All silver solder joints must be physically, as well as  chemically, clean. The joint must be free of dirt, grease, oil,  and/or paint. After removing the dirt, grease, etc., any oxide  (rust and/or corrosion) should be removed by grinding or  filing the piece until bright metal can be seen. During the  soldering operation, the flux continues to keep the oxide away  from the metal and aid in the flow of the solder. The three recommended types of joint for silver soldering  are lap, flanged, and edge. With these, the metal is formed  to furnish a seam wider than the base metal thickness and  provide the type of joint that holds up under all types of  loads.  [Figure 5-27] The oxy-acetylene flame for silver soldering should be a  soft neutral or slightly reducing flame. That is, a flame with  a slight excess of acetylene. During both preheating and  application of the solder, the tip of the inner cone of the  flame should be held about  1 ⁄ 2 -inch from the work. The flame  should be kept moving so that the metal does not overheat. When both parts of the base metal are at the correct  temperature, the flux flows and solder can be applied  directly adjacent to the edge of the seam. It is necessary to  simultaneously direct the flame over the seam and keep it  moving so that the base metal remains at an even temperature.  The TIG process as it is known today is a combination of  the work done by General Electric in the 1920s to develop  the basic process, the work done by Northrop in the 1940s  to develop the torch itself, and the use of helium-shielding  gas and a tungsten electrode. The process was developed for  welding magnesium in the Northrop XP-56 flying wing to  eliminate the corrosion and porosity issues with the atomic  hydrogen process they had been using with a boron flux. It  was not readily used on other materials until the late 1950s  when it found merit in welding space-age super alloys. It was  also later used on other metals, such as aluminum and steel,  to a much greater degree.  Modern TIG welding machines are offered in DC, AC, or  with AC/DC configurations, and use either transformer or  inverter-based technology. Typically, a machine capable of  AC output is required for aluminum. The TIG torch itself has  changed little since the first Northrop patent. TIG welding  is similar to oxy-fuel welding in that the heat source (torch)  is manipulated with one hand, and the filler, if used, is  manipulated with the other. A distinct difference is to control  the heat input to the metal. The heat control may be preset  and fixed by a machine setting or variable by use of a foot  pedal or torch-mounted control. Several types of tungsten electrode are used with the TIG  welder. Thoriated and zirconiated electrodes have better  electron emission characteristics than pure tungsten, making  them more suitable for DC operations on transformer-based  machines, or either AC or DC with the newer inverter-based  machines. Pure tungsten provides a better current balance  with AC welding with a transformer based machine, which  is advantageous when welding aluminum and magnesium.  The equipment manufacturers’ suggestions for tungsten type  and form should be followed as this is an ever changing part  of the TIG technology. The shape of the electrode used in the TIG welding torch  is an important factor in the quality and penetration of the  weld. The tip of the electrode should be shaped on a dedicated  grinding stone or a special-purpose tungsten grinder to avoid  contaminating the electrode. The grinding should be done  longitudinally, not radially, with the direction of stone travel  away from the tip.  Figure 5-28  shows the effects of a sharp  versus blunt electrode with transformer-based machines.  When in doubt, consult the machine manufacturer for the  latest up-to-date suggestions on tungsten preparation or if  problems arise. The general guidelines for weld quality, joint fit prior to  welding, jigging, and controlling warp all apply to this  process in the same regard as any other welding method. Of  particular note are the additional process steps that sometimes  must be taken to perform a quality weld; these are dealt within  their appropriate sections. Gas Tungsten Arc Welding (TIG Welding) Figure 5-28.  Effects of sharp and blunt electrodes. Figure 5-27.  Silver solder joints. 5-22 Solder Solder Solder Solder Lap joint Flanged butt joint Edge joint Easy arc starting Handles less amperage Wider arc shape Good arc stability Less weld penetration Shorter electrode life Usually harder to start the arc Handles more amperage Narrower arc shape Potential for arc wander Better weld penetration Longer electrode life Sharper Electrode Blunter Electrode