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.
Par 4-91 Page 4-77 FIGURE 4-34. FINGER PATCH REPAIRS FOR MEMBERS DENTED AT A CLUSTER
d. Mem bers Dented in a Bay. Repair dented, bent, cracked, or otherwise damaged tubular members by using a split-sleeve reinforcement. Carefully straighten the damaged member, and in the case of cracks, drill No. 40 (0.098 inch) stop holes at the ends of the crack.
4-92. REPAIR BY WELDED SLEEVE. This repair is outlined in Figure 4-35. Select a length of steel tube sleeve having an inside diameter approximately equal to the outside diameter of the damaged tube and of the same material, and at least the same wall thickness. Diagonally cut the sleeve reinforcement at a 30-degree angle on both ends so that the minimum distance of the sleeve from the edge of the crack or dent is not less than 1-1/2 times the diameter of the damaged tube. Cut through the entire length of the reinforcement sleeve, and separate the half-sections of the sleeve. Clamp the two sleeve sections to the proper positions on the affected areas of the original tube. Weld the reinforcement sleeve along the length of the two sides, and weld both ends of the sleeve to the damaged tube. (See Figure 4-35.) The filling of dents or cracks with welding rod in lieu of reinforcing the member is not acceptable.
FIGURE 4-35. MEMBERS DENTED IN A BAY (REPAIRS BY WELDED SLEEVE)
DENTED OR BENT TUBE
CRACKED TUBE
REINFORCEMENT TUBE SPLIT NOTE:
LOCALLY DENTED OR
BENT MEMBERS SHOULD
FIRST BE REFORMED IN CLAMP.
REINFORCEMENT SLEEVE TO BE OF SAME
MATERIAL AND AT LEAST THE SAME GAUGE AS TUBE BEING REPAIRED.
1-1/2 A ‹WELD 11-1/2 A *
AS ALTERNATIVE TO SPLIT TUBE,
A TWO-PIECE REINFORCEMENT
SLEEVE MAY BE FORMED FROM STEEL SHEET OF THE SAME MATERIAL 300 AND AT LEAST THE SAME GAUGE AS 1A → • 1A ~ THE DAMAGED TUBE. USE FISHMOUTH 1-1/2AM •1-1/2A•
ENDS AND FOUR ROSETTE WELDS AS SHOWN.
Par 4-92 Page 4-78
Par 4-93 Page 4-79
4-93. REPAIR BY BOLTED SLEEVE. Do not use bolted-sleeve repairs on welded steel-tube structure unless specifically authorized by the manufacturer or the FAA. The tube area removed by the bolt holes, in this type of repair, may prove critical.
4-94. WELDED-PATCH REPAIR. Dents or holes in tubing may be repaired by using a patch of the same material, one gauge thicker. (See Figure 4-36.)
FIGURE 4-36. WELDED-PATCH REPAIR
a.
Dented Tubing.
(1) Dents are not deeper than 1/10 of tube diameter, do not involve more than 1/4 of the tube circumference, and are not longer than tube diameter.
(2) Dents are free from cracks, abrasions, and sharp corners.
(3) The dented tubing can be substantially reformed, without cracking, before application of the patch.
b. Punctured Tubing. Holes are not longer than tube diameter and involve not more than 1/4 of tube circumference.
4-95. SPLICING TUBING BY INNER-SLEEVE METHOD. If the damage to a structural tube is such that a partial replacement of the tube is necessary, the inner-sleeve splice is recommended; especially where a smooth tube surface is desired. (See Figure 4-37.)
a. Make a diagonal cut when removing the damaged portion of the tube, and remove the burr from the edges of the cut by filing or similar means. Diagonally cut a replacement steel tube of the same material and diameter, and at least the same wall thickness, to match the length of the removed portion of the damaged tube. At each end of the replacement tube allow a 1/8-inch gap from the diagonal cuts to the stubs of the original tube. Select a length of steel tubing of the same material, and at least the same wall thickness, and of an outside diameter equal to the inside diameter of the damaged tube. Fit this inner-sleeve tube material snugly within the original tube, with a maximum diameter difference of 1/16 inch. From this inner-sleeve tube material cut two
Par 4-95 Page 4-80 sections of tubing, each of such a length that the ends of the inner sleeve will be a minimum distance of 1-1/2-tube diameters from the nearest end of the diagonal cut.
b. If the inner sleeve fits very tightly in the replacement tube, chill the sleeve with dry ice or cold water. If this is insufficient, polish down the diameter of the sleeve with emery cloth.
Tack the outer and inner replacement tubes using rosette welds. Weld the inner sleeve to the tube stubs through the 1/8-inch gap, forming a weld bead over the gap.
4-96. SPLICING TUBING BY OUTER-SLEEVE METHOD. If partial replacement of a tube is necessary, make the outer-sleeve splice using a replacement tube of the same diameter.
Since the outer-sleeve splice requires the greatest amount of welding, it should be used only when the other splicing methods are not suitable. Information on the replacement by use of the outer-sleeve method is given in Figure 4-38 and Figure 4-39.
FIGURE 4-37. SPLICING BY INNER-SLEEVE METHOD ALLOW 1/8" GAP BETWEEN ORIGINAL AND ROSETTE WELD- REPLACEMENT TUBES FOR WELDING.
ORIGINAL TUBE 30 ISLEEVE TUBE INSIDE 1/4A -A- I1/2A
REPLACEMENT TUBE
ROSETTE WELD FOR WELDIN CAP 1/4A TUBE - /1/2A --1/2A Par 4-96 Page 4-81
FIGURE 4-38. SPLICING BY OUTER-SLEEVE METHOD (REPLACEMENT BY
WELDED OUTSIDE SLEEVE) 30° SLEEVE 30
ROSETTE WELDS MAY -- WELD
BE OMITTED WHEN SLEEVES FIT TIGHTLY.
- REPLAGEMENT TUBE
ORIGINAL TUBE , FISH-MOUTH 1/2A - SLEEVE 1/2A 1/4A
ROSETTE WELDS MAY AWELD
BE OMITTED WHEN SLEEVES FIT TIGHTLY.
- REPLACEMENT TUBE
ORIGINAL TUBE
FOUR ROSETTE WELD HERE FIRST ORIGINAL TUBE WELDS → 11/40 +1/244 30 • 1/8" GAP T1'½A-71/29 1½A → FOR WELDING ALTERNATIVE SPLIT SLEEVE SPLICE ~ IF OUTSIDE DIAMETER OF ORIGINAL TUBE IS LESS THAN 1 INCH, SPLIT SLEEVE MAY BE MADE FROM STEEL TUBE OR SHEET STEEL. USE SAME MATERIAL OF AT LEAST THE SAME GAUGE.
Par 4-96 Page 4-82
Par 4-96 Page 4-83 FIGURE 4-39. TUBE REPLACEMENT AT A STATION BY WELDED
OUTER SLEEVES
a.
Remove the damaged section of a tube utilizing a 90-degree cut. Cut a replacement steel tube of the same material, diameter, and at least the same wall thickness to match the length of the removed portion of the damaged tube. This replacement tube must bear against the stubs of the original tube with a total tolerance not to exceed 1/32 inch. The outer-sleeve tube material selected must be of the same material and at least the same wall thickness as the original tube.
The clearance between inside diameter of the sleeve and the outside diameter of the original tube may not exceed 1/16 inch.
b. From this outer-sleeve tube material, cut diagonally (or fishmouth) two sections of tubing, each of such length that the nearest end of the outer sleeve is a minimum distance of 1–1/2-tube diameters from the end of the cut on the original tube. Use a fishmouth sleeve wherever possible. Deburr the edges of the sleeves, replacement tube, and the original tube stubs.
c. Slip the two sleeves over the replacement tube, align the replacement tube with the original tube stubs, and slip the sleeves over the center of each joint. Adjust the sleeves to suit the area and provide maximum reinforcement.
d. Tack weld the two sleeves to the replacement tube in two places before welding. Apply a uniform weld around both ends of one of the reinforcement sleeves and allow the weld to cool;
then, weld around both ends of the remaining reinforcement tube. Allow one sleeve weld to cool before welding the remaining tube to prevent undue warping.
4-97. SPLICING USING LARGER DIAMETER REPLACEMENT TUBES. The method of splicing structural tubes, as shown in Figure 4-40, requires the least amount of cutting and welding. However, this splicing method cannot be used where the damaged tube is cut too near the adjacent cluster joints, or where bracket-mounting provisions make it necessary to maintain
Par 4-97 Page 4-84 the same replacement tube diameter as the original. As an aid to installing the replacement tube, squarely cut the original damaged tube leaving a minimum short stub equal to 2-1/2-tube diameters on one end and a minimum long stub equal to 4-1/2-tube diameters on the other end.
Select a length of steel tube of the same material and at least the same wall thickness, having an inside diameter approximately equal to the outside diameter of the damaged tube. Fit this replacement tube material snugly around the original tube with a maximum diameter difference of 1/16 inch. From this replacement tube material, cut a section of tubing diagonally (or fishmouth) of such a length that each end of the tube is a minimum distance of 1-1/2-tube diameters from the end of the cut on the original tube. Use a fishmouth cut replacement tube wherever possible. Deburr the edges of the replacement tube and original tube stubs. If a fishmouth cut is used, file out the sharp radius of the cut with a small round file. Spring the long stub of the original tube from the normal position, slip the replacement tube over the long stub, and then back over the short stub. Center the replacement tube between the stubs of the original tube. Tack weld one end of the replacement tube in several places, then weld completely around the end. In order to prevent distortion, allow the weld to cool completely, then weld the remaining end of the replacement tube to the original tube.
FIGURE 4-40. SPLICING USING LARGER DIAMETER REPLACEMENT TUBE
4-98. REPAIRS AT BUILT-IN FUSELAGE FITTINGS. Make splices in accordance with the methods described in paragraphs 4-86 through 4-92. Repair built-in fuselage fittings in the manner shown in Figure 4-41. The following paragraphs outline the different methods as shown in Figure 4-41.