Q.8 What do you mean by soldering write its application? (AKTU. 2010 - 11)
Ans. Soldering: -
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a relatively low melting point. Soft soldering is characterized by the melting point of the filler metal, which is below 400 °C (752 °F). The filler metal used in the process is called solder.
Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is distinguished from welding by the base metals not being melted during the joining process. In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and bond to the materials to be joined by wetting action. After the metal cools, the resulting joints are not as strong as the base metal, but have adequate strength, electrical conductivity, for many uses. Soldering is an ancient technique mentioned in the Bible and there is evidence that it was employed up to 5000 years ago in Mesopotamia.
Applications: -
Small figurine being created by soldering. One of the most frequent applications of soldering is assembling electronic components to printed circuit boards (PCBs). Another common application is making permanent but reversible connections between copper pipes in plumbing systems. Joints in sheet metal objects such as food cans, roof flashing, rain gutters and automobile radiators have also historically been soldered, and occasionally still are. Jewelry components are assembled and repaired by soldering. Small mechanical parts are often soldered as well. Soldering is also used to join lead came and copper foil in stained glass work. Soldering can also be used to effect a semi-permanent patch for a leak in a container cooking vessel.
Some examples of solder types and their applications are tin-lead (general purpose), tin-zinc for joining aluminium, lead-silver for strength at higher than room temperature, cadmium-silver for strength at high temperatures, zinc-aluminium for aluminium and corrosion resistance, and tin-silver and tin-bismuth for electronics.
Q.9 Describe briefly brazing process. (AKTU. 2010 - 11)
Ans. Brazing: -
Brazing is the coalescence of a joint with the help of a filler metal whose liquidus temperature is above 450C and is below the solidus temperature of the base metal. The filler metal is drawn into the joint by means of capillary action (entering of fluid into tightly fitted surfaces).
Brazing is a much widely used joining process in various industries because of its many advantages. Dissimilar metals, such as stainless steel to cast iron can be joined by brazing. Almost all metals can be joined by brazing except aluminium and magnesium which cannot easily be joined by brazing.
Because of the lower temperatures used there is less distortion in brazed joints. Also, in many cases the original heat treatment of the plates being joined, is not affected by the brazing heat. The joint can be quickly finished without much skill. Because of the simplicity of the process it is often an economical joining method with reasonable joint strength. The brazed joints are reasonably stronger, depending on the strength of the filler metal used.
But the brazed joint is generally not useful for high temperature service because of the low melting temperature of the filler metal. The colour of the filler metal in the brazed joint also, may not match with that of the base metal.
Because the filler metal reaches the joint by capillary action, it is essential that the joint is designed properly. The clearance between the two parts to be joined should be critically controlled. Another important factor to be considered is the temperature at which the filler metal is entering the joint. While designing a brazed joint, care is to be taken to see that the differences in the coefficients of thermal expansion of the two pieces to be joined, are properly accounted for. If there is too much of clearance, the capillary forces may not be sufficient to draw the filler metal into the joint, whereas insufficient clearance may have too small an amount of filler metal to give rise to any effective strength.
In brazing, joints need to be extremely clean. Any grease or oil present in the joint prevents the flow of filler metal. Hence the joint should be thoroughly cleaned using proper solvents. Oxides and scales present are removed by acid pickling. Fluxes are added into the brazed joint to remove any of the oxides present or prevent the formation of the oxides so that the base metal and the filler metal remain pure during the joining. The fluxes generally used are combinations of borax, boric acid, chlorides, fluorides and tetra borates and other wetting agents. The fluxes used for ferrous materials are mixtures of borax and boric acid in a paste form. A popular composition is 75% borax and 25% boric acid. Alkaline bifluorides are used for brazing of stainless steel, aluminium or beryllium copper alloys. A special flux containing sodium cyanide is used in brazing tungsten to copper. The fumes from all these fluxes are harmful and therefore a proper ventilation and care in use, are essential.
Enough flux should be applied at the joint so that it lasts throughout the brazing session. The method of application of the flux is by spraying, brushing or with the help of a pressurised application.
Depending on the type of base metals brazed, a number of filler metals are available. The joint obtained in brazing is by means of the diffusion of the filler metal into the base material, associated with the surface alloying. Copper based materials are generally used for brazing ferrous materials. Copper alloys having high zinc (70%) content are not extensively used because of their brittleness, but those with less zinc content are used for brazing of various steels. Aluminium-silicon filler materials are used for brazing aluminium.
Silver brazing makes use of a silver based filler metal. Silver brazing is used to give high strength (tensile strength up to 900 MPa) joints. Though originally used for jewellery applications, silver brazing is now extensively used in industrial applications. They can be used with a large range of materials, but because of its high cost it is used in only special applications requiring high strength and high temperature servive.
Heat sources that are used’ for brazing are: a molten bath of brazing filler metal, oxyacetylene torch, controlled atmosphere furnace, electrical resistance heating and induction heating. The main points to be noted are that the joint should be maintained in proper fit with cleanliness before heating the joint for brazing.
A general brazing method is to apply flux to the joint, after it has been properly prepared, heat the joint to bring it to the liquidus temperature of the filler material, and apply the filler material so that it flows into the crevice by capillary forces. The filler metal on solidification gives the necessary joint strength.
