General Guidelines For Repairing Three-Dimensional Aluminum Features
- Procedure code:
- Developed For Hspg (Nps - Sero)
- Metal Materials
- Last Modified:
A. This procedure provides general guidelines for making simple repairs to three dimensional aluminum features. For repairs to sheet aluminum features see procedure 05010-08-R, "General Guidelines for the Repair of Sheet Metal Aluminum Features."
B. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
- These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).
C. For general information on the characteristics, uses and problems associated with aluminum, see 05010-08-S.
A. Technical Report #C-6: "Aluminum and its Alloys," The Aluminum Association, September 1978, fourth printing, February 1984.
B. Technical Report #22: "Aluminum Soldering Handbook," The Aluminum Association, December 1985.
C. "Metal's in America's Historic Buildings: Uses and Preservation Treatments", U.S. Department of Interior - National Park Service - Preservation Assistance Division, Chapters 8 and 18, Revised 1993.
A. Protect aluminum architectural elements with non- absorptive, insulating coating to prevent direct contact with corrosive agents.
B. Protect aluminum architectural elements from rainwater run-off from wood and copper roofs and copper gutters.
C. Where aluminum touches masonry, coat with a heavy-bodied bituminous paint followed by two coats of aluminum metal and masonry paint.
D. When aluminum is painted for cosmetic reasons and there is no incompatibility with other building materials, prime with a zinc chromate primer and apply two finish coats of compatible paint from the same manufacturer. DO NOT USE lead-based paints or paints with copper containing anti-fouling agents.
A. Aluminum to match existing in finish, alloy (unless failure was caused by the use of an unsuitable alloy for the conditions), temper, thickness, color and appearance.
B. Filler as appropriate for base metal and joining technique being used. Consult manufacturer.
C. Flux as appropriate for metal and joining technique being used. Consult manufacturer.
A. Gas tungsten-arc or gas metal-arc equipment as appropriate for welding or brazing repair.
B. Torch, iron or hot plate as appropriate for soldering repair.
C. Fire extinguisher
A. Verify the aluminum type, i.e. wrought or cast, and alloy prior to the installation of replacement material.
- Many, but not all wrought and cast aluminum features may be successfully soldered. Consult manufacturer.
- Failure of feature may be the result of using the wrong type of alloy for the environmental conditions encountered.
- Alloy composition affects solderability and choice of flux. Consult manufacturer.
- Filler alloy and base metal alloy must also be compatible. Consult manufacturer.
A. Surface Preparation: Surfaces to be soldered must be soil free, with only a thin oxide layer which can be displaced by hot flux during soldering procedure.
3.03 ERECTION, INSTALLATION, APPLICATION
- For temperatures above 8400F, brazing must be done under shop conditions only by an experienced craftsperson.
- Use brazing only on those items where joint design allows for the complete removal of flux residue.
B. Field Welding:
NOTE: WELDING SHOULD BE EXECUTED ONLY BY A SKILLED WELDER UNDER CAREFUL SUPERVISION. NOTE: USE CAUTION IN HANDLING FLAME TOOLS WHEN WELDING. THE DANGER OF SETTING THE STRUCTURE ON FIRE IS ALWAYS PRESENT. COMPLY WITH FIRE, SAFETY AND ENVIRONMENTAL PROTECTION REGULATIONS. CAUTION: DURING WELDING THE METAL BECOMES VERY HOT AND CAN UNDERGO TREMENDOUS THERMAL SHOCK.
- For large sections, welding should take place off site. The piece must be removed and transported to a workshop where it can be preheated before welding and postheated after welding to ensure a gradual temperature change within the metal.
- Use gas tungsten-arc or gas metal-arc welding processes only.
- If the aluminum has anodized coating, the coating must first be removed from the surfaces to be welded to permit proper fusion of the surfaces.
- Use materials and methods that minimize distortion and develop strength and corrosion resistance of base metals.
- Obtain fusion without undercut or overlap.
- Remove welding flux immediately.
- At exposed connections, finish exposed welds and surfaces smooth and blended so that no roughness shows after finishing and contour of welded surface matches those adjacent.
- Advantages of welding:
- Arc welding produces a strong, durable connection and, if properly executed, is at least as strong as the surrounding metal.
- It is faster and less expensive than threaded connections, which require drilling a pilot and then tapping to accommodate screws or bolts.
- Welding is the most preferred for the attachment of the decorative castings and for other non-structural repairs for economic reasons and because it allows to preserve the original damaged elements, which otherwise would have to be replaced.
- Disadvantages of welding:
- In cases where the original attachments are bolted, the use of this method may result in internal stresses (welds cannot move with seasonal expansion/ contraction cycles) which may in turn lead to further breaks.
- Welding may leave a 'bead' along the surface of the connection which may be unacceptable in some restoration projects, even though much of the weld may later be ground down, depending on the location and the welding material.
- Oxyfuel-gas Welding:
NOTE: WELDING SHOULD BE EXECUTED ONLY BY A SKILLED WELDER UNDER CAREFUL SUPERVISION.
- Use ONLY under shop conditions to insure complete removal of corrosive fluxes.
- Metallic bond (gas) welding is more reliable than fusion (arc) welding in repairing large sections because a lower temperature is used and heat is applied and removed at a slower rate.
- Provides the following advantages over either brazing or welding:
- Works with lower temperatures than either brazing or welding so that the risk of thermally induced stresses or heat distortion is greatly reduced.
- Repairs can be easily done in the field and require less skill on the part of the operator than brazing or welding.
- Techniques have been developed which allow for soldering to be achieved with flux, without flux, or without solder.
- Consult manufacturer to determine best method of soldering for situation, and flux or filler alloy (solder) to be used.
- Repair (See also procedure 05010-07-R, "Procedures for Soldering Sheetmetal").
- Coat thoroughly cleaned surfaces with suitable flux.
- Hold surfaces to be joined a few thousandths of an inch apart.
- Place suitable solder in or near joint and apply heat until flux reacts and solder flows into the joint.
- After joint cools thoroughly clean joint to remove any traces of flux. Method of cleaning will depend on type of flux used.
- Exposed solder may be ground, filed, polished, painted, or electroplated as required to match surrounding surfaces.
NOTE: DO NOT ANODIZE UNLESS THE SOLDER IS FIRST PAINTED OR SEALED WITH ANOTHER COATING.
NOTE: DO NOT USE CHEMICAL FINISHING TREATMENTS (USED TO BRIGHTEN BARE ALUMINUM) CONTAINING NITRIC ACID WITH ZINC-BASED SOLDER.