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Historic Preservation - Technical Procedures

Spectitle:

General Guidelines For Repairing Three-Dimensional Aluminum Features

Procedure code:

0501009R

Source:

Developed For Hspg (Nps - Sero)

Division:

Metals

Section:

Metal Materials

Last Modified:

02/24/2012

Details:

General Guidelines For Repairing Three-Dimensional Aluminum Features



GENERAL GUIDELINES FOR REPAIRING THREE-DIMENSIONAL ALUMINUM
FEATURES


PART 1---GENERAL

1.01 SUMMARY

    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:

         1.   Safety Precautions

         2.   Historic Structures Precautions

         3.   Submittals

         4.   Quality Assurance

         5.   Delivery, Storage and Handling

         6.   Project/Site Conditions

         7.   Sequencing and Scheduling

         8.   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.

1.02 REFERENCES

    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.

1.03 MAINTENANCE

    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 antifouling agents.


PART 2---PRODUCTS

2.01 MATERIALS

    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.

2.02 EQUIPMENT

    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


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Verify the aluminum type, i.e. wrought or cast, and alloy
         prior to the installation of replacement material.

         1.   Many, but not all wrought and cast aluminum
              features may be successfully soldered.  Consult
              manufacturer.

         2.   Failure of feature may be the result of using the
              wrong type of alloy for the environmental
              conditions encountered.

         3.   Alloy composition affects solderability and choice
              of flux.  Consult manufacturer.

         4.   Filler alloy and base metal alloy must also be
              compatible.  Consult manufacturer.

3.02 PREPARATION

    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

    A.   Brazing:  

         1.   For temperatures above 8400F, brazing must be done
              under shop conditions only by an experienced
              craftsperson.  

         2.   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.

         1.   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.  

         2.   Use gas tungsten-arc or gas metal-arc welding
              processes only.  

         3.   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.

         4.   Use materials and methods that minimize distortion
              and develop strength and corrosion resistance of
              base metals.

         5.   Obtain fusion without undercut or overlap.

         6.   Remove welding flux immediately.

         7.   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.

         8    Advantages of welding:

              a.   Arc welding produces a strong, durable
                   connection and, if properly executed, is at
                   least as strong as the surrounding metal.  

              b.   It is faster and less expensive than threaded
                   connections, which require drilling a pilot
                   and then tapping to accommodate screws or
                   bolts.  

              c.   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.

         9.   Disadvantages of welding:

              a.   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.

              b.   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.

    C.   Oxyfuel-gas Welding:  

         NOTE:  WELDING SHOULD BE EXECUTED ONLY BY A SKILLED
         WELDER UNDER CAREFUL SUPERVISION.

         1.   Use ONLY under shop conditions to insure complete
              removal of corrosive fluxes.

         2.   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.  

    D.   Soldering:

         1.   Provides the following advantages over either
              brazing or welding:

              a.   Works with lower temperatures than either
                   brazing or welding so that the risk of
                   thermally induced stresses or heat distortion
                   is greatly reduced.

              b.   Repairs can be easily done in the field and
                   require less skill on the part of the operator
                   than brazing or welding.

              c.   Techniques have been developed which allow for
                   soldering to be achieved with flux, without
                   flux, or without solder.

              d.   Consult manufacturer to determine best method
                   of soldering for situation, and flux or filler
                   alloy (solder) to be used.

         2.   Repair (See also procedure 05010-07-R, "Procedures
              for Soldering Sheetmetal").

              a.   Coat thoroughly cleaned surfaces with suitable
                   flux.

              b.   Hold surfaces to be joined a few thousandths
                   of an inch apart.

              c.   Place suitable solder in or near joint and
                   apply heat until flux reacts and solder flows
                   into the joint.

              d.   After joint cools thoroughly clean joint to
                   remove any traces of flux.  Method of cleaning
                   will depend on type of flux used.

         3.   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.

                         END OF SECTION