Isostatic Laminator | Diffusion Bonding | Diffusion Welding | Hot Pressure Welding | Hot Isostatic pressing (HIP)

What is diffusion bonding?

Diffusion bonding or welding is a solid state joining process wherein coalescence of the faying surfaces is produced by the application of pressure and elevated temperature to carefully cleaned and mated metal surfaces so that they actually grow together by atomic diffusion. Diffusion welding takes longer welding time.

History:

The process developed in the early 1970’s. The process created based on the goldsmith’s gold and copper foil bonding to produce “filled gold”. At first, gold foil placed above the copper foil. Simultaneously a weight is placed over the coil and from the furnace constant heat is applied to make the strong bond.

Diffusion welding process involves two steps or stages:

First stage:

To accomplish the task of diffusion bonding, metal to metal contacts of the two pieces to be diffused. This is done by the application of pressure that deforms the substrate roughness and disrupts and disperses the above mentioned surface layers and contaminants.

Second stage:

It involves diffusion and grain growth to complete the weld and ultimately eliminate the interface formed in the previous stage. The second stage induces complete metallic bonding across the area of contact.

In order to increase diffusion rate, moderate heating temperatures (usually below 1100°C) are used. Without applying heat if it takes many hours to perform a certain bonding, with the application of heat, the time element will be cut to a few hours or minutes.

Diffusion welding process steps:

  1. Two typical work piece surfaces to be diffusion welded can be seen
  2. The individual peaks which make up the roughness are deformed by the application of increasing pressure
  3. At places where the surfaces move together under shear, the oxide films are disrupted and metal to metal contact takes place
  4. After metal to metal contact is established, the atoms are within the attractive force fields of each other and hence a high strength joint is generated. The joint resembles a grain boundary
  5. A planer interracial boundary being thermodynamically unstable tends to migrate to a more stable configuration if conditions permit

Diffusion welding methods:

They are

  1. Gas pressure bonding
  2. Vacuum fusion bonding
  3. Eutectic fusion bonding

Gas pressure bonding:

Parts to be joined are placed together in intimate contact and then heated to around 815°C. During heating, an inert gas pressure is built up over all the surfaces of the parts to be welded. Non ferrous metals are joined with the help of gas pressure bonding method.

Vacuum fusion bonding:

Parts to be joined are pressed together mechanically or hydraulically. A hydraulic press used for diffusion welding resembles that employed in forging and is equipped to pressurize from three directions.

Eutectic fusion bonding:

It is a low temperature diffusion welding process. A thin plate of filler metal is kept between the base metal to be joined. As the pieces are heated to an elevated temperature, the filler material diffuses and forms a eutectic compound with the parent metals.

Advantages of diffusion welding:

  1. Continuous, leak tight welds can be formed
  2. The process is well suited for welding dissimilar metals and ceramics
  3. Number of welds can be made simultaneously
  4. Weld-ability is largely independent of material thickness

Limitations of diffusion welding:

  1. Opposing surfaces must be mated in size to within a few angstroms of each other in order to achieve a satisfactory metal bond
  2. Diffusion welding requires a relatively long, time consuming thermal cycle
  3. Diffusion welding is not classified as a mass production process
  4. The cost of the machine is usually high. Simultaneous application of heat and compressive force in vacuum environment are the major problem in constructing the machine.

Applications of diffusion welding:

  1. Fabrication of composite materials
  2. Copper liner fabrication in the liquid rocket combustion chamber
  3. Stainless steel window frame fabrication in the land based missile
  4. Internal channel liners for the liquid hydrogen cooling

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Hobby Aluminum Casting | Sand Casting Aluminum | Green Sand Casting Process

CO2 Sand Casting process

CO2 sand casting or Hobby Aluminum casting is suitable for urgent and heavy orders with high accuracy and good surface finish on castings required. Hobby mould casters used this method because in this method elimination of core drying machine, but it is not an economical process. The only expense hobby mould caster encounters are the cost of the CO2 cylinder, regulator, and hoses and hand held applicator gun or nozzle. The operation is fast and the possibility of rejections is less. CO2 is a sand molding technique and it uses grain sand which is mixed with a solution of sodium silicate that acts as binding agent. CO2 gas is used to harden the mass of sand after the mould has been made. The chemical reaction takes place is explained below:

H2O + Na2SiO3 + CO2 à Na2CO3 + SiO2 (Silica Gel in Colloidal state)

In this process, CO2 gas is passed through sand mix (0.25% moisture content Sodium Silicate), then the sodium silicate sand becomes stiff gel which gives enough strength for the mould. Additives used are Coal powder, Wood Flour, Sea coal, Dextrin, Kaolin clay, Aluminium oxide and Invert Sugar.

Properties of Additives:

  1. Kaolin clay is used for mould stability
  2. Aluminium oxide is used for hot strength of bonded sand
  3. Invert Sugar is used after pouring to reduce the retained strength of the mould or core which removes from the mould with less shaking.

Steps involved

  1. Mould material is pure Silica, 3-5% of liquid base binder of sodium silicate, mulled for 3 to 4 minutes
  2. Now rammed around pattern in moulding box
  3. CO2 gas forced into mould at about 1.4 to 1.5 kg/cm2 for a predetermined time (15 to 30 seconds)
  4. Apply refractory coating, system ready for pouring
  5. Use either wood or metal pattern
  6. Suitable pressure reducing valves are used to reduce pressure of gas
  7. Pattern rubbed with graphite before ramming to overcome removal of pattern from mould

Advantages:

  1. Low raw material cost
  2. Fast production rate
  3. No core or mould baking equipments
  4. Semi skilled workers enough
  5. Hollow moulds can be easily made
  6. Easily mechanized

Disadvantages:

  1. Difficult to reclaim the used sand
  2. Poor collapsibility due to inorganic nature of bond
  3. Moisture susceptibility
  4. Less suitable for non ferrous casting

Applications:

  1. For larger cores preparation
  2. Heavy or thick walled castings
  3. Producing cores of Fe, Al, Cu base alloys