Concepts of surface Integrity
Surface integrity is the surface
condition of a workpiece after being modified by a manufacturing process. The
surface integrity of a workpiece or item changes the material's properties. The
consequences of changes to surface integrity are a mechanical engineering
design problem, but the preservation of those properties are a manufacturing
consideration.
Surface integrity can have a
great impact on a parts function; for example, Inconel 718 can have a fatigue
limit as high as 540 MPa (78,000 psi) after a gentlegrinding or as low as 150
MPa (22,000 psi) after electrical discharge machining (EDM).
There are two aspects to surface
integrity: topography characteristics and surface layer characteristics. The
topography is made up of surface roughness, waviness, errors of form, and
flaws. The surface layer characteristics that can change through processing
are: plastic deformation, residual stresses, cracks, hardness, overaging, phase
changes, recrystallization, intergranular attack, and hydrogen embrittlement.
When a traditional manufacturing process is used, such as machining, the
surface layer sustains local plastic deformation.
The processes that affect surface
integrity can be conveniently broken up into three classes: traditional
processes, non-traditional processes, and finishing treatments. Traditional
processes are defined as processes where the tool contacts the workpiece surface;
for example: grinding, turning, and machining. These processes will only damage
the surface integrity if the improper parameters are used, such as dull tools,
too high feed speeds, improper coolant or lubrication, or incorrect grinding
wheel hardness. Nontraditional processes are defined as processes where the
tool does not contact the workpiece; examples of this type of process include
EDM, electrochemical machining, and chemical milling. These processes will
produce different surface integrity depending on how the processes are
controlled; for instance, they can leave a stress-free surface, a remelted
surface, or excessive surface roughness. Finishing treatments are defined as
processes that negate surface finishes imparted by traditional and non-traditional
processes or improve the surface integrity. For example, compressive residual
stress can be enhanced via peening or roller burnishing or the recast layer
left by EDMing can be removed via chemical milling.
Finishing treatments can affect
the workpiece surface in a wide variety of manners. Some clean and/or remove
defects, such as scratches, pores, burrs, flash, or blemishes. Other processes
improve or modify the surface appearance by improving smoothness, texture, or
color. They can also improve corrosion resistance,wear resistance, and/or
reduce friction. Coatings are another type of finishing treatment that may be
used to plate an expensive or scarce material onto a less expensive base
material.
Variables
Manufacturing
processes have five main variables: the workpiece, the tool, the machine tool,
the environment, and process variables. All of these variables can affect the
surface integrity of the workpiece by producing:
· High
temperatures involved in various machining processes
· Plastic
deformation in the workpiece (residual stresses)
· Surface
geometry (roughness, cracks, distortion)
· Chemical
reactions, especially between the tool and the workpiece
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