Joining Properties Joining metals structuraly by welding, brazing, or soldering, or by such mechanical means as riveting or bolting, is a tremendous help in design and fabrication. When all other properties are equal, material that can be welded has the advantage. Shock and Fatigue Properties Aircraft metals are subject to both shock and fatigue (vibrational) stresses. Fatigue occurs in materials that are exposed to frequent reversals of loading or repeatedly applied loads, if the fatigue limit is reached or exceeded. Repeated vibration or bending will ultimately cause a minute crack to occur at the weakest point. As vibration or bending continues, the crack lengthens until complete failure of the part occurs. This is termed “shock and fatigue failure. ” Resistance to this condition is known as shock and fatigue resistance. It is essential that materials used for critical parts be resistant to these stresses. The preceding discussion of the properties and qualities of metals is intended to show why you must know which traits in metals are desirable and which are undesirable to do certain jobs. The more you know about a given material, the better you can handle airframe repairs. METAL WORKING PROCESSES When metal is not cast in a desired manner, it is formed into special shapes by mechanical working processes. Several factors must be considered when determining whether a desired shape is to be cast or formed by mechanical working. If the shape is very complicated, casting will be necessary to avoid expensive machining of mechanically formed parts. On the other hand, if strength and quality of material are the prime factors in a given part, a cast will be unsatisfactory. For this reason, steel castings are seldom used in aircraft work. There are three basic methods of metal working. They are hot-working, cold-working, and extruding. The process chosen for a particular application depends upon the metal involved and the part required, although in some instances you might employ both hot- and cold-working methods in making a single part. Hot-Working Almost all steel is hot-worked from the ingot into some form from which it is either hot- or cold-worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the interior is still molten. The ingot is then placed in a soaking pit, which retards loss of heat, and the molten interior gradually solidifies. After soaking, the temperature is equalized throughout the ingot, which is then reduced to intermediate size by rolling, making it more readily handled. The rolled shape is called a bloom when its sectional dimensions are 6 x 6 inches or larger and approximately square. The section is called a billet when it is approximately square and less than 6 x 6 inches. Rectangular sections that have width greater than twice the thickness are called “slabs.” The slab is the intermediate shape from which sheets are rolled. HOT-ROLLING.—Blooms, billets, or slabs are heated above the critical range and rolled into a variety of shapes of uniform cross section. The more common of these rolled shapes are sheets, bars, channels, angles, I-beams, and the like. In aircraft work, sheets, bars, and rods are the most commonly used items that are rolled from steel. As discussed later in this chapter, hot-rolled materials are frequently finished by cold-rolling or drawing to obtain accurate finish dimensions and a bright, smooth surface. FORGING.—Complicated sections that cannot be rolled, or sections of which only a small quantity is required, are usually forged. Forging of steel is a mechanical working of the metal above the critical range to shape the metal as desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained. Pressing is used when the parts to be forged are large and heavy, and this process also replaces hammering where high-grade steel is required. Since a press is slow acting, its force is uniformly transmitted to the center of the section, thus affecting the interior grain structure as well as the exterior to give the best possible structure throughout. Hammering can be used only on relatively small pieces. Since hammering transmits its force almost instantly, its effect is limited to a small depth. Thus, it is necessary to use a very heavy hammer or to subject the part to repeated blows to ensure complete working of the section. If the force applied is too weak to reach the center, the finished forging surface will be concave. If the center is properly worked, the surface will be convex 1-25