In addition to the current and time requirements, correct pressure must be applied to the metal while it is plastic from the
electrical heating. Pressure can be divided into two steps. The first is the application of initial squeeze to press the
surfaces together; the second is the forge force which is applied to force the plasticized metals together. The best control
of pressure is available in pneumatic operated mechanisms since they are capable of both rapid and accurate pressure
follow up. Closer edge spacing and smaller flanges are made possible because magnesium spot welding generally
requires less pressure than aluminum. Also because of this fact, smaller off-set electrodes may be used in confined
areas. Table 4-25 lists some spot weld machines and Table 4-26 lists recommended spacing data for spot welding.
4-83. There are some conditions to be avoided if functionally sound joints are to result.
Dissimilar alloys should not be spot welded together, especially those that have very different properties.
Avoid use of spot welded assemblies in areas where very high vibration is encountered.
In welding together material of different thicknesses, when high strength joints are desired, avoid exceeding a
thickness ratio of 2.5 to 1. (If such welding is necessary use DC equipment and place the thicker piece against the
Do not join three pieces together unless the thicknesses are within 25% of each other and the thickest piece is in
the middle. Table 4-27 reflects typical shear strengths of spot welds in some alloys.
4-84. Spot Welding Equipment Requirements. Machines used in this type welding may be AC single and 3-phase,DC
stored energy and dry disk rectifier. They should incorporate high current capacity, accurate welding time control and
positive pressure control if quality strong welds are expected. The design of the electrodes used with the welding
machine is particular ly important. Both dome shaped and flat tipped over are used. The tip radius may vary from 2 to 8
inches in the dome shaped. The flat type tips range from 3/8" to 1-¼ '-' in diameter and are used when mark free
surfaces are a requirement. Electrodes are made of high conductivity copper alloy, R.W.M.A. Group A, Class 1 or 2.
Water cooling of these to within 3/8" of the tip is necessary and intermittent cooling occurring only when the weld is made
is the better means of maintaining constant tip temperature control. Keep the electrode contact surfaces clean and
4-85. Spot welding Procedures. As in arc and gas welding the initial step of surface preparation is important if a good
weld is expected. The surface should be clean with an even and low resistance (50microhms or less). Use water cooled
high conductivity tips and keep them clean, smooth and free of condensation.
4-86. In surface preparation (prior to welding) the material may be cleaned with stainless steel, aluminum oxide pads,
cloth, or stainless steel rotary wire brushes. Do not use steel wonder brushes Other, than these in mechanical cleaning
since the electrically charged tip will attract metal particles. Chemical cleaning is recommended if possible and
especially if a large production is contemplated. A low even surface resistance, half as much tip cleaning and a longer
allowable period between cleaning and actual welding will result ii material is chemically rather than mechanically
cleaned. Mechanically cleaned material should be welded at once; chemically cleaned material can go as long as 24
hours before being welded. Chemical cleaning is accomplished in the following manner:
Vapor degrease or alkali clean the surface.
Dip the material in a 30 second bath proportioned to 0.67-1.3 fluid ounce of sulfuric acid (nitric or hydrochloric
are substituted) to each gallon of water to remove oxide deposits. Rinse in cold water.
Finally immerse the material in a 30 second room temperature bath in 1/3 ounce chronic acid per one gallon of
water; or a 3-minute bath in 24 ounce chromic acid and 1/16 fluid ounce of concentrated sulfuric acid to each gallon of
Alloys containing thorium will be immersed for 3-8 minutes; Tanks for these baths are of ceramic,
rubber lined, stainless steel or 1100 aluminum.
Rinse material in cold running water then in hot water.
4-87. Only material with an oiled or acid pickled surface finish should be used for spot welding due to the surface etch
imparted to magnesium by a chrome pickle finish. It is difficult to remove this etch.
4-88. Welding with single phase AC machines is accomplished using electronic current and timing control and slope
control of wave must be maintained. If the electricity supply line is used to power other equipment and therefore subject
to voltage drop, the welding machine should be equipped with either a signal device to show a poor weld has been made
or a nonfiring device to prevent welding, when a detrimental voltage drop occurs. Prevention of cracks in spot welds can
be avoided by post heat application to the welds. About 2 or 3 cycles of heat is effective in preventing such cracks.
Slope control, as illustrated by Figure 4-4 will also assist in controlling weld cracking. Precise timing other heat and
pressure application with variable control of heat and pressure intensity results in welding with minimum possibility of
cracking and spitting. Slope control enables single phase machines to weld alloys normally welded by stored energy