Chapter 13 - Design Considerations for Spot Welding
Spot welding is often selected for joining sheet metal fabrications, stampings
and assemblies because it is fast, reliable and economical. However, numerous design
considerations can affect the quality and cost of the weld, among them: size of the spot
weld, accessibility, positioning, materials and thicknesses being joined, and the number
of spots needed to attain the desired strength.
Figure 1. In resistance spot and projection welding,
two sheet metal parts under pressure are heated by electrical resistance, melting the
metal and forming a weld "nugget." Circular in shape in a plan view, the nugget
has an oblong cross-section that, ideally, penetrates both thicknesses almost equally.
This section will focus primarily on resistance spot welding
(RSW) and resistance projection welding (RPW) since these processes are most commonly used
due to their speed and flexibility. See Figure 1, for schematics of RSW and RPW.
Applications include attachment of reinforcing braces and
stiffeners, functional brackets, hinges and other parts. Often, spot welding is the method
of choice for assembly of entire enclosures, cabinets and multipart assemblies.
Thickness of the majority of parts joined by spot welding
ranges up to 1/8 in. (3 mm) for each member, although parts up to 1/4 in. (6 mm) thick
have been successfully spot welded.
General Design Considerations
Based on the experience of stampers and fabricators, certain
general recommendations can facilitate spot welding of a sheet metal design, no matter
what metalforming process is used to make it. It is always useful to consult with the
metalformer in the design stage when questions arise regarding the part design,
application of spot welding or, control of spot welding cost for a particular design.
Knowledgeable designers avoid overspecifying the number of
welds, weld size, and location. After evaluating strength requirements, it usually
suffices to specify "a minimum number of spot welds equally spaced," thereby
leaving the most economical positioning up to the metalformer.
Even though spot welding is a very costeffective way of
fastening sheet materials, if other joining methods are also specified, it may be more
economical to redesign so that one or the other method is eliminated.
Dimensional precision is often overspecified, sometimes
unintentionally. CAD systems, for example, specify three or four digits of precision
unless instructed otherwise. Where possible, spot welding should be shown schematically
without dimensions.
Weld Size and Strength
Weld size (nugget diameter) is typically slightly less than
the diameter of the impression the electrode creates on the material. These dimensions and
other spot welding parameters are given in Table I for aluminum, carbon and stainless
steel. For simplicity, such standards can be specified by the designer as the controlling
print information on spot welds.
Base metal strength and spot weld strength are interrelated.
Table I gives realistic strength expectations for design purposes. For economy, avoid
over-specification of welds.
In applications where space is limited, specifying one weld
can produce a stronger bond than two spots, which may be limited in size and integrity
because of contraints in positioning, accessibility and shunting effects (current loss).
Typically, diameters of spot welds range from about 1/8 in.
(3 mm) to l/2 in. (13 mm) depending on the thicknesses of the workpieces and the material.
When the size of spot welds is designated, the designer should specify only one size
throughout an assembly in the interest of manufacturing economy and total part cost.
Weldability of Materials
Low carbon steel is one of the most readily spot welded
materials, as well as being the most commonly used material for stampings and
fabrications. It can be spot welded to many ferrous and non-ferrous alloys with varying
success, depending on the combination of metals joined.
Higher carbon and low-alloy steels can also be spot welded,
although with reservations, because of a tendency to form harder welds, which may degrade
weld performance. As carbon content increases, so does brittleness, with an associated
propensity for cracking and weld separation.
In addition, higher strength steels may require special
techniques or treatments like tempering after welding. Spot weldability of HSLA
(high-strength low-alloy) steels is directly related to composition and type of
microalloying elements. It is advisable to check with the supplier before specifying spot
welding here.
Stainless steels are spot weldable, some grades more readily
than others. Austenitic grades of the 300 series are the most commonly welded types,
followed by ferritic. Martensitic stainlesses are the least common because welded joints
are always much more brittle. All stainless steels require careful adjustment of welding
parameters and/or special methods to obtain optimum quality welds.
Highly conductive materials like aluminum require very high
power to form quality spot welds. However aluminum alloys are routinely spot welded (see
Table I for weldability). Here, cleanliness is much more of a concern than with low-carbon
steels because of aluminum's rapid surface oxidation characteristics.
For optimum quality and weld performance, expensive cleaning
procedures to remove surface oxide are required. For demanding applications, equipment to
monitor surface resistivity from lot to lot is necessary to assure consistency of quality.
This leads to a related consideration. If aluminum has been
chosen for an important reason, such as lightweight or high strength-to-weight ratio, the
added expense of ensuring a high-quality weld should be justified. If it has not,
re-evaluation of the original material selection is in order or, perhaps, another
assembling method should be considered.
Spot welding of very dissimilar metals, such as aluminum and
steel, is generally not possible because of different melting characteristics and
conductivities.
Some types of coated low-carbon steels require special
techniques. Steels plated with chrome and nickel for electrical conductivity can usually
be resistance welded as readily as uncoated material. Aluminum, tin, zinc and terne-coated
steels are also spot weldable with special precautions and welding equipment.
Some coatings can emit poisonous fumes that must be safely
handled when spot welded, thereby increasing cost. Spot welding of coated substrates
creates burn marks in the coating which can be unsightly and may corrode in severe
environments. Designers should carefully consider the product's appearance and service
requirements before specifying spot welding of pre-plated materials.
Thickness of Mating Parts
Ideally, equal thicknesses of two sheet metal parts to be
joined produces an evenly distributed weld nugget within the two layers. When this is not
practical, materials of different thicknesses can also be joined and produce a centered
weld nugget by using a larger electrode on the thicker member.
At a ratio above about 3-to-1 (thickest to thinnest member),
spot welding becomes difficult. At this point, another joining method should be
considered--for example, projection welding.
Note that weld deformation is always greater on the thinner
member. For this reason, stiffeners and brackets spot welded to cosmetic parts should be
thinner than or equal in thickness to the exposed surface material.
Weld Proximity and Spacing
Recommended spacing between welds and distances from a spot
weld to component edges and other part features should be followed to obtain optimum weld
quality and strength.
- Weld-to-weld spacing should be a minimum of 10 material
thicknesses. For 0.060 sheet steel, that's about 0.6 in. (15 mm). Ideally, 20 times
material thickness is recommended to reduce shunting effects with a minimum spacing of 1/2
in. (13 mm). See Table I for minimum and recommended weld spacing for various material
thickness of aluminum, carbon and stainless steel.
- Weld-to-edge distance should also conform to a minimum
dimension that is a function of the weld diameter. Generally, the center of a spot weld
(its location point) is positioned one to two diameters away from the edge of the part
being welded or from a feature in the part being welded depending on thickness of material
(Figure 2).
Figure
2. Recommended minimum spacing between spot welds and edges of parts to be joined also
applies to slots and holes in the workpieces.
If this minimum-dimension requirement is not followed, a poor-quality weld,
distortion of the parts being joined, or no weld at all may result. See Figures 3, 4 and
5.
Figure
3. Improper weld. Excess metal has been expelled from the weld causing the weld to deform
due to excessive indentation and surface cracks.
Figure 4. Existence of deep pits is cause for
rejection, as are cracks and burned metal in the weld.
Figure
5. Excessive edge bulge, which caused weld and base metal to crack, leads to a rejectable
weld.
Figure 6. When a part incorporating a formed feature
like a bend is joined to another sheet metal part, sufficient clearance must be maintained
to form a quality spot weld.
Positioning and Accessibility
If possible, spot welding of sheet metal components should be
restricted to joining flat, coplanar surfaces. Spot welding for assembly of mating parts
in multiple planes should be limited to parts smaller than a "bread box" that
are easy to handle.
With large, heavy parts, another fastening method, welding
process or possibly a redesign should be considered unless production quantities support
the initial expense of specialized spot welding equipment.
Figure
7. Specifying spot welds on short flanges could make access nearly impossible.
Although single- and double-bend electrode tips are available
to reach confined weld locations, a small flange dimension may restrict access, and
thereby prevent a successful spot weld. Such is the case with C-shaped parts or U-shaped
channels with short flanges (Figure 7). For instance, specifying a 1/4 in. (6 mm) diameter
weld on a 3/8 in. (10 mm) flange not only violates spacing considerations, but also makes
it very difficult for the operator to access the weld location.
Cosmetics
Good design practices attempt to limit spot welding on
appearance or cosmetic surfaces. While textured paints can be used to hide small electrode
marks on finished surfaces, grinding, or filling and grinding, is often required and can
double the cost of the welding operation.
Often, structural elements such as stiffeners are required to
reinforce large cosmetic surfaces. For these applications, designers should select
material which is thinner than the material from which the appearance part is fabricated.
This assures that weld shrinkage will occur on the noncosmetic part which helps to control
the cost of filling and abrasive finishing.
Plating Spot Welded Parts
Spot welding creates overlapping seams which, when immersed
in electroplating solutions, trap solution residues through capillary action. This creates
two problems. First, the residue often leaves plating salt deposits which are unsightly
and which, in extreme cases, may require touch up or manual removal at increased cost.
Second, the metal in the seam is unprotected and can corrode severely in harsh
environments.
When designing spot welded assemblies for electroplating,
consideration must be given to plating drainage, enclosed seams and pockets, overlapping
seams and other areas where solutions may be trapped or where special cleaning or
processing techniques may be required. When these operations are combined, early
consultation with an experienced supplier is crucial.
Spot Welded Fasteners
Weld nuts and weld studs are commonly used to provide a means
for subsequent fastening of additional components and assemblies, or for periodic removal
of service parts for maintenance and repair. When specifying welded fasteners, care should
be taken not to tightly tolerance concentricity or perpendicularity to a datum plane,
since this drastically increases cost. Weld fasteners located by holes punched by prior
stamping operations are an accurate and generally preferred location method.
For maximum cost-effectiveness, select weld nuts and studs of
one size that will be used throughout the assembly. This helps to keep set-ups to a
minimum and increases manufacturing throughput.
Nuts located by holes are typically within ±0.006 (0.15 mm)
of the original hole location. Studs can be located to ±0.020 (0.51 mm) with simple
fixturing. Closer tolerances require more sophisticated and costly fixtures.
Positive Location of Workpieces
Precise locations of spot welded parts is a cost-related
process and should be considered during the design. Part positioning involves either
extensive fixturing or, preferably, selfalignment through built-in stamped features like
holes and tabs. With the latter method, the location is predetermined by the accuracy of
alignment features.
Figure
8. Half shear or extruded button, with their mating holes, can accurately align components
for spot welds where tolerances are important.
The most preferred and most easily achieved method for
accurately self-fixturing parts is the half sheared or extruded cylindrical button and
matching hole in the mating part. (Figure 8). One mating hole should be 0.003 in. (0.08
mm) larger in diameter than the extrusion and the second hole should be slotted by 0.040
in. (1.02 mm) minimum to allow for normal fabrication tolerances as shown in the drawing.
Another alternative is to produce a lanced tab in a punching process. Mating parts can
then be brought up to it and located in position.
Knowledgeable designers recognize such cost-saving and
quality-improvement methods and specify them in the manufacturing process. The consistency
attainable with such methods surpasses that of sophisticated jigs and fixtures but the
greatest value is the cost efficiency. Additionally, these techniques can be used for
fillet welding applications, and mechanical assemblies.
Projection Welding
A refinement of resistance spot welding is resistance
projection welding (RPW). It makes use of projections previously formed on the workpiece
to reduce the power required to make a resistance weld. Consequently, multiple welds can
be made more easily at the same time, and thicker sections can be joined more readily than
in RSW. Other advantages include reduced shunting effects, closer weld-to-weld spacing and
welding of workpieces with smaller flanges.
Projection welding can be used on low-carbon, low-alloy and
stainless steels, as well as on aluminum. Typically, thicknesses up to 0.125 in. (3.18 mm)
can be joined. Thin workpieces--from 0.010 in. (0.25 mm) up to 0.022 in. (0.56 mm)--may
require special equipment. Below 0.010 in. (0.25 mm), resistance spot welding is
recommended, because on this thin material the projections would collapse before the
fusion temperature is reached.
Figure 9. Button-and cone-type welding projections
and typical dimensions.
The two major types of welding projections appear in Figure
9, along with commonly used projection sizes, which are normally based on the thickness of
the thinner material to be welded. In general, projections should be positioned as shown
in Figure 10 to optimize strength and accessibility.
Figure
10. Recommended positioning for projection welds made on various thicknesses of sheet
metal.
While projection welding can be less expensive than
resistance spot welding, workpiece alignment is more critical, and heights of projections
with simultaneous welds need to be closely controlled-- typically, within 0.003 in. (0.08
mm) of each other.
Tolerance Considerations
Required restrictive tolerances should be indicated on the
part drawing, while less important dimensions should be designated with appropriately
relaxed tolerances. One way to do this is to expand the tolerance block on drawings by
including:
English Units
.X ± 0.100 (2.54 mm)
.XX ± 0.020 (0.5l mm) (dedicated tools)
.XX ± 0.030 (0.76 mm) (universal tools)
.XXX ± 0.010 (0.25 mm)
Figure
11. Recommended method for specifying spot welds and projection welds.
When specifying attachment of structural parts such as
stiffeners use the most liberal tolerance (X ± 0.100), since this reduces or eliminates
fixturing costs for such unimportant dimensions. Critical dimensions, for such details as
hole diameters, coplanar hole-to-hole spacing, etc., should be separately toleranced.
Established by the American Welding Society (AWS), the
recommended method for specifying resistance spot welds and projection welds appears in
Figure 11.
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Excerpt taken from Design
Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
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