Martha K. Raymond
Staff Editor
Metals or plastics that hug the smooth contours of sleek new products, such as automobiles, look as though they consist of one piece. Often there’s a lot of assembly hardware that joins parts together, but it’s hidden in joints or behind panels. Besides this mechanical fasteners approach to joining, engineers can weld assemblies together to obtain smooth appearances. Welding and other heat-related joining methods are also very reliable and produce strong joints. So, engineers choosing a fastening system for new applications could find it helpful to look at some recent developments in the field of welding and heat-driven fastening.
Some developments include welding dissimilar lightweight automotive alloys and joining light-gauge aluminum sheet. Many companies are also looking for the convenience of portable welding systems that work in the field, whether on a ship or an oil rig. One of the most significant trends is to develop systems for welding high-strength stainless steels. Welding heat often causes grain boundary corrosion that can decay a weld. “Materials drive many of the advances in the welding industry. For example, stainless steel is tough to weld, especially the higher-strength alloys,” says Steven Sumner, a product-development manager at Lincoln Electric Co., Cleveland.
Among the tools developed to handle stainless are all-position flux-cored electrodes. In addition to welding stainless, they simplify welding procedures in nonconventional locations. Part of a special family of consumables, they promote easy slag removal and manipulation of the puddle while welding vertically and overhead. This is important because many welds in the field must take place out of posittion. Lincoln’s approach was to develop one electrode able to work anywhere in a shipyard. Fabricators then need not stock several different electrodes for working in different orientations. The welds also have a good appearance because, with proper blends of shielding gas, the electrodes produce little or no spatter and create flat, high-quality welds.
One reason, Blue Max All-position Fluxcored electrodes work well is because the slag covers the weld to secure it, and the cooling rate is controlled to yield the correct grain structure, according to Sumner.
These all-position electrodes are recommended for general fabrication of structures and pipes. A typical example is in oil-drilling platforms, where they facilitate precise welds and little wastage. Their special features also allow for a relatively high deposition rate. They are characterized by a smooth arc, weld puddle fluidity, and wetting action.
Another development aimed at simplifying operations is software called WaveDesigner. It lets operators easily modify the wave shape of the welding current. The idea is to make programmable waveform controllers that control welding power easy to use.
An operator designs a waveform through a series of straightforward steps. The first is to select a wave shape approximating the desired one. Editing begins with the selection of a workpoint and wire feed speed. Next comes the setting of pulse variables and arc length. Starting and ending routines get adjusted if necessary. A mouse click then lets the software automatically set the wave-shape parameters. A few click-and-drag operations will change a waveform displayed on the screen. A pulse waveform editor lists values for wire-feed speed, peak amps, peak time, background amps, and background time. Typing in a value or clicking and dragging makes a change. A modification on the screen changes the bead size almost immediately. Another way to change bead profile is to change the frequency by clicking on the end of the whole wave shape and stretching it out.
Waveform manipulations help produce good welds in different materials. “The base material has a significant impact on the welding qualities and welding wire chemistry,” says Lincoln’s Sumner. For example, the melt-off rate, the resistivity, and the thermal conductivity of the wire all differ depending on the base material for which it is intended. With no distinction made between base materials, the power source would otherwise try to run stainless as it would aluminum. In contrast, the Lincoln software lets the operator define one program for stainless and another for aluminum. Each routine can be saved, recalled later, and modified if needed.
When welding aluminum, operators try not to mark or deform the wire, even when it’s soft 4043 aluminum. To this end, one system that straightens kinks or bends in wire coming off the spool is a MIG 5XL Mongoose. It is a highperformance push-pull welding system, from Esab Welding and Cutting Products, Florence, Ky.
A planetary drive in both the wire feeder and gun feeds aluminum up to 0.0625 in. in diameter and steel and flux-core wires to 0.45 in. in diameter. The MIG 5XL planetary drive consists of two hyperbolically ground rollers and feeds wire up to 50 ft. Specialized electronic circuitry synchronizes the speed of the gun and wire-feed motors. Feed rollers rotate 360° around the weld wire providing consistent wire feed. A tension spring and centrifugal arms automatically set correct roll pressure. Feed rolls needn’t change to handle wires of a different size or type, though an easy-to-use spring and/or inlet-outlet guide change may be necessary.
FRICTION WELDS
It used to be difficult to join aluminum and other alloys in an environmentally responsible manner. But The Welding Institute (TWI) in Cambridge England, developed a new process, friction-stir welding (FSW), which is a clean method that welds these materials without using consumables. FSW is patented, so some details of it cannot be disclosed except to licensed users. But some unique characteristics of FSW are described by Joe DeVito, product manager at Esab, which joined TWI, as an original research and development contributor.
Friction-stir welding is a mechanical joining method that doesn’t use heat or an arc. FSW employs a machine tool that passes down between two plates, fusing the plates by plastic flow, never quite reaching the melting point of the material. The process doesn’t join by melting, so it doesn’t need filler metals and shielding gases. This eliminates gas porosity and oxide entrapments to produce welds with mechanical properties typically superior to those of conventional welds.
The FSW process makes welds in a single pass and in any position. There’s no special surface preparation, such as machining or etching. In addition, FSW is a simple, energyefficient, computer-controlled process. The equipment can be easily adapted to existing machine tools and doesn’t require special power supplies.
Friction-stir welding joins aluminum alloys that are in the 2xxx to 7xxx series. The process also joins lead, copper, and titanium alloys.
Light-weight alloys aren’t the only materials now fostering advances in fastening technology. New developments in shipbuilding with highstrength steel are also having an impact. The Naval Sea Systems Command (Navsea), Washington, D.C., is considering using HSLA-65 steel as the primary hull and secondary structural material for future surface combatants. The material is stronger than current highstrength steels and will handle higher stresses. One key benefit of the material is that it may let shipbuilders use thinner plates. The resulting vessels will weigh less, cost less to build, and be cheaper to operate. The Navy figures it could save $25 million for each DD21/SC21-class ship it builds out of HSLA-65.
The Navsea objective is to establish optimum welding procedures for HSLA-65 ship construction that include a selection of consumables and verification of welded joint performance. Of course Navsea and shipbuilders have no production or service experience with the new material. To address this problem, the National Center for Excellence in Metalworking Technology is defining optimum welding procedures using existing consumables compatible with HSLA-65 steel. So far, commercially available consumables have been tested for strength and toughness properties.
JOINING PLASTICS
Plastics are becoming more popular as consumers continue to demand products such as lightweight battery packs for cellular phones and pagers. The right assembly method helps keep joints lightweight, as well. One such method is ultrasonic welding. Because the joining method uses friction to develop heat, joint integrity is determined by either increasing the level of friction or increasing pressure by rubbing two parts of the assembly together. Pressure is a critical element because too much causes the parts to vibrate as an integral structure with no heating. Too little pressure generates too little contact friction or heating. Determining the correct pressure is the challenge when developing joining techniques for new materials and advanced stainless steels, Plastics are becoming more popular as consumers continue to demand products such as lightweight battery packs for cellular phones and pagers. The right assembly method helps keep joints lightweight, as well. One such method is ultrasonic welding. Because the joining method uses friction to develop heat, joint integrity is determined by either increasing the level of friction or increasing pressure by rubbing two parts of the assembly together. Pressure is a critical element because too much causes the parts to vibrate as an integral structure with no heating. Too little pressure generates too little contact friction or heating. Determining the correct pressure is the challenge when developing joining techniques for new materials and advanced stainless steels, according to Jeffrey Frantz, applications and acoustic engineering manager at Branson Ultrasonics Corp., Danbury, Conn. When working with new materials, manufacturers tweak friction settings to find the combination that creates sufficiently strong bonds.
Ultrasonic bonding joins materials when a resonant tool called a horn vibrates two plastic materials against each other, transmitting pulses to the part. Vibration heats and fuses the parts together. To get enough vibrational amplitude and power to melt thermoplastics, ultrasonic assembly takes place at 20 kHz. However, higher frequencies that produce less vibration can also join thermoplastics, especially engineering thermoplastics such as reinforced polymers.
Ultrasonic plastic welders provide consistent bonds for plastic assemblies large and small. Sonic welds are possible in dissimilar materials, but the melting temperatures of both materials must be quite close. Otherwise only the lowertemperature melting material will soften and there won’t be a bond. One ultrasonic welding machine, a welder SureWeld 20, has the power supply integrated into the welding head. The integrated unit provides a lower cost option compared to other ultrasonic welders that include an expensive stand-alone power supply.
Ultrasonic bonding joins material in a number of different ways. For example, ultrasonic welders can bond plastic to plastic or to synthetic fabrics. The equipment can also insert metal parts, such as threaded inserts, into plastic. The integrated welder stakes, welds, rivets, and spot welds plastic assemblies in seconds. It is optimized for bonding large plastic parts or hard-to-weld materials, and is especially useful in the automotive, appliance, packaging, toy, and housewares industries.
Another category of nontraditional joining besides sonics is electromagnetic welding. The process uses inductive energy to heat a magnetically active weld material. This material is made by compounding ferromagnetic particles within a thermoplastic matrix. The technique briefly exposes the joint to an oscillating magnetic field developed by a set of conductive work coils. The material hits fusion temperature within a matter of seconds and flows into the joint cavity. Heat within the material transfers by conduction to the abutting part surfaces causing fusion at the joint interface.
The technique quickly establishes fusion temperatures within the joint without creating excessive induced stress in the assembled parts. It is fast, reliable and overcomes some inherent disadvantages of other conventional bonding techniques. In practice, the time needed for fusion ranges from less than a sec for small part assemblies to as much as 10 to 30 sec for large part assemblies requiring 2 to 20-ft bond lines.
Electromagnetic welding, commonly referred to as the Emaweld process, can process structural, hermetic or pressure-tight welds on almost all thermoplastic materials, for large or small components, using automatic, semiautomatic, or manual operating procedures. The magnetic Emaweld material comes in extruded profiles such as strand, tape, or sheet, plus injection- molded preforms to conform exactly with the joint contour.
Electromagnetic welding creates stronger bonds than many conventional welding techniques. These techniques typically use fiberreinforced and filled materials that often replace part of a thermoplastic matrix, resulting in resin-poor areas.
Another benefit is that electromagnetic welding handles large parts, producing up to a 20-ft bond line in one shot. The key is the strategic placement of the work coils and corresponding joint design.