Defects in Rolled Plates and Sheets

Surface Defects:

Impurities and foreign material inclusions in the original cast material can cause defects like as scale, corrosion, scratches, fractures, pits, and gouges on the surface of rolled sheets.

Inadequate circumstances during material preparation or rolling operation are also possible causes of such problems.

Wavy edges, zipper fractures, edge cracks, alligatoring, folds, and laminations are examples of structural flaws. These flaws can be further divided into the following categories:

i. Defects caused by roll bending:

• Rolls deflect as if they were straight beams loaded transversely (with rolling loads). As a result, the strip’s edges are compressed more than the core piece, making the edges thinner than the centre.
• The strip elongates more near the margins than at the centre because the drop in thickness is turned into an increase in length.
• However, because the material is continuous, tensions inside the material are adjusted.
• Compressive strains are experienced on the material’s borders, while tensile strains are experienced in the centre.
• Wavy edges form on the sheet because the edges are prevented from extending freely in the longitudinal (rolling) direction.
• Zipper cracks in the centre region of the strip are generated by differential strain and the material’s weak ductility at room temperature.
• The foregoing two flaws can be mitigated to some extent by giving the rolls camber.
• i.e., the diameter of the rolls is somewhat larger in the middle than it is at the ends. The rollers will offer a straight uniform gap to the strip under load situations.

 ii. Defects caused by inhomogeneous deformation: 

• This is the most common source of edge cracks. The material’s thickness decreases proportionately as it is squeezed under the rolls, but its length and width rise (lateral spread).
• The decrease in thickness for elements towards the centre will primarily be translated to lengthening, whereas for components near the edges, a portion of the decrease in thickness will be translated to lengthening and the remainder will be converted to lateral spread.
• As a result, the increase in length at the centre will be greater than at the edges, resulting in rounded sheet ends.
  
• The material towards the edges will be under strain, while the material at the centre will be compressed, due to the continuity of material elements.
• This can result in the production of edge cracks or, in extreme cases, a sheet split in the middle.
• Alligatoring is a frequent fault induced by non-uniform material deformation during the rolling process.
• The middle plane of the rolled sheet looks to be ruptured.
• Alligatoring is a complicated occurrence that is caused by a number of causes.
• The majority of rolled sheet flaws degrade the edge quality. Slitting the edges of the sheets is how these flaws are removed in practise.

Process of Extrusion

• Extrusion is a compression method that forces work metal through a die aperture to generate a specified cross-sectional shape.
• Aluminum, copper, steel, magnesium, and polymers are among the materials that can be extruded. Extrusion works best with aluminium, copper, and polymers.
• Squeezing toothpaste from its plastic tube is a similar process.

Advantages include:

1. Extrusion, especially hot extrusion, allows for a wide range of shapes.
2. Cold and warm extrusion improve grain structure and strength qualities.
3. Extremely tight tolerances, especially in cold extrusion, are conceivable.
4. In some extrusion processes, there is minimal or no waste material.

Limitations: 

• The extruded part’s cross section must be consistent throughout its length.
• Even when hot extruded, the force required is considerable.
• The extrusion technique has high tooling and setup costs, but the actual manufactured product is less expensive when produced in large quantities.

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