Final answer:
The selected filler metal in welding often differs from the base materials due to the need to achieve enhanced mechanical properties, corrosion resistance, and to match thermal expansion coefficients, ensuring strong and stable welds.
Step-by-step explanation:
It is not uncommon for the selected filler metal in welding to have a different chemical composition than the base materials being welded. The reason for this lies in the need to address specific welding characteristics, such as achieving desirable mechanical properties, ensuring corrosion resistance, overmatching the strength of the base metals, or compensating for dilution by the base materials. For example, when welding steel, filler metals are often designed to produce alloys that have specific characteristics like enhanced strength, ductility, and toughness that the base metal may lack in its pure form, or to maintain integrity against certain environmental factors.
Moreover, the thermal expansion coefficient of different metals and their weldments is a critical factor to consider during welding. For instance, when a metal implant is placed in the human body, researchers must find a metal coating with a thermal expansion coefficient similar to the metal to prevent thermal stress, which can lead to cracking at the interface. Similarly, in welding, using a filler metal that can accommodate the thermal expansion characteristics of the base materials ensures a strong and stable weld joint, matching the expansion and contraction rates during the heating and cooling phases of welding.