Contribution of nanoindentation and tribology to investigation of welds

The quality of welds depends on the materials used and welding parameters. To help optimizing the welding process, it is necessary to perform very local mechanical characterization of the weld. Nanoindentation and tribology are useful tools for analyzing the effects of the welding process on hardness, elastic-plastic properties and wear resistance.

Welding is an important metallurgical process used to

join two or more components together, usually for the

creation of more complex structures that cannot be

molded directly (tubular frames or structures), for

attaching functional parts to a larger component

(hinges, etc.) or for surface protection. The most

commonly welded materials are metals and

thermoplastics. There are many welding processes

with the most widely used being arc and gas welding.

Other methods such as friction stir welding,

electron/laser beam welding or ultrasonic welding exist

and are employed in cases of welding special materials

or in special environments. During welding, two

parts are heated or melted at the joining surfaces and

fused together using a filler material. Crucial process

parameters for arc welding are arc voltage, passing

current, welding speed, weld geometry, shield gas

composition, and electrode feed speed. These

parameters determine the extent of the thermally

affected material around the weld, the so-called “heat

affected zone” (HAZ). The HAZ is a part of material

close to the weld that has been heated during the

welding process below the melting point but high

enough to undergo microstructural changes. These

changes can lead to changes in mechanical properties

such as increased hardness and decreased yield

strength. Due to the microstructural changes the HAZ

is more prone to cracking and corroding and therefore

the HAZ usually acts as the weakest structural point of

the component. Therefore, it is crucial to understand

and minimize the undesired thermal effects of welding.

The typical dimensions of the weld and the HAZ are

hundreds of micrometers up to several millimeters. To

study the local changes due to the welding process, the

methods of instrumented indentation are preferred

because they offer suitable spatial resolution. For

example, the Anton Paar Micro Combi Tester (MCT3)

or Nano Hardness tester (NHT3) can measure

hardness and elastic modulus or local indentation

stress-strain characteristics in different areas of the

weld or HAZ. To study the causes of weld cracking,

measurements of representative stress-strain curves

around the crack tip can be performed. For cases when

the weld is in frictional contact with another component

or the environment, its tribological and wear properties

are also important. The wear and friction properties can

easily by measured by a tribometer which measures

the coefficient of friction and can be used for wear rate


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