Par 4-77 Page 4-65
4-77. PREPARATION FOR WELDING.
a. Hold elements to be welded in a welding jig or fixture which is sufficiently rigid to prevent misalignment due to expansion and contraction of the heated material and which positively and accurately positions the pieces to be welded.
b. Clean parts to be welded with a wire brush or other suitable method prior to welding.
Do not use a brush of dissimilar metal, such as brass or bronze on steel. The small deposit left by a brass or bronze brush will materially weaken the weld, and may cause cracking or subsequent failure of the weld. If the members are metallized, the surface metal may be removed by careful sandblasting followed by a light buffing with emery cloth.
4-78. INSPECTION OF A COMPLETED WELD. Visually inspect the completed weld for the following:
a. The weld has a smooth seam and uniform thickness. Visual inspection shall be made of the completed weld to check for undercut and/or smooth blending of the weld contour into the base metal.
b. The weld is tapered smoothly into the base metal.
c. No oxide has formed on the base metal more than 1/2 inch from the weld.
d. There are no signs of blowholes, porosity, or projecting globules. Many military specifications, as well as ASTM codes, specify acceptable limits of porosity and other types of defects that are acceptable.
e. The base metal shows no signs of pitting, burning, cracking, or distortion.
f. The depth of penetration insures fusion of base metal and filler rod.
g. The welding scale is removed. The welding scale can be removed using a wire brush or by sandblasting. Remove any roll over, cold lab, or unfused weld metal. Check underside of welded joint for defects.
4-79. MICROFISSURES. Cracks in parts and materials can vary from tiny microfissures that are visible only with magnification, to those easily identified by unaided eyes. Microfissures are the worst type of defect for two reasons: they are often hard to detect, and they produce the worst form of notch effect/stress concentration. Once they form, they propagate with repeated applications of stress and lead to early failures. Every possible means should be used to detect the presence of cracks, and ensure their complete removal before welding operations proceed.
(See Figure 4-26.)
Par 4-79 Page 4-66 FIGURE 4-26. COMMON DEFECTS TO AVOID WHEN FITTING AND WELDING
AIRCRAFT CERTIFICATION CLUSTER 4-
80. NONDESTRUCTIVE TESTING. Nondestructive testing or evaluation is advisable in critical applications. Nondestructive testing methods such as; magnetic particle, liquid penetrant, radiography, ultrasonic, eddy current, and acoustic emission can be used; however, they require trained and qualified people to apply them.
4-81. PRACTICES TO GUARD AGAINST. Do not file or grind welds in an effort to create a smooth appearance, as such treatment causes a loss of strength. Do not fill welds with solder, brazing metal, or any other filler. When it is necessary to weld a joint which was previously welded, remove all of the old weld material before rewelding. Avoid welding over a weld, because reheating may cause the material to lose its strength and become brittle. Never weld a joint which has been previously brazed.
4-82. TORCH SIZE (Oxyacetylene welding). When using oxyacetylene welding, the torch tip size depends upon the thickness of the material to be welded. Commonly used sizes, proven satisfactory by experience, are shown in Table 4-13.
TABLE 4-13. TORCH TIP SIZES Thickness of Steel (in inches) Diameter of Hole in Tip Drill Size
0.015 to 0.031 0.026 71
0.031 to 0.065 0.031 68
0.065 to 0.125 0.037 63
0.125 to 0.188 0.042 58
0.188 to 0.250 0.055 54
0.250 to 0.375 0.067 51
4-83. WELDING RODS AND ELECTRODES. Use welding rods and electrodes that are compatible with the materials to be welded. Welding rods and electrodes for various applications have special properties suitable for the application intended. Lap welds are used in shear applications. The weld throat of the fillet weld is considered the plane 45 degrees to the surface
Par 4-83 Page 4-67 plane of the sheet being welded and is equal to 0.707 times the thickness of the sheet stock. (See Figure 4-27.)
FI GURE 4-27. LAP WELD STRENGTH CALCULATION 4-
84. ROSETTE WELDS. Rosette welds are generally employed to fuse an inner reinforcing tube (liner) with the outer member. Where a rosette weld is used, drill a hole, (in the outside tube only) of sufficient size to insure fusion of the inner tube. A hole diameter of approximately onefourth the tube diameter of the outer tube serves adequately for this purpose. In cases of tightfitting sleeves or inner liners, the rosettes may be omitted. Rosette weld edge distance is 1/2 the diameter of the tube, as measured from the edge of the rosette hole to the end of the inside and outside tube. Rosettes shall not be considered when determining the strength of a welded form.
Drill an 1/8-inch hole in the lower tube in the center of the intended rosette weld so the heat does not burn away the outer tube. This small hole tends to bleed off the heat from the torch and keeps the size of the rosette small.
4-85. HEAT-TREATED MEMBERS. Certain structural parts may be heat treated and, therefore, could require special handling. In general, the more responsive an alloy steel is to heat treatment, the less suitable it is for welding because of its tendency to become brittle and lose its ductility in the welded area. Weld the members which depend on heat treatment for their original physical properties by using a welding rod suitable for producing heat-treated values comparable to those of the original members. (See paragraph 4-74.) After welding, heat treat the affected members to the manufacturer’s specifications.
Par 4-86 Page 4-68
4-86. TYPES OF WELDING.
a. Gas Welding. A fuel gas such as acetylene or hydrogen is mixed inside a welding torch with oxygen to produce a flame with a temperature of around 6,300 °F (3,482 °C). This flame is used to melt the materials to be welded. A filler rod is melted into the puddle of molten metal to reinforce the weld. When highly reactive metals such as aluminum are gas welded, they must be covered with flux to exclude oxygen from the molten metal and keep oxides from forming which would decrease the strength of the weld. (An illustration of a carburizing flame, a neutral flame, and an oxidizing flame is shown in Figure 4-28.)
FIGURE 4-28. BASIC GAS-WELDING FLAMES: EACH HAS A DISTINCTIVE SHAPE, COLOR, AND SOUND; NEUTRAL FLAME IS THE MOST USED
b. S hielded Metal Arc Welding (SMAW). This method is the most familiar and common type and is known in the trade as stick welding. A metal wire rod coated with a welding flux is clamped in an electrode holder connected to the power supply with a heavy electrical cable. The metal to be welded is also attached to the power supply. The electrical power is supplied to the work at a low voltage and high current and may be either AC or DC, depending upon the type of welding being done. An arc is struck between the rod and the work and produces heat in excess of 10,000 °F, which melts both the material and the rod. As the flux melts, it releases an inert gas which shields the molten puddle from oxygen in the air and prevents oxidation. The molten flux covers the weld and hardens to an airtight slag cover that protects the weld bead as it cools. This slag must be chipped off to examine the weld.
c. Gas Metal Arc Welding (GMAW). This method of welding was formerly called Metal Inert Gas (MIG) welding, and is an improvement over stick welding because an uncoated wire electrode is fed into the torch and an inert gas such as argon, helium, or carbon dioxide flows out around the wire to protect the puddle from oxygen. The power supply connects between the torch and the work, and the arc produces the intense heat needed to melt the work and the electrode.
Low-voltage high-current DC is used almost exclusively with GMAW welding. GMAW is used more for large-volume production work than for aircraft repair.
Par 4-86 Page 4-69
d. Gas Tungsten Arc Welding (GTAW). This is the form of electric arc welding that fills most of the needs in aircraft maintenance. It is more commonly known as tungsten inert gas (TIG) welding.
(1) Rather than using a consumable electrode such as is used in both of the other two methods we have discussed, the electrode in TIG welding is a tungsten rod. (In earlier procedures using this form of welding, a carbon electrode was used, but it has been replaced almost exclusively with tungsten.)
(2) The 250+ amp arc between the electrode and the work melts the metal at 5,432 °F, and a filler rod is manually fed into the molten puddle. A stream of inert gas such as argon or helium flows out of the torch and envelopes the arc, thereby preventing the formation of oxides in the puddle.
(3) The versatility of TIG welding is increased by the power supply that is used. DC of either polarity or AC may be used. (See Figures 4-29 and 4-30.)
FIGURE 4-29. SET TIG WELDER TO DC CURRENT, STRAIGHT POLARITY FOR
WELDING MILD STEEL, STAINLESS STEEL, AND TITANIUM
Par 4-86 Page 4-70 FIGURE 4-30. SET TIG TO AC CURRENT FOR WELDING ALUMINUM
AND MAGNESIUM 4-
87. ELECTRIC-RESISTANCE WELDING. Many thin sheet metal parts for aircraft, especially stainless steel parts, are joined by one of the forms of electric resistance welding, either spot welding or seam welding.
a. Spot Welding. Two copper electrodes are held in the jaws of the spot welding machine, and the material to be welded is clamped between them. Pressure is applied to hold the electrodes tightly together, and electrical current flows through the electrodes and the material.
The resistance of the material being welded is so much higher than that of the copper electrodes that enough heat is generated to melt the metal. The pressure on the electrodes forces the molten spots in the two pieces of metal to unite, and this pressure is held after the current stops flowing long enough for the metal to solidify. Refer to MIL-HDBK-5 or MMPDS-10 for joint construction and strength data. The amount of current, pressure, and dwell time are all carefully controlled and matched to the type of material and the thickness to produce the correct spot welds. (See Figure 4-31.)
Par 4-87 Page 4-71 FIGURE 4-31. IN SPOT WELDING, HEAT IS PRODUCED BY ELECTRICAL RESISTANCE BETWEEN COPPER ELECTRODES; PRESSURE IS
SIMULTANEOUSLY APPLIED TO ELECTRODE TIPS TO FORCE METAL TOGETHER TO COMPLETE FUSING PROCESS; SPOT-WELD NUGGET SIZE IS
DIRECTLY RELATED TO TIP SIZE
b. S eam Welding. Rather than having to release the electrodes and move the material to form a series of overlapping spot welds, a seam-welding machine is used to manufacture fuel tanks and other components where a continuous weld is needed. Two copper wheels replace the bar-shaped electrodes. The metal to be welded is moved between them, and electric pulses create spots of molten metal that overlap to form the continuous seam.
4-88. BRAZING. Brazing refers to a group of metal-joining processes in which the bonding material is a nonferrous metal or alloy with a melting point higher than 425 ºC (800 °F), but lower than that of the metals being joined. Brazing includes silver brazing (erroneously called silver soldering or hard soldering), copper brazing, and aluminum brazing.
NOTE: Never weld over a previously brazed joint.
a. Brazing requires less heat than welding and can be used to join metals that are damaged by high heat. However, because the strength of brazed joints is not as great as welded joints, brazing is not used for structural repairs on aircraft. In deciding whether brazing of a joint is justified, it should be remembered that a metal, which will be subjected to a sustained high temperature in use, should not be brazed.
b. A brazing flux is necessary to obtain a good union between the clean base metal and the filler metal. There are a number of readily available manufactured fluxes conforming to American Weld Society (AWS) and AMTECH specifications.
Par 4-88 Page 4-72
c. The base metal should be preheated slowly with a mild flame. When it reaches a dull-red heat (in the case of steel), the rod should be heated to a dark (or purple) color and dipped into the flux. Since enough flux adheres to the rod, it is not necessary to spread it over the surface of the metal.
d. A neutral flame is used in most brazing applications. However, a slightly oxidizing flame should be used when copper-zinc, copper-zinc-silicon, or copper-zinc-nickel-silicon filler alloys are used. When brazing aluminum and its alloys, a neutral flame is preferred, but if difficulties are encountered, a slightly reduced flame is preferred to an oxidizing flame.
e. The filler rod can now be brought near the tip of the torch, causing the molten bronze to flow over a small area of the seam. The base metal must be at the flowing temperature of the filler metal before it will flow into the joint. The brazing metal melts when applied to the steel and runs into the joint by capillary attraction. In braze welding, the rod should continue to be added, as the brazing progresses, with a rhythmic dipping action; so that the bead will be built to a uniform width and height. The job should be completed rapidly and with as few passes of the rod and torch as possible.
f. When the job is finished, the metal should be allowed to cool slowly. After cooling, remove the flux from the parts by immersing them for 30 minutes in a lye solution.
(1) Copper brazing of steel is normally done in a special furnace having a controlled atmosphere, and at a temperature so high that field repairs are seldom feasible. If copper brazing is attempted without a controlled atmosphere, the copper will probably not completely wet and fill the joint. Therefore, copper brazing in any conditions other than appropriately controlled conditions is not recommended.
(a) The allowable shear strength for copper brazing of steel alloys should be 15,000 KSI for all conditions of heat treatment.
(b) The effect of the brazing process on the strength of the parent or base metal of steel alloys should be considered in the structural design. Where copper furnace brazing is employed, the calculated allowable strength of the base metal, which is subjected to the temperatures of the brazing process, should be in accordance with Table 4-14.
TABLE 4-14. CALCULATED ALLOWABLE STRENGTH OF BASE METAL Material Allowable Strength Heat-treated material (including normalized) used in “as-brazed” condition Mechanical properties of normalized material Heat-treated material (including normalized) reheat-treated during or after brazing Mechanical properties corresponding to heat treatment performed
(2) Alloys commonly referred to as silver solders melt above 425 °C (800 °F), and when using them the process should be called silver brazing.
Par 4-88 Page 4-73
(a) The principal use of silver brazing in aircraft work is in the fabrication of high-pressure oxygen lines and other parts which must withstand vibration and high temperatures. Silver brazing is used extensively to join copper (and its alloys), nickel, silver, various combinations of these metals, and thin steel parts. Silver brazing produces joints of higher strength than those produced by other brazing processes.
(b) It is necessary to use flux in all silver-brazing operations, because of the necessity for having the base metal chemically clean, (without the slightest film of oxide to prevent the silver-brazing alloy from coming into intimate contact with the base metal).
(c) The joint must be physically and chemically clean, which means it must be free of all dirt, grease, oil, and paint. After removing the dirt, grease, and paint, any oxide should be removed by grinding or filing the piece until bright metal can be seen. During the soldering operation, the flux continues the process of keeping oxide away from the metal and aids the flow of solder.
(d) In Figure 4-32, three types of joints for silver brazing are shown: flanged butt, lap, and edge joints. If a lap joint is used, the amount of lap should be determined according to the strength needed in the joint. For strength equal to that of the base metal in the heated zone, the amount of lap should be four to six times the metal thickness.
FIGURE 4-32. SILVER BRAZING JOINTS (
e) The oxyacetylene flame for silver brazing should be neutral, but may have a slight excess of acetylene. It must be soft, not harsh. During both preheating and application of the solder, the tip of the inner cone of the flame should be held about 1/2 inch from the work.
The flame should be kept moving so that the metal will not become overheated.
(f) When both parts of the base metal are at the right temperature (indicated by the flow of flux), brazing alloy can be applied to the surface of the under or inner part at the edge of the seam. It is necessary to simultaneously direct the flame over the seam, and keep moving it so that the base metal remains at an even temperature.
Par 4-88 Page 4-74
(3) The torch can be shut off simply by closing the acetylene off first and allowing the gas remaining in the torch tip to burn out. Then turn off the oxygen valve. If the torch is not to be used again for a long period, the pressure should be turned off at the cylinder. The hose lines should then be relieved of pressure by opening the torch needle valves and the working pressure regulator, one at a time, allowing the gas to escape. Again, it is a good practice to relieve the oxygen pressure and then the acetylene pressure. The hose should then be coiled or hung carefully to prevent damage or kinking.
(4) Soft soldering is used chiefly for copper, brass, and coated iron in combination with mechanical seams; that is, seams that are riveted, bolted, or folded. It is also used where a leak-proof joint is desired, and sometimes for fitting joints to promote rigidity and prevent corrosion. Soft soldering is generally performed only in very minor repair jobs. This process is used to join electrical connections because it forms a strong union with low electrical resistance.
(a) Soft solder gradually yields under a steadily applied load and should not be used unless the transmitted loads are very low. It should never be used as a means of joining structural members.
(b) A soldering iron is the tool used in soldering. Its purpose is to act as a source of heat for the soldering operation. The bit, or working face, is made from copper since this metal will readily absorb heat and transmit it to the work. Figure 4-33 shows a wedge-shaped bit.
FIGURE 4-33. ELECTRIC SOLDERING IRON (
c) To tin the soldering iron, it is first heated to a bright red, and then the point is cleaned (by filing) until it is smooth and bright. No dirt or pits should remain on its surface. After the soldering iron has been mechanically cleaned, it should be reheated sufficiently to melt solder and chemically cleaned by rubbing it firmly on a block of sal ammoniac (ammonium chloride). Rosin flux paste may also be used. Solder is then applied to the point and wiped with a clean cloth.
(d) A properly tinned copper iron has a thin unbroken film of solder over the entire surface of its point.
(e) Soft solders are chiefly alloys of tin and lead. The percentages of tin and lead vary considerably in various solder, with a corresponding change in their melting points, ranging from 145-311 ºC (293–592 ºF). Half-and-half (50/50) solder is a general purpose solder and is most frequently used. It contains equal proportions of tin and lead, and it melts at approximately 182 ºC (360 ºF).
Par 4-90 Page 4-75
(f) The application of the melted solder requires somewhat more care than is apparent. The parts to be soldered should be locked together or held mechanically or manually while tacking. To tack the seam, the hot copper iron is touched to a bar of solder, then the drops of solder adhering to the copper iron are used to tack the seam at a number of points. The film of solder between the surfaces of a joint must be kept thin to make the strongest joint.
(g) A hot, well-tinned soldering copper iron should be held so that its point lies flat on the metal (at the seam), while the back of the copper iron extends over the seam proper at a 45-degree angle, and a bar of solder is touched to the point. As the solder melts, the copper iron is drawn slowly along the seam. As much solder as necessary is added without raising the soldering copper iron from the job. The melted solder should run between the surfaces of the two sheets and cover the full width of the seam. Work should progress along the seam only as fast as the solder will flow into the joint.
4-89. AIRCRAFT PARTS NOT TO BE WELDED.
a. Brace Wires and Cables. Do not weld aircraft parts whose proper function depends upon strength properties developed by cold-working. Among parts in this classification are streamlined wire and cables.
b. Brazed and Soldered Parts. Do not weld brazed or soldered parts as the brazing mixture or solder will penetrate and weaken the hot steel.
c. Alloy Steel Parts. Do not weld alloy steel parts such as aircraft bolts, turnbuckle ends, etc., which have been heat treated to improve their mechanical properties.
d. Nos. 2024 and 7075 Aluminum. Do not weld these two aluminum alloys (that are often used in aircraft construction) because the heat from the welding process will cause severe cracking. The 2024 aluminum is most often used in wing skins, fuselage skins, and in most structured airframe parts. The 7075 aluminum is most often used in machined fittings such as wing spar attachments, landing-gear attachments, and other structural parts.
4-90. WELDING ROD SELECTION. Most aircraft repair shops that are prepared to make weld repairs should have the basic selection of welding rods available. The best rods to stock, the metals they weld, and the AWS specification numbers are shown in Table 4-15.
Par 4-90 Page 4-76 TABLE 4-15. CHART SHOWING WELDING FILLER ROD SELECTION Welding Rod # AMS Spec. AWS Spec. Welds These Metals 4130 AMS 6457 AWS A5.18 Mild steel, 4130 steel 4140 AMS 6452 AWS A5.28 4140 steel 4043 AMS 4190 AWS A5.10 Most weldable aluminum 308L AMS 5692 AWS A5.9 304 stainless steel 316L AMS 5692 AWS A5.9 316 stainless steel AZ61A AMS 4350 AWS A5.19 AZ61A magnesium ERTi-5 AMS 4954 AWS A5-16 Titanium
4-91. REPAIR OF TUBULAR MEMBERS.
a. Inspection. Prior to repairing tubular members, carefully examine the structure surrounding any visible damage to insure that no secondary damage remains undetected.
Secondary damage may be produced in some structure, remote from the location of the primary damage, by the transmission of the damaging load along the tube. Damage of this nature usually occurs where the most abrupt change in direction of load travel is experienced. If this damage remains undetected, subsequent normal loads may cause failure of the part.
b. Location and Alignment of Welds. Unless otherwise noted, welded steel tubing may be spliced or repaired at any location along the length of the tube. To avoid distortion, pay particular attention to the proper fit and alignment.
c. Members Dented at a Cluster. Repair dents at a steel-tube cluster joint by welding a specially formed steel patch plate over the dented area and surrounding tubes. (See Figure 4-34.)
To prepare the patch plate, cut a section of steel sheet of the same material and thickness as the heaviest tube damaged. Trim the reinforcement plate so that the fingers extend over the tubes a minimum of 1.5 times the respective tube diameter. (See Figure 4-34.) Remove all the existing finish on the damaged cluster-joint area to be covered by the reinforcement plate. The reinforcement plate may be formed before any welding is attempted, or it may be cut and tack-welded to one or more of the tubes in the cluster joint, then heated and formed around the joint to produce a smooth contour. Apply sufficient heat to the plate while forming so that there is generally a gap of no more than 1/16 inch from the contour of the joint to the plate. In this operation avoid unnecessary heating, and exercise care to prevent damage at the point of the angle formed by any two adjacent fingers of the plate. After the plate is formed and tack-welded to the cluster joint, weld all the plate edges to the cluster joint.