Fig. 6.29. Gas Metal Arc Welding (GMAW) Schematic
Heat input during welding should be kept as low as practical. For multiple bead or multiple layer welds, several small beads should be used instead of a few large heavy beads. Aluminum and titanium readily form high melting oxide compounds. Even under ideal welding conditions, aluminum and titanium oxides form and float to the top of the molten weld puddle. When depositing multiple pass weldments, the oxide particles can accumulate to the point that a scale is formed that can inhibit proper fusion. Also, flakes of oxide scale can become entrapped in the weldment. The flakes can act as mechanical stress raisers and significantly reduce joint efficiency and service life. The oxide film should be removed by abrasive blasting, or by grinding, when it becomes heavy enough to be visually apparent on the weld surface. Wire brushing is not recommended, because it does not actually remove the oxide film but only polishes it, hiding it from sight.
Another welding process used is linear friction welding, in which blades are attached to a hub to produce an integrated one piece engine rotor. In this method, one part is oscillated back and forth in a straight line, with a force applied normal to the plane of oscillation. The oscillatory motion, combined with the pressure, creates the heat needed to soften and upset the metal, forming a solid state joint in which the original surfaces are expelled as flash. Although machining this flash is an exacting process, in some situations, this still produces a significant cost savings as compared to machining the entire assembly from a large forging.
Since the aging cycle(s) used for the precipitation hardening alloys can introduce residual stresses that can cause joint cracking, the precipitation hardened alloys are brazed in the solution treated condition and then aged after brazing. Nickel brazing alloys produce brazed joints with strength and oxidation resistance for service at temperatures up to 2000° F. While the joints have good strength, often approaching that of the base metal (Fig. 6.30), they generally exhibit only moderate ductility.
Nickel brazing alloys (Table 6.4) contain 70-95% nickel. Generally, these alloys contain boron and/or silicon, which are melting point depressants and act as oxide reducing agents. In many commercial brazing filler metals, the levels are 2-3.5% boron and 3-10% silicon. Chromium, in amounts up to 20%, is often present to provide oxidation and corrosion resistance; however, higher amounts tend to lower the joint strength. Braze alloys are often produced in the form of powder (200 mesh or finer). They are also available as powder impregnated sheets that can be cut or formed into rings, washers, and other shapes. Powder can be applied directly to the joints; however, it is usually mixed with a binder and applied as a slurry by brushing, spraying, or dipping. Acrylic resins are often used as binders, because they do not leave a residue that could contaminate the joint. It is important that the slurry be allowed to thoroughly dry before brazing.
Four factors that are important in achieving high strength brazed joints include: (1) joint design, (2) close control of joint clearances, (3) elimination of flux inclusions and unfused areas, and (4) effective wetting of the base material by the brazing alloy.
• Lap joints which load the brazed joint in shear are much stronger than butt joints that load the joint in tension.
• For high strength joints, joint clearances should be small. Clearances should be between 0.0005 and 0.005 in. Slightly larger clearances, along with some defects, such as flux inclusions or incomplete brazed areas, can be tolerated if the lap joint is designed with an increased overlap (i.e., 3X the thickness). The best approach is to design parts that are self-locating; however, if this is not possible, then mechanical fixturing, riveting, bolting, or spot welding can be used.
• In all brazing operations, it is essential that the parts are thoroughly cleaned and that all oxides have been removed from the surfaces. Grinding is frequently used to insure the removal of oxide films. To improve wetting, fluoride based fluxes can be used, or the surfaces can be plated with nickel. It is imperative that all flux residue be removed after brazing to avoid corrosive attack of the base metal. Flux residue is generally glass like and quite tenacious, requiring grinding, chipping, or abrasive blasting for removal. Fluxes should not be used in a vacuum environment.
• Precipitation hardening alloys present several difficulties not normally encountered with solid solution alloys. Precipitation hardening alloys often to
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