Knowledge Base
Topics related to brazing to aid with learning and understanding the technical aspects of
brazing, how brazing works, why it works, along with useful hints and tips.
Joint Design
What type of brazed joint should you design? There are many kinds of joints. But our
problem is simplified by the fact that there are only two basic types the
butt and the lap. The rest are essentially modifications of these two. Let's look first at the butt
joint, both for flat and tubular parts.
As you can see, the butt joint gives you the advantage of a single thickness at the
joint. Preparation of this type of joint is usually simple, and the
joint will have sufficient tensile strength for a good many applications. However, the strength of the
butt joint does have limitations. It depends, in part, on the amount of bonding surface, and in a butt
joint the bonding area can't be any larger than the cross section of the thinner member.
Now let's compare this with the lap joint, both for flat and tubular parts.
The first thing you'll notice is that, for a given thickness of base metals, the bonding area of the lap
joint can be larger than that of the butt joint and usually is. With larger bonding areas, lap
joints can usually carry larger loads.
The lap joint gives you a double thickness at the
joint, but in many applications (plumbing connections, for
example) the double thickness is not objectionable. And the lap joint is generally self-
supporting during the brazing
process. Resting one flat member on the other is usually enough to maintain a uniform
joint clearance. And, in tubular
joints, nesting one tube inside the other holds them in proper alignment for brazing. However,
suppose you want a joint that has the advantages of both types; single thickness at the
joint combined with maximum tensile strength. You can get this combination by designing the
joint as a buttlap joint.
True, the buttlap is usually a little more work to prepare than straight
butt or lap, but the extra work can pay off. You wind up with a single thickness
joint of maximum strength. And the joint is usually self supporting when assembled for brazing.
Figuring the proper length of lap
Obviously, you don't have to calculate the bonding area of a butt joint. It will be the cross-
section of the thinner member and that's that. But lap
joints are often variable. Their length can be increased or decreased. How long should a lap joint
be? The rule of thumb is to design the lap joint to be three times as long as the
thickness of the thinner joint member.
A longer lap may waste brazing filler metal and use more
base metal material than is really needed, without a
corresponding increase in joint strength. And a shorter lap will lower the strength of the
joint. For most
applications, you're on safe ground with the "rule of three." More specifically, if you know the appr
oximate tensile strengths of the base members, the lap length required for optimum
joint strength in a silver brazed joint is as follows:
Tensile strength of weakest member
Lap length = factor x W
(W = thickness of weakest member)
35,000 psi 241.3 MPa 2 x W
60,000 psi 413.7 MPa 3 x W
100,000 psi 689.5 MPa 5 x W
130,000 psi 896.3 MPa 6 x W
175,000 psi 1,206.6 MPa 8 x W
Note: ksi x 6.8948 = 1 MPa
If you have a great many identical assemblies to braze, or if the
joint strength is critical, it will help to figure
the length of lap more exactly, to gain maximum strength with minimum use of
brazing materials. The
formulas given below will help you calculate the optimum lap length for flat and for tubular joints.
Figuring length of lap for flat joints
X = Length of lap
T = Tensile strength of weakest member
W = Thickness of weakest member
C = Joint integrity factor of .8
3 of 6 2/3/2010 9:44 AM
L = Shear strength of brazed filler metal Let's see how this formula works, using an example.
Problem: What length of lap do you need to join .050" annealed Monel sheet to a metal of equal or
greater
strength?
Solution:
C = .8
T = 70,000 psi (annealed Monel sheet)
W = .050"
L = 25,000 psi (Typical shear strength for silver brazing filler metals)
X = (70,000 x .050) /(.8 x 25,000) = .18" lap length
Problem in metric: What length of lap do you need to join 1.27 mm annealed Monel sheet to a met
al of
equal or greater strength?
Solution:
C = .8
T = 482.63 MPa (annealed Monel sheet)
W = 1.27 mm
L = 172.37 MPa (Typical shear strength for silver brazing filler metals)
X = (482.63 x 1.27) /(.8 x 172.37)
X = 4.5 mm (length of lap)
Figuring length of lap for tubular joints
X = Length of lap area W = Wall thickness of weakest member D = Diameter of lap area
T = Tensile strength of weakest member C = Joint integrity factor of .8 L = Shear strength of
brazed filler metal Again, an example will serve to illustrate the use of this formula.
Problem: What length of lap do you need to join 3/4" O.D. copper tubing (wall thickness .064") to 3
/4" I.D.
steel tubing?
Solution:
W = .064"
D = .750"
C= .8
T = 33,000 psi (annealed copper)
L = 25,000 psi (a typical value)
X = (.064 x (.75 .064) x 33,000)/(.8 x .75 x 25,000)
X = .097" (length of lap)
Problem in metric: What length of lap do you need to join 19.05 mm O.D. copper tubing (wall thick
ness
1.626 mm] to 19.05 mm I.D. steel tubing?
Solution:
W = 1.626 mm
D = 19.05 mm
C = .8
T = 227.53 MPa (annealed copper)
L = 172.37 MPa (a typical value)
X = (1.626 x (19.05 1.626) x 227.53)/(.8 x 19.05 x 172.37)
X = 2.45 mm (length of lap)
Designing to distribute stress
When you design a
brazed joint, obviously you aim to provide at least minimum adequate strength for the
given application. But in some
joints, maximum mechanical strength may be your overriding concern. You
can help insure this degree of strength by designing the
joint to prevent concentration of stress from weakening the joint. Motto -
spread the stress. Figure out where the greatest stress falls. Then impart
flexibility to the heavier member at this
point, or add strength to the weaker member. The illustrations below
suggest a number of ways to spread the stress in a brazed joint.
To sum it up when you're designing a
joint for maximum strength, use a lap or scarf design (to increase joint area) rather than a
butt, and design the parts to prevent stress from being concentrated at a single point. There
is one other technique for increasing the strength of a
brazed joint, frequently effective in brazing small part assemblies. You can create a stress-
distribution fillet, simply by using a little more brazing filler metal than you normally would, or
by using a more "sluggish" alloy. Usually you don't want or need a fillet in a brazed
joint, as it doesn't add materially to joint strength. But where it contributes to spreading
joint stresses, it pays to create the fillet.
Designing for service conditions
In many brazed joints, the chief requirement is strength. And we've discussed various ways of achi
eving joint strength. But there are frequently other service requirements which may influence the
joint design or filler metal selection. For example, you may be designing a
brazed assembly that needs to be electrically conductive. A silver
brazing filler metal, by virtue of its silver content, has very little tendency to increase
electrical resistance across a properlybrazed joint. But you can further insure minimum resistance
by using a close joint clearance, to keep the layer of filler metal as thin as
possible. In addition, if strength is not a prime
consideration, you can reduce length of lap. Instead of the customary "rule of three," you can redu
ce lap length to about 11/2 times the crosssection of the thinner member. If the
brazed assembly has to be pressure tight against gas or liquid, a lap
joint is almost a must, since it withstands greater pressure than a butt joint. And its broader
bonding area reduces any chance of leakage. Another consideration in designing a joint
to be leak proof is to vent the assembly. Providing a vent during the brazing
process allows expanding air or gases to escape as the molten filler metal flows into the
joint. Venting the assembly also prevents entrapment of flux in the
joint. Avoiding entrapped gases or flux reduce the potential for leak paths. If possible, the
assembly should be self-
venting. Since flux is designed to be displaced by molten filler metal entering a joint,
there should be no sharp corners or blind holes to cause flux entrapment. The
joint should be designed so that the flux is pushed completely out of the joint
by the filler metal. Where this is not possible, small holes may be drilled into the
blind spots to allow flux escape. The joint is completed when molten filler metal appears at
the outside surface of these drilled holes.
To maximize corrosion resistance of a joint, select a
brazing filler metal containing such elements as silver, gold or
palladium, which are inherently corrosion resistant. Keep
joint clearances close and use a minimum
amount of filler metal, so that the finished joint will expose only a fine line of
brazing filler metal to the atmosphere. These are
but a few examples of service requirements that may be demanded of your brazed
assembly. As you can see both the joint design and filler metal selection must
be considered. Fortunately, there are many filler metals and fluxes available to you -
in a wide range of compositions, properties and
melting temperatures. The selector charts that appear later in this
book can help you choose filler metals and fluxes that best meet the service requirements of the
joints you design. The Technical Services Department at Handy & Harman/Lucas-
Milhaupt is available to help answer any questions you may have with regard to your specific
brazing application, joint design and/or filler metal selection.
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