Par 4-62 Page 4-61
(5) Immediately electroplate the part using any solutions, except copper or silver. Both of these require undercoats.
(6) Rinse and dry.
n. Plating on Cast Iron and High-Carbon Steels (Steels Heat Treated to 180,000 psi).
(1) Electroclean the area using DC until water does not break on the surface. This electroclean process should be accomplished at 12 V to 20 V, using the appropriate electrocleaning solution.
(2) Rinse the area thoroughly in cold, clean tap water.
(3) Reverse-current etch at 8 V to 10 V, using the appropriate etching solution, until a uniform gray is obtained.
(4) Rinse thoroughly.
(5) Remove surface smut with 15 V to 25 V using the appropriate activating solution.
(6) Rinse thoroughly.
(7) Electroplate immediately, using any of the solutions, except copper or silver (both of these require undercoats).
(8) Rinse and dry.
o. Plating on Ultrahigh Strength Steels (Heat Treated Above 180,000 psi).
(1) Electroclean the area using reverse current until water does not break on the surface.
This electroclean process should be accomplished at 8 V to 12 V using the appropriate electroclean solution.
(2) Rinse the area thoroughly in cold, clean tap water.
(3) Immediately electroplate the part, using either nickel, chromium, gold, or cadmium.
Other metals require an undercoat of one of the above. Plate initially at the highest voltage recommended for the solution so as to develop an initial barrier layer. Then reduce to standard voltage.
(4) Rinse and dry.
(5) Bake the part for 4 hours at 375 °F ±25 °F.
NOTE: Where the solution vendor provides substantiating data that hydrogen embrittlement will not result from plating with a particular solution, then a postbake is not required. This substantiating data can be in the form of aircraft industry manufacturer’s process specifications, military specifications, or other suitable data.
Par 4-62 Page 4-62 NOTE: Acid etching should be avoided, if possible. Where etching is absolutely necessary, it should always be done with reverse current. Use alkaline solutions for initial deposits.
p. Dissimilar Metals and Changing Base. As a general rule, when plating two dissimilar metals, follow the plating procedure for the one with the most steps or activation. If activating steps have to be mixed, use reverse current activation steps prior to DC activation steps.
q. Plating Solution Selection.
(1) Alkaline and neutral solutions are to be used on porous base metals, white metals, high-strength steel, and for improved coating ability. Acid solutions are to be used for rapid buildup and as a laminating structure material in conjunction with alkaline-type solutions.
(2) Chrome brush-plating solutions do not yield as hard a deposit as bath-plating solutions. The hardness is about 600 Brinell as compared to 1,000 Brinell for hard chrome deposited from a tank.
(3) Silver-immersion deposits will form with no current flowing on most base metals from the silver brush-plating solutions. Such deposits have poor adhesion to the base metal.
Consequently, a flash of a more noble metal should be deposited prior to silver plating to develop a good bond.
(4) In general, brush plating gives less hydrogen embrittlement and a lower fatigue strength loss than does equivalent tank deposits. However, all brush-plated, ultrahigh strength steel parts (heat treated above 180,000 psi) should be baked, as mentioned, unless it is specifically known that embrittlement is not a factor.
r. Qualification Tests. All brush-plated surfaces should be tested for adhesion of the electrodeposit. Apply a 1-inch wide strip of 3M Scotch Flatback Masking Tape 250, or an approved equal, with the adhesive side to the freshly plated surface. Apply the tape with heavy hand pressure and remove it with one quick motion perpendicular to the plated surface. Any plating adhering to the tape should be cause for rejection.
s. Personnel Training for Quality Control. Manufacturers of selective-plating equipment provide training in application techniques at their facilities. Personnel performing selective plating must have adequate knowledge of the methods, techniques, and practices involved. These personnel should be certified as qualified operators by the manufacturers of the products used.
4-63.–4-73. [RESERVED.]
Par 4-74 Page 4-63 SECTION 5. WELDING AND BRAZING
4-74. GENERAL. This section covers weld repairs to aircraft and component parts only.
Observe the following procedures when using such equipment as gas tungsten arc welding (GTAW), Gas Metal Arc Welding (GMAW), plasma arc welding, and oxyacetylene gas welding.
When repairs of any of these flight-critical parts are required, it is extremely important to make the weld repairs equal to the original weld. Identifying the kind of metal to be welded, identifying the kind of welding process used in building the part originally, and determining the best way to make welded repairs are of utmost importance.
a. Welding is one of the three commonly used methods of joining metals without the use of fasteners. Welding is done by melting the edges of two pieces of metal to be joined and allowing the molten material to flow together so the two pieces will become one.
b. Brazing is similar to welding in that heat is used to join the material; but rather than melting, the metal is heated only enough to melt a brazing rod having a much lower melting point. When this brazing rod melts, it wets the surfaces to be joined, and when it cools and solidifies, it bonds the pieces together.
c. Soldering is similar to brazing except that brazing materials normally melt at temperatures above 425 ºC (800 °F), while solders melt at temperatures considerably lower.
d. The next step in making airworthy weld repairs is to decide the best process to use, considering the available state-of-the-art welding equipment, and then deciding the correct weld-filler material to use. Before any weld repairs can be made, the metal parts to be welded must be cleaned properly, fitted and jigged properly, and all defective welds must be removed to prepare for an aircraft quality weld repair.
e. Finally, after the weld is completed, the weld must be inspected for defects. All these things are necessary in order to make an airworthy weld repair.
f. Aircraft Welding Qualifications. Four groups of metals a person can be certified and qualified to use are:
(1) Group 1, 4130 Steel.
(2) Group 2, Stainless Steel.
(3) Group 3, Aluminum
(4) Group 4, Titanium.
g. For other group listing of metal the welder may qualify, refer to AMSSTD1595.
h. Most large business or agencies conduct their own certification tests, or they have an outside testing lab validate the certification tests.
Par 4-75 Page 4-64
4-75. EQUIPMENT SELECTION. Use the welding equipment manufacturer’s information to determine if the equipment will satisfy the requirements for the type of welding operation being undertaken. Disregarding such detailed operating instructions may cause substandard welds. For example, when using GTAW equipment, a weld can be contaminated with tungsten if the proper size electrode is not used when welding with DC reverse polarity. Another example, the depletion of the inert gas supply below the critical level causes a reduction in the gas flow and will increase the danger of atmospheric contamination.
a. Electric welding equipment versatility requires careful selection of the type of current and polarity to be used. Since the composition and thickness of metals are deciding factors, the selection may vary with each specific application. Metals having refractory surface oxide films
(i.e., magnesium alloys and aluminum and its alloys), are generally welded with AC, while DC is used for carbon, low alloy, noncorrodible, and heat-resisting steels. General recommendations covering current and polarity are shown in Table 4-12.
TABLE 4-12. CURRENT AND POLARITY SELECTION FOR INERT GAS WELDING
MATERIAL
ALTERNATING
CURRENT (AC) DIRECT
CURRENT (DC) With High-Frequency Stabilization Straight Polarity Magnesium up to 1/8 in. thick 1 N.R.
Magnesium above 3/16 in. thick 1 N.R.
Magnesium castings 1 N.R.
Aluminum up to 3/32 in. thick 1 N.R.
Aluminum over 3/32 in. thick 1 N.R.
Aluminum castings 1 N.R.
Stainless steel 1 Low carbon steel, 0.015 to 0.030 in. 1 Low carbon steel, 0.030 to 0.125 in. N.R. 1 1 Recommended N.R. Not Recommended
b. Oxyacetylene gas equipment is suitable for welding most metals. It is not, however, the best method to use on such materials as stainless steel, magnesium, and aluminum alloys;
because of base metal oxidization, distortion, and loss of ductility.
NOTE: If oxyacetylene is used for welding stainless steel or aluminum, all flux must be removed, as it may cause corrosion.
4-76. ACCURATELY IDENTIFY THE TYPE OF MATERIAL TO BE REPAIRED. If positive identification of the material is not possible, contact the aircraft manufacturer or subject the item to a metallurgical laboratory analysis. Before any welding is attempted, carefully consider the weldability of the alloy, since all alloys are not readily weldable. The following steels are readily weldable: plain carbon (of the 1000 series), nickel steel (of the SAE 2300 series), chrome-nickel alloys (of the SAE 3100 series), chrome-molybdenum steels (of the SAE 4100 series), and low nickel-chrome-molybdenum steel (of the SAE 8600 series).
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.