1. HAZ - Feeling the Heat
2. Diffusion Welding
3. Porosity - The Friendly Foe
HAZ - Feeling the Heat
Heat affected zone (HAZ) is the volume of material surrounding the fusion zone whose properties are altered by exposure to heat generated during welding. Volume of HAZ depends on total heat input, rate of heat input, thermal conductivity of the materials being welded, and rate of heat loss due to cooling mechanisms. Arc welding processes typically introduce a lot of heat leading to large volume of fusion zone and a large HAZ. On the other hand, welding processes such as electron beam welding, laser welding and resistance welding produce a fairly smallHAZ.
While all eyes are on the fusion zone, it is the HAZ that can throw you a curve ball (or a googly for those who follow the game of Cricket). Exposure to a high enough temperature can produce multiple changes in the HAZ that can affect mechanical and corrosion properties. Parts that have been strengthened by work hardening can see their strength drop locally in the HAZ due to annealing and grain growth. Such annealing in not always bad since the softer HAZ could potentially counter some of the stresses induced in the fusion zone. Grain growth can become potential problem if you are welding very thin metal foils and trying to make a hermetic seal; grains can grow big enough to match the thickness of the foil and provide a leak path along the grain boundaries for Helium to pass through.
Austenitic stainless steels are particularly susceptible to multiple problems in theHAZ. One of them is liquation cracking where a liquid film forms along the grain boundary in the partially melted zone adjacent to the fusion zone. Liquation can occur due to segregation of impurities to grain boundaries. Such cracking can be avoided by ensuring sufficient ferrite is present as second phase in the austenitic matrix. Cracking in HAZ is also observed in austenitic stainless steels where
Cr-carbides form along the grain boundaries thus reducing corrosion resistance leading to attack along grain boundaries in chloride containing environments. Using stabilized grades with Nb and Ti which have greater affinity for carbon thus reducing formation of Cr-carbides is an option. Other options to reduce corrosion include use of low-carbon grades, annealed base materials, low weld heat input, and solution heat treatment after welding.
As the name implies, formation of a weld depends on diffusion of atoms across the weld interface. Since diffusion is a time dependent process, diffusion welding is perhaps the slowest of all welding processes taking up to 4 hours to form the bond. Interestingly enough, when made correctly, it is perhaps the best process to form a weld joint that most closely matches properties of the base metal. Diffusion welding is a solid state process where the parts are held tightly together and heated to less than 80% of the melting point of the lower melting component. Diffusion welding is the most sensitive of all welding processes to surface contamination and oxide layers on metals and alloys. Since atoms have to be in intimate contact across the interface for diffusion to take place, the mating parts require very careful surface preparation to ensure smoothness and flatness. Important process parameters include pressure, temperature, and time.
As the parts to be welded are heated under pressure, asperities on the surface exceed yield stress leading to localized deformation which then produces greater contact across the weld interface. In the second phase of the process, extensive diffusion occurs across the weld interface. Grain boundaries cross over the weld interface and produce a homogenous solid albeit with some porosities trapped along the original boundary. In the last phase, diffusion leads to reduction/elimination of the porosities. A soft filler layer is sometimes used to assist with the first phase of the process and reduce the need for high pressure. Diffusion brazing is another variant where alloy combination or filler layer is used to form an in-situ braze, usually a eutectic, which wets both surfaces. Typically, additional time is allotted to allow the braze to completely diffuse into the matrix. The process can be stopped anytime during the second or third phase depending on weld requirements. For example, it is not necessary to completely dissolve the voids if the application does not call for such a microstructure. Applications for diffusion welding include Ti alloy structures for aerospace components and for bonding powders to metal surface for biomedical applications. Perhaps the most common application is sintering of ceramic and metal powders to form a dense solid for cutting tools and other related applications. Given the cost of sample preparation, long process duration, as well as suitability for a particular alloy combination, it is not surprising that diffusion welding is not a very commonly used process. But it just might be what the doctor ordered for your application.
Porosity - The Friendly Foe
Of all the weld defects that can be listed, porosity is the most benign. Given the circular shape, they do not cause any excessive rise in weld stress. Another advantage is that, as opposed to cracks, porosities do not grow once formed during the welding process. Porosity is the only defect that is allowed by AWS fusion weld specifications (within certain limits of size and number) even in welds of highest quality for aerospace applications. Porosities form in welds for many reasons. In resistance welds, they can form as the metal expands and then freezes before the pore can collapse; one can often find one big pore right in the center of the weld. Pores can form in welds where high vapor pressure elements such as zinc in galvannealed steels have a hard time escaping from the molten metal. Pores can form in aluminum welds with moisture contamination since aluminum does not have any solubility for hydrogen as the molten metal cools down; there is rarely any gas pressure remaining in the pores to be worried about. So if some small amount of porosity is the only issue you are worried about, you really have nothing much to worry about.