Welding Small Components
Welding tiny components presents some unique problems related to thermal effects. For example, the steels used in such components have melting temperatures in excess of 2500°F, while local surface temperatures at the weld may exceed 3000°F. Extreme care must be taken to avoid damaging adjacent components with heat from the weld zone.
Other problems include weld and base metal cracking, distortion, or the deposit of metallic vapors on electrical surfaces, which can destroy a sensitive component's ability to function.
Weld beads must have smooth transitions with the base metal to avoid the development of stress concentration areas, which lead to early fatigue failures. And undercutting, concavity, lack of penetration, and other visual defects are critical.
Some tiny components, such as sensors, frequently include diaphragms or other parts as thin as 0.001". This thinness offers little resistance to bending or buckling. Obviously, these parts are easily distorted from the heat of welding unless special precautions are taken.
Thin diaphragms are particularly sensitive to variations in the welding process. Small diameter, circumferential welds create expansion and contraction that cause buckling deformation and tensile stresses, jeopardizing the contact necessary for welding. When separation occurs, it causes instantaneous burn-through and melt-back.
Occasionally, difficult-to-weld materials are involved. Hastelloy, beryllium, brass, and aluminum are typical. Heat treatable alloys require special consideration during the development of weld parameters to preserve the special hardness, strength, or corrosion resistance characteristics for which the material was selected.
In some instances, the weld joint is a combination of thin and thick materials producing a heat unbalance during welding. With some welding techniques, the thinner detail would reach its melting temperature and melt back before the thicker member.
Heating unbalances can also occur when materials of dissimilar conductivity are welded. An example is joining copper to stainless steels. Not only do they differ in thermal conductivity, but also in their melting temperatures.
Each metal joining process has its unique characteristics. When selecting a process for a specific joining application, such questions such as depth of penetration, joint preparation, cleaning, inert gas or vacuum shielding, weld joint accessibility, proximity to heat sensitive materials, productivity, and cost must be answered.
Though electron beam, laser beam, plasma arc, and gas tungsten arc are the dominant choices for miniature component welding, electron beam and laser are the ideal technical choices. Their precise narrow welds and low total energy input prevent distortion and minimize heat-affected zones. Both processes produce welds of high metallurgical quality.
In view of these significant advantages, one might question why any welding process other than electron beam or laser would be considered. Perhaps the most comprehensive answer is cost. The capital investment starts at $150,000. Depending on energy output levels, automation, number, range of motion, and other levels of sophistication, the cost can quite easily exceed several hundred thousand dollars.
Because of this high initial capital equipment cost, manufacturers must give serious consideration to utilization rates if they plan to acquire electron beam or laser welding for in-house use. This problem is easily solved by using those job shops specializing in the use of this high tech welding equipment.
Let's take a look at the characteristics of different welding options.
Laser Welding--Laser welding's focused energy beam diameters are 0.030" or considerably less. As a result, weld joint fit-ups must be machined to close tolerances. Joint gaps less than 0.004" are desirable, with a goal of zero gap. Gaps exceeding 0.005" quickly become unweldable--there is simply no fusion material available. Occasionally, filler wire or shims can be secured along the joint, adding greatly to the cost of the process. Automatic filler wire feed is very difficult because it must be precisely fed into a very small weld pool with exact and unforgiving timing.
However, when the workpiece can be designed and fabricated in anticipation of using either electron beam or laser, the processes are unrivaled. Their lack of distortion, minimal defects, high metallurgical quality, and, especially for lasers, exceptional potential for high volume automation, can make these processes very cost effective.
The primary reason for the laser's automation capability is their freedom of movement. Laser focusing and beam delivery optics are readily incorporated in computer programmed, multi-axes motion systems. High volume production assemblies can be manipulated under the programmed welding optics. Also, the laser emits a beam of light as its welding energy, free of restraining cables, electrical connections, and arc length limitations.
Second, other than cleanliness, special environments are not needed for the laser to function. If inert gas shielding is needed to protect the workpiece from oxidation, it is easily added and controlled in the programmed weld parameters. Freedom from the physical limits of vacuum chambers enables large workpieces to be welded.
Third, the raw, collimated laser beam can be projected great distances to time sharing workstations.
The laser's most significant limitation is weld penetration. Generally, the process is limited to a thickness of less than 0.100". Although very high power lasers are available to increase penetration, techniques such as "keyholing" are more cost effective for processes such as drilling, trepanning, and cutting. Fortunately, weld penetrations of less than 0.100" meet most tiny component requirements.
Electron Beam Welding--Electron beam welding has penetration capability exceeding 10". Materials with low thermal conductivity can be penetrated by the stream of focused electrons to even greater thickness. In this regard, the electron beam process has no rival.
Electron beam and laser beam welding are readily adapted to small production lots and prototypes. In fact, their computer-stored parameters are quickly set up for repeated small or single piece production lots.
Also, their modern vacuum chambers, with advanced seals and high performance pumping systems, can be evacuated rapidly. Thus, the electron beam process becomes an economical joining process. With a microscope viewer, the electron beam becomes a super accurate tool with resolutions of beam placement of about 0.001". Thin, critical welds have become commonplace for this process.
Along with laser welding, electron beam welding offers an excellent solution to the heat imbalance problem associated with thin/thick material weld joints. Their precisely targeted beams can be focused on the thicker detail, thus compensating for the heating unbalance. This avoids using fixtures to cool the thin component and decrease the heat unbalance.
Plasma Arc Welding--Plasma arc and gas tungsten arc welding equipment is relatively inexpensive. Although they cannot duplicate the energy density, precision, and limited peripheral effects of the laser and electron beam welding processes, they are well suited for small assemblies designed in anticipation of their use. Of these two production workhorses, plasma arc welding more closely satisfies the need for the welding precision usually required for small components.
The plasma arc and the plasma are internal to the torch. The plasma, at temperatures as high as 30,000F near the tungsten electrode, exits from the torch through a small diameter, aerodynamically designed constricting orifice. The orifice collimates the plasma and dramatically concentrates its energy into a beam-like, high velocity configuration.
The constrained plasma column produces narrow welds with the ability to penetrate deeper than the gas tungsten arc process at the same energy levels. More importantly, the higher energy density of the concentrated plasma more rapidly heats the weld joint and significantly reduces the heat affected zone next to the weld.
Plasma arc equipment and power supplies incorporate all the electronic control circuitry and components necessary for repeatable weld processes. These features include control of pulse rates, pulse profiles, current sloping, and arc polarity. Automatic arc length control is also integrated to accommodate variations in weld joint elevation.
Gas Tungsten Arc Welding--The use of gas tungsten arc welding became prominent in the 1940s when the aircraft industry recognized its advantages over gas welding. Helium was the first inert gas used for electrode and weld joint shielding. As a result, the process was called Heliarc. Later, argon was substituted for the helium and the process was labeled "TIG" for Tungsten Inert Gas, recognizing the use of either helium or argon inert gasses. The name has been further refined to Gas Tungsten Arc Welding (GTAW) since other gasses, such as hydrogen, are sometimes combined.
The GTAW process continues to be a production workhorse in industries where metal welding is required. Although most often performed manually, it sometimes is assembled into fully automated systems.
In the hands of a skilled welder, the process is extremely flexible. The welder's skill enables him to readily adapt to joint configurations and compensate for fit-up tolerances and other variables.
Joint gaps, mismatch, and other irregularities unacceptable for precision automated machine welding, are easily accommodated by the skilled welder. Unlike the machine dependent laser and electron beam welding processes, there is no need for precise and consistent joint preparation.
The intrinsic value of manual welding, whether GTAW or plasma arc, is the ability to produce high quality welds with maximum flexibility. Machine welding requires a defined parameter, limiting in-process manipulation and correction.
Immediate process availability for production, minimal development, and low capital equipment cost are somewhat offset by the labor intensity, with quality dependent on the skill and consistency of the welder. Transfer of an operation from one welder to another, shift to shift, or plant to plant can become difficult.
The broad arc of the gas tungsten arc process, and its much lower rate of energy input to the workpiece, result in a wide, large volume melt and a more extensive heat affected zone. These conditions can easily become the source for distortion, cracking, porosity and stresses.
The above characteristics, which limit GTAW for precision and critical, heat sensitive component welds, initiated the development of the plasma arc welding process.
Choosing the Best Process
It is impossible to state without reservation which welding process should be used for maximum efficiency in a given production application. There can be many specific variables and subtleties. There are, however, some generalizations to help define a given process.
Electron beam is a candidate for penetration beyond 0.250" without preparation of the weld joint with filler metal. Both electron beam and laser are good choices for critical, heat sensitive weld joints and widely dissimilar materials.
When distortion to any degree is unacceptable, laser, electron beam, and plasma arc are suitable choices. For high volume, long production run welding of small component assemblies, laser offers the best approach.
For maximum flexibility, immediate use, minimal development, lower critical joint tolerances, and low capital investment, plasma arc and gas tungsten arc are the dominant choices.
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