domande d'esame maurizio vedani

Failure and controls of metals

Thanks to:

• Agazzi Nicholas
• Armanini Linda
• Giulio Benedini

Surface and bulk defects in aluminum

In this cast made of Al, we can find surface and bulk defects. For the surface defects, like sink, blister, cold shot, joints or vortex, ejection mark and thermal fatigue, we use the liquid penetrant method because it is very cheap, simple, and fast at detecting cracks. Otherwise, if we want to know the size of defects, we can use Eddy current NDT because Al is conductive and in this way, we can also check defects under the subsurface (usually depths until 37% of surface density).

Detection methods for bulk defects

For bulk defects, like shrinkage porosity, gas porosity, inclusions like aluminum oxide, and hot tears in the geometrically constrained zones (stress concentration factor), the X-rays method is used. We cannot use cheaper methods like ultrasound because the cast has a complex geometry with a lot of voids, and usually, the surfaces of casts are wrinkled, creating scattering of US-waves. Also, magnetic particles cannot be used because Al isn’t magnetic.

Surface and bulk defects in steel

For surface defects, we can use magnetic particle NDT because steel is ferromagnetic if it is not austenitic, or Eddy current, but it is less effective because steel is a conductor but less conductive than Al or Cu. For bulk defects, we can use UTS because it is cheaper, and the structure is very simple without complex geometry. We cannot use X-rays because the sample is too thick, and the technique is very expensive. We expect to find shrinkage porosity or gas porosity and inclusion in the bulk and hot tears where there are constrained zones.

For surface defects, we can use magnetic particles because steel is magnetic, or liquid penetrants, but in that way, it is more difficult because we expect to have a lot of cracks on the surface. For bulk defects, we use US with inclined probes or x-ray, but the last one is more expensive. The most probable defects that we can find are porosity gas, cold and hot cracks, slag inclusions, and incomplete penetration or lack of fusion. Hot tears occur where there are constrained zones, usually in weld metal zones, and cold cracks in heat-affected zones due to residual stresses or phase transformation.

X-ray production and application

X-rays are produced by heating a tungsten filament, which emits electrons (thermionic effect), accelerated by an anode. The rays produced hit the object and cross it with an intensity depending on the density and thickness of the material. They blacken a screen according to their intensity, giving a 2D image of the 3D object. In this way, it is possible to detect surface as well as bulk defects. In particular, it can be used in the presence of welding defects (inclusions, cracks, bandings, lack of penetration), foundry products defects (shrinkage, porosity), inclusions, and incorporated elements of corrosion.

Advantages and disadvantages of X-rays

• Advantages: both surface and bulk defects, widely available, wide range of materials.
• Disadvantages: no information about the position of the defect, requires qualified operators.

Quality indicators show the sensitivity of the analysis and are of two types:

• Wire type parameters: 16 wires with different diameters are scanned. The smallest diameter detected indicates sensitivity.
• Plaque type parameters: 3 holes of T, 2T, and 4T diameter are scanned. The hole shown with the best resolution gives information about sensitivity.

Distortions and residual stresses

Distortions are the macroscopic effect of residual stresses; they can be reversible or irreversible. The first ones are usually caused by cycling of heating and cooling, homogeneous in all volume, but there is no phase modification and so no change in shape and size when the sample comes back to room temperature. The irreversible distortions can be divided into:

• Size distortions: usually caused by phase transformation during cooling or tempering (martensite transformation), they can be solved during finishing operations or with the correct tolerance of the parts.
• Shape distortions: usually caused by inhomogeneous thermomechanical cycles during heating or cooling. Causes during heating include large thickness reductions (different heating rates), relief of residual stresses, high-temperature plastic deformation due to weight or stacking of parts (creep). Causes during cooling include large thickness reduction, high-temperature plastic deformation, and cooling efficiency of quenching media.

Distortions are caused by residual stresses from manufacturing operations, handling of parts at high temperature, quenching, and hot or cold rolling.

Mechanisms for toughening of brittle materials

These two types of extrinsic mechanisms are both for toughening brittle materials, making the crack path more tortuous. Microscopic mechanisms try to affect the crack path using residual stresses or particle distributions:

• Crack deflection and meandering: the crack path can be deviated, promoting the formation of second phases with suitable dimensions that oppose the stress field of the crack, hindering its propagation along a straight line and making it tortuous, or promoting the formation of smaller particles that, breaking, affect the crack path.
• Zone shielding: the stress field near the crack tip is reduced by means of phase transformation, development of microcracks, or microvoids.

Mesoscopic mechanisms try to optimize the fibrous texture of the material to deviate crack propagation from a straight path:

• Crack arrester: there is a delamination ahead of the crack tip that reduces triaxiality (to plane stress) and crack-tip acuity, leading to less stress intensification, which contributes positively to arrest the propagation of the crack.
• Crack divider: instead of a bulk piece, the crack propagates across different planes, so the triaxiality is lost, and we reach the better toughness condition of plane stress.

Ultrasonic testing method

The US testing method exploits the piezoelectricity of materials like quartz, which is their ability to expand or contract in the presence of an electrical signal.