Quality of welded joint

The welding process permits the transfer of much energy in a very short time (about 10-100 milliseconds), avoiding excessive heating of the remainder of the sheets. In order to clearly understand in which way the heat is transferred, it is necessary to study what happens in the crystal structure of the alloys involved after the melting metal, how the grains recrystallizes and how their influence the weld joint’s strength.

4.3.1 Metallurgical principles

The aim of this chapter is to analyze the metallurgical principles governing the aspects of RSW and how they are critical in understanding the formation of the structure of welded joints. During welding, the solidification of the liquid part begins as usual with the nucleation and proceeds with the growth of the crystals. This last process is directly influenced by the heat dissipation into electrodes and metal sheets: the size, type and

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orientation of the generated crystals depends on the direction and the rate of cooling. If the rate is too quicky, the micro-segregation phenomena is arisen. In the spot weld, a stratify layers with different crystalline structure and chemical compositions take place, because an equilibrium composition distribution is not achieved due to an insufficient diffusion rate. So, at the end of this process, a different composition between the core and the outer layer are formed and the differences between them increases as much as increase, in the phase diagrams, the distance between the liquidus and solidus lines. The only ways to decrease these differences, is increasing the diffusion rate or increasing the time span useful to the solidification. In the alloys subjected to the welding process, the elements are involved in another phenomena, called segregation. It is the chemical-physical phenomenon where in a solid solution formed during solidification of a liquid one, the liquid component that solidifies with an upper melting point, solidifies with the native structure without the interference of the other component. So, due to the different melting point of the elements that formed the alloy, this process takes place as the solid-liquid interface advances into the solid-liquid, and as result, the concentration of the element with lower melting point is increased in the remaining melt of alloying elements. [29]

A huge problem can occur in aluminum alloys, when at grain boundaries certain compounds, rejected from solid solution, are the last to solidify due to their lower melting temperatures. If the metal is stretched by thermal stresses during the process or by an external load, a crack can be generated at the grains surrounded by such liquid as the liquid has no stretch under these conditions. This problem can be solved in the RSW thanks to the right pression performed by electrodes.

Solidification starts at the boarders of the heat-affect zone (HAZ), where partially melted grains become nuclei of the new solid grains’ growth, with columnar orientation. The central zone of the HAZ, solidifies last, on condition that the melt liquid volume is smaller than the solid volume surrounding it. This becomes the new site for the equiaxed grains’

growth, oriented depending on the versus of the cooling rate. The energy dissipation, enabled in the liquid volume if the quantity of heat that goes out is bigger than the quantity that flows into, depends on the source of heating around it: the water-cooled electrodes and the cool metal sheets, that transfer the energy from the sides. Keeping in mind this influence, three different conditions can appear. The first one (Fig. 49) is the ideal one, in which the solidification occurs in uniform way from the electrode and sheets’ sides. In

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this configuration, if cracks and voids are generated in the central portion of the nugget, they don’t affect the performance of the welding.

Figure 49-Grains in a uniform solidified joint [25]

A different structure is obtained when the electrodes are overcooled. Looking at figure 50, it can be seen long columnar grains in electrodes direction and smaller grains from the sides. This configuration takes places when the cooling rate is bigger in vertical direction, so due to faster solidification from top to the bottom, the last small liquid volume solidifies near to the original interface of the metal sheets. The situation is significantly dangerous because a deficit of volume is source of cracks between the sheets and due to the lower solidification rate at the lateral directions, they can be very close to the HAZ, since the reduced volume of liquid is located at the periphery of the spot weld.

Figure 50- Structure obtained with overcooled electrodes condition [25]

The third case is verified when the cooling rate is faster in longitudinal direction rather than in vertical one. This can happen if the contact between electrodes and metal sheets is located in a small area or electrodes are subject to wear. The result is shown in figure 51. It is clear that the last liquid volume with equiaxed grains solidifies at the center of the nugget, since the most heat is dissipated through the sheets. In this area cracks and

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voids are located mostly, due to the reduced dimension of the mentioned volume, as before. Being far from the periphery of the spot weld, the effect on its strength, is neglected. However, the propagation originated from these discontinuities can become a problem if they arrive close to the edge of the nugget. [25]

Figure 51-structure of the nugget with cooling rate faster along longitudinal direction [25]