Effects Of Copper Brazing On The Mechanical Properties By R. W. McCord (EAA 45540)
778 Coleman Avenue, Apt. D Menlo Park, California Design Checker-Lockheed, Sunnyvale, California
I
HAD BEEN CONSIDERING using some brazing on my EAA "Biplane" fuselage currently under construction, thinking that for fairing clips, throttle quadrant mount, and possibly other non-primary structure, the lower heat required for this operation would be beneficial to the 4130 tubing. Most of these items would be brazed to the primary structure. Reading in the EAA Welding Manual, I found on pg. 46: "Because of its higher melting point, grade D brazing alloy . . . is preferable with chrome vanadium and chrome molybdenum steel." To me, this sentence is approval to braze 4130 steel. In an attempt to learn all I could about brazing, I went into the Lockheed Design Manual, and was surprised to find on pg. 6.302 that 4130 was conspicuously absent in the list of steel alloys approved to braze. There is a reference to a "Stress Memo No. 72b", dated June 1, 1946, which gives strength of brazed joints and, more important, "effects of brazing on parent materials due to decarburization". This document brings to light some rather startling information (to me, at least) regarding the lowering of the mechanical properties of 4130 steel, especially in sections up to .065-in. thickness. I think everyone is aware of the lower joint strength in brazing, but the purpose of this article is to put forth the actual "numbers" of the lowered strength of the parent material adjacent to a brazed joint. I'm wondering how many homebuilders have done some
brazing — maybe just fairing clips — in which the primary structure was involved. If there is just one other who has not "designed-in" the 40 percent reduction in Tensile Yield strength of the 4130 steel in his structure, that's too many. The accompanying data sheet shows the relative losses of mechanical properties of the 4130 tubing under discussion from the welding and brazing operations. With fusion welding, there is a loss of approximately 20 percent in Tensile Yield strength, from 75,000 psi to approximately 60,000 psi, but the loss due to copper brazing is 40 percent, down to 45,000 psi. If a tension loaded member were designed with a safety factor of six using the as-received yield strength, after welding you would have a 4.8, and after copper brazing you would have only a 3.6 safety factor. Remember, this is the strength of the adjoining and joint area parent material, not the strength of the joint itself. Referring to Airplane Structures (Niles & Newell, 2nd Edition, Pg. 24), flight tests on a "150-hp Trainer" recorded maximum G loads in a "sharp pull-out" of: 3.9 G's @ 86 - 105 mph 4.3 G's @ 106 - 125 mph In this case (used strictly for "eye-ball comparison" of a similar weight/hp/speed plane) the member in question might well have been subject to tension loads in excess of its yield strength and while not in excess of its breaking (ultimate) strength, might put the structural integrity of the firframe in jeopardy. Some interesting information regarding silver brazing is included in the aforementioned Memo. Maybe we are missing a process which might well have satisfactory applications; however, the joint must be designed properly to provide adequate area to compensate for the reduced shear strength of the joint. £)
PARENT MATERIAL STRENGTH WITHIN 3 INCHES OF JOINT ( IN PSI)
Fusion Weld Copper Braze Silver Braze
58 MAY 1972
Ult. Ten.
%Loss
81,000 65,000 95,000
15 32
None
Yield Ten.
% Loss
Approx. 60,000 45,000 60,000
Approx. 20 40 20
Joint Shear Strength
Shear Area @ Joint (Area Of Welded Joint @ 100)
50,000 15,000 15,000
100 330 330
