forming (conformado) geometry, microstructure and materials forming vs. castings?: even when modern...

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FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and can compete with traditionally forged components, there are THREE REASONS why, for many products, forming is preferable to castings: - GEOMETRY - MICROSTRUCTURE - Some materials are difficult to process as liquids (ceramics, refractory metals)

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Page 1: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

FORMING (Conformado)

Geometry, microstructure and materials

FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and can compete with traditionally forged components, there are THREE REASONS why, for many products, forming is preferable to castings:

- GEOMETRY

- MICROSTRUCTURE

- Some materials are difficult to process as liquids (ceramics, refractory metals)

Page 2: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Geometry: products with one dimension significantly different in size from the others (long products like rails or thin products like car body panels) are difficult to produce by casting and are usually made by a forming process.

Forming processes also have geometrical limitations. In general, products containing cavities or complex re-entrants features are difficult to produce by forming.

Casting and Forming will be in competition with each other only for products which possess geometries obtainable by either method.

Microstructure: Microstructure determines the properties of the final product!. Sound castings exhibiting excellent mechanical properties can be produced with good casting design BUT … these are usually made from short freezing range eutectic alloys. IT IS FAR MORE DIFFICULT TO PRODUCE SOUND CASTINGS IN LONG FREEZING RANGE ALLOYS (why ????).

Do we have to restrict our choice to eutectics?! Most of the metallurgical tricks we can use to get the product properties we want (precipitaion hardening, martensitic transformation, etc.) are generally available ONLY IN LONG FREEZING RANGE ALLOYS

Easy way of producing an alloy: homogenously mix the elements in the liquid state BUT ... THIS HOMOGENEOUS MIXTURE IS NOT RETAINED ON SOLIDIFICATION AND A CAST MICROSTRUCTURE IS PRODUCED. Even relatively fine microstructures contain compositional and microstructural inhomogeneities which are significantly larger than can be tolerated in many products. THEN ….?

Page 3: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

THEN ….?: HEAT TREATMENT to decrease compositional gradients BUT … MICROSTRUCTURAL INHOMOGENENEITIES ARE MOST EFFICIENTLY REMOVED BY PLASTIC DEFORMATION

THEREFORE: Forming Processes are often used to convert CAST MICROSTRUCTURES into more homogeneous WROUGHT MICROSTRUCTURES !

WROUGHT ALLOY PRODUCTION

Microstructure of a cast low alloy steel ingot

The data listed in engineering design reference books correspond to WROUGHT MATERIALS even when,

virtually, all alloys have been originally obtained by casting, i.e., the COARSE GRAIN STRUCTURE, SEGREGATION and POROSITY often found in cast metals have been removed by HOT WORKING.

The raw material for most FORMING and CUTTING processes is already in this WROUGHT condition!

Continuous casting

Page 4: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

To produce a WROUGHT MICROSTRUCTURE a forming process must remove both: SEGREGATION and POROSITY: best achieved by one of the following HOT WORKING processess: ROLLING, EXTRUSION or FORGING to break up the dendrite structure decreasing the effective diffusion distance and healing up any porosity

Large deformations are needed to produce wrought structures directly from ingot to product: large plastic strains. Example: HOT EXTRUSION with extrusion ratios (reduction in area) of 200:1.

Processes that impose less plastic strain on the material such as CLOSED DIE FORGING or SHEET METAL FORMING require the raw material to have a wrought structure!!!!: they MUST use EXTRUDED or ROLLED bar or sheet stock as input material.

In Summary:

- Forming processes require high forces and resilient tooling. The tooling costs often need to be spread over large production volumes.

- Forming is often preferred over Casting for reasons of:

* Product geometry

* Microstructure (and therefore, Properties!)

- In Casting it is EASY to produce COMPLEX SHAPES but DIFFICULTDIFFICULT to produce GOOD GOOD MICROSTRUCTURESMICROSTRUCTURES

Page 5: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Classifying Forming Processes

Forming processes involve the INTERACTION of a WORKPIECE (the starting material) with some form of TOOLING to produce a change in shape: based on the ability of some materials to FLOW PLASTICALLY in the solid state

- Little or no waste

- Large machines, expensive tooling

- Large production quantities are normally necessary to justify forming as a production route

A FIRST CLASSIFICATION is based on the GEOMETRY of the WORKPIECE: 3D or 2D or:

BULK DEFORMATION PROCESSES: the workpiece is subjected to triaxial stresses which are usually compressive in nature, hindering failure processes and improving formability. Examples: forging (card F1), rolling (F3) and extrusion (F4)

SHEET DEFORMATION PROCESSES: the workpiece surface area to thickness ratio is relatively high. Both the workpiece and the imposed stresses can be considered 2D. Examples: sheet metal forming (F2), vacuum forming (F6) and blow molding (F7).

Sheet material is usually produced first by continuous bulk processes such as rolling followed by “blanking“ and only then the material is pressed into closed dies.

Page 6: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and
Page 7: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and
Page 8: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and
Page 9: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and
Page 10: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Stress Systems in:

sheet processes bulk processes

Sheet deformation processes are best approximated as plane stress

Bulk deformation processes are best approximated as plane strain

Page 11: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Most sheet deformation processes involve a combination of bending and

stretching. Stress state can be approached as plane stress. The consequence is that forming loads are not particularly high in sheet processes and Forming Presses are more often limited by their maximum deformation than by their maximum load.

In bulk deformation processes: constrain in one direction (z direction in

previous picture) resulting in close to plane strain condition: the workpiece becomes longer and no wider!. The same constraint operates in rolling. Therefore there is a finite stress along the z direction. Plane strain processes involve a hydrostatic stress component wich is normally compressive and this effectively hinders failure processes promoting formability.

Page 12: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Load and Flow Modelling:

The main interest in modelling plastic flow is in predicting loads for bulk forming, since sheet forming loads are relatively low.

Approximations to material stress-strain curves: rigid-plastic; elastic-plastic; rigid-plastic with work hardening; elastic-plastic with work hardening, Hollomon ( = K n ); etc.

Work and Heat of Deformation:

total work of deformation

ideal work of deformation

work of friction

redundant work

efficiency of the deformation process

Page 13: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Ideal Work of Deformation: Uideal

Redundant work: Uredundant

+

+Work of Friction: Ufriction

Tool – Workpiece Interaction:

* Friction

* Acting Loads

f

dU ideal

0

.

redundant work in extruding sheet material through a die

Total WORK of DEFORMATION

Utotal = Uideal + Ufriction + Uredundant

total

ideal

U

UProcess Efficiency:

c

U

c

UT idealtotal

...max Temperature Increase:

RISK !!

Hot Shortness

Hot shortness in Al alloy extrusions

Page 14: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Distribution of tooling pressure in plane strain forging: the friction hill

PLANE STRAIN FORGING

)(

2exp)( ax

hYxpxy

From Equilibrium + Tresca (low friction):

h

aYxp

2exp0

Maximum tooling pressure:

h

aYp

1

Mean tooling pressure:

h

awaYwapF

1).2().2(

Tooling load:

Page 15: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

Friction in close die forging

h

arYF

3

21 2

For a cylindrical workpiece:

Page 16: FORMING (Conformado) Geometry, microstructure and materials FORMING vs. CASTINGS?: Even when modern castings can possses good structural integrity and

ROLLING

Use of front and back tension to reduce friction in rolling

Friction Hill in rolling

2121

21

h

b

h

rYp

Mean rolling pressure:

Strip rolling in four stand continuous mill

Backing rolls