SURFACE POROSITY.—  Surface  porosity  usu- ally results from atmospheric contamination. It can be caused by a clogged nozzle, shielding gas set too low or too high, or welding in a windy area. To avoid surface porosity, you should keep the nozzle clean of spatter, use the correct gas pressure, and use a protective wind shield when welding in a windy area. CRATER POROSITY.— Crater  porosity  usually results from pulling the torch and gas shield away before the crater has solidified. To correct this problem, you should reduce the travel speed at the end of the joint. You also may try reducing the tip-to-work distance. COLD LAP.— Cold laps often result when the arc does not melt the base metal sufficiently. When cold lap occurs, the molten puddle flows into an unwelded base metal. Often this results when the puddle is allowed to become too large. To correct this problem, you should keep the arc at the leading edge of the puddle. Also, reduce the size of the puddle by increasing the travel speed or reducing the wire-feed speed. You also may use a slight whip motion. LACK OF PENETRATION.— Lack of penetra- tion usually results from too little heat input in the weld zone. If the heat input is too low, increase the wire-feed speed  to  get  a  higher  amperage.  Also,  you  may  try reducing the wire stick-out. BURN-THROUGH.—  Burn-through (too much penetration) is caused by having too much heat input in the weld zone. You can correct this problem by reducing the wire-feed speed, which, in turn lowers the welding amperage. Also you can increase the travel speed. Burn- through  can  also  result  from  having  an  excessive amount of root opening. To correct this problem, you increase  the  wire  stick-out  and  oscillate  the  torch slightly. WHISKERS.— Whiskers  are  short  pieces  of  elec- trode wire sticking through the root side of the weld joint. This is caused by pushing the wire past the leading edge of the weld puddle. To prevent this problem, you should cut off the ball on the end of the wire with side cutters before pulling the trigger. Also, reduce the travel speed and, if necessary, use a whipping motion. GMA WELDING COMMON METALS You  can  use  the  welding  equipment  and  techniques for gas metal-arc welding to join all types of metals; however, as we discussed in the GTAWprocess, each of the  metals  requires  a  unique  welding  method.  In  this section, we discuss some of the welding methods asso- ciated with a few of the more commonly welded metals. Carbon Steels The majority of welding by all methods is done on carbon steels. When you are using GMA to weld carbon steels, both the spray-arc and short-arc methods may be applied. For spray-arc welding, a mixture of 5-percent oxygen  with  argon  is  recommended.  As  we  mentioned earlier, this mixture provides a more stable arc. Also you may use a mixture of argon and CO₂ or straight CO₂. Straight CO₂ is often used for high-speed production welding;  however,  with  CO₂ the arc is not a true spray arc. For short-arc welding, a 25-percent CO₂ and  75-per- cent argon mixture is preferred. For GMA welding of thin materials (0.035 inch to 1/8 inch), no edge preparation is needed and a root opening of 1/16 of an inch or less is recommended. For production  of  adequate  welds  on  thicker  material,  some beveling is normally required. When welding plates 1/4 of an inch or greater in thickness, you should prepare a single or double-V groove with 50- to 60-degree in- cluded  angle(s). Aluminum The joint design for aluminum is similar to that of steel; however, aluminum requires a narrower joint spacing  and  lower  welding  current  setting. The short-arc welding method is normally used for out-of-position welding or when welding thin materials because short-arc produces a cooler arc than the spray type arc. When welding thinner material (up to 1 inch in thickness), you should use pure argon. The spray-arc welding method is recommended for welding thicker materials. With spray arc, more heat is produced to melt the wire and base metal. When you are welding thicker material (between 1 and 2 inches) a mixture  of  90-percent  argon  and  10-percent  helium  is recommended. The helium provides more heat input and the argon provides good cleaning action. Stainless  Steel DCRP with a 1- or 2-percent oxygen with argon mixture is recommended for most stainless steel weld- ing. In general, you weld stainless steel with the spray- arc welding method and a pushing technique. When welding stainless steel up to 1/1 6 of an inch in thickness, you should use a copper backup strip. For welding thin materials in the overhead or vertical positions, the short- arc  method  produces  better  results. 8-22