint will spread quickly into the surrounding area. Second, T2219 has a relatively low melting point, so any extra heat in the surrounding area tends to melt the material and cause the part to deform or disintegrate. Finally, melted aluminum has a relatively low viscosity such that the keyhole is prone to collapse. This all adds up to make T2219 a very porosity prone alloy and difficult to weld properly.
Normally, the electron beam weld parameters can be adjusted to compensate for the quirks of the material, but AMS2680 is very specific and stringent.
Aerospace Material Specification 2680 was created to control the electron beam welding process utilized for fatigue critical applications, such as gas bottles used in aerospace applications. The following describes the overall use of the specification:
“This specification defines the procedures and requirements for joining metals and alloys using the electron beam welding process….These procedures are used typically for high quality, electron-beam welding of aerospace components, the failure of which could cause loss of the aerospace vehicle or one of its major components, loss of control, or significant injury to occupants of a manned aerospace vehicle, but usage is not limited to such applications.”
AMS2680 has many specific requirements for joint preparation, cleaning, EB weld processing in vacuum, operator training, machine calibration, and part inspection, to name a few. But a few requirements of the specification make this application particularly challenging. The part had to have a full penetration weld that was X-ray inspected to assure that the existence of any porosity did not exceed the limits defined in the specification.
The reason for a full penetration weld and no porosity has to do with stress concentration in a part subjected to varying loads. The loads on the walls of a gas bottle constantly vary as it undergoes pressurization and depressurization. Internal pressure causes the walls to flex, with stresses being distributed evenly across the cross section of the material. However, a partial penetration weld, or pores left behind from a low quality weld, causes the stresses to concentrate in these areas. The stress concentration load can easily be higher than the material’s strength, causing cracks. Combined with the varying load, the cracks would eventually open and the part would fail in a catastrophic manner, possibly causing loss of life.
Can the problem be solved by using less power? No: AMS2680 calls for a full penetration weld, so high power is required to achieve the desired penetration. Can the feed rate be increased or decreased? No: fast feed rates cause the keyhole to collapse in, creating porosity, and slow feed rates lead to too much energy flowing into the surrounding material, resulting in a very large area of melt, which deforms the entire part.
There were a number of elements involved in solving this particular welding challenge. As part of its standard operating procedure, EB Industries optimized the focus, power, and feed rate of its EB welder. A little bit of drift in any parameter can cause problems, but the company’s machines are tightly maintained and fully computer controlled so that any weld performed is highly accurate and repeatable.
The key trick is adding a copper heat sink around the part in the area of the weld. The heat sink allowed EB Industries to put a significant amount of energy directly into the area of the weld to achieve full penetration, as per AMS2680. Outside of the weld area, the heat sink bleeds off the excess heat, minimizing the chance of deforming the gas bottle. The result is a full penetration weld with a keyhole that closes up quickly enough to prevent any porosity in the joint.
John DeLalio is a mechanical engineer and the Director of Business Development for EB Industries, Farmingdale, N.Y.
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