Soldering & Brazing

Soldering

A solder must be capable of wetting ie alloying with the metals to be joined and at the same time have a freezing range appreciably lower so that the work itself is in no danger of fusion or deterioration of mech properties.

Alloys based on S_n or P_l fulfill most of these requirements for a wide range of materials since S_n will allot readily with iron and steel Cu and its allots, Ni and its allots and lead.

At the same time tin-lead l\alloys posses mechanical toughness and melt at temperature between (183-250deg C) which is comfortably below the point at which deterioration in properties of the metals to be joined take place.

Solders are of two types = tin mans solder (62 Sn 38pl) (solidifies quickly)

= Plumeters solder (33 Sn 67pl) (solidifies over an extended phase) between 183-250Deg C.

flux – for iron and steel – HCL + Zn (granulated)

for electrical – organicflux – lactic acid, oleic acid

Brazing

it is used to join similar or dissimilar metals using a filler metal and flux without melting the base metals.

Brazing alloys melt at temperature above 450Deg C as compared to solders and form a stronger integral lord.

Fluxes- Lorax, horic acid, chlorides, florides.

The fluxes used for ferrous materials are mixtures of horax and horix acid in paste form. 

Filler metals

Ferrous- Cu based materials with Zn

Alluminium – Al-Si

Silva brazing – silver based filler metal

              Cu      Zn        Ag      other elements    freezing range   applications

ALI       4           -            -       Al-86 si-10            535-595      Aluminum + some 

                                                                                                   Al alloys

AGI      15         16          50      cd-19                      620-640    Cu + Cu alloys, Ms 

                                                                                                   alloy steels

AG 7    28            –         72                                          780      same as above

CP2      91.5         –          2           P-6.5                   645-740  Cu+Cu base alloys

NK5      -             -           -           Ni-Rem            980-1070  MS, SS, Ni+ Ni alloys

                                                   Si-3.5, B-1.8

Purpose of flux

1)    It dissolves and floats away oxides from metal surface.

2)    It protects the heated surfaces from oxidation by ambient air by combining with O₂ at the surface.

3)    It reduces the surface tension of brazing alloy to improve the wetting and capillary action of the alloy.

4)    It medicates the proper brazing temp has been reached when flux is molten.

Surface preparation

Interface must be clean with any OH, DIRT, Paint or grease.

Organic solvents – C CL₄, acetone may be used but before welding all must be evaporated.

Oxides present on surfaces- eliminated by the use of flux. The flux is expected to react with oxides present and form flow density slaf which would float on top of tee molten metal pool protecting it from further oxidation.

Another requirement of welding is a filler metal. Expect of resistance welding, all other process require a filler metal to fill the gap