Page 44 - MetalForming March 2023 - PMA Chairman Jeff Aznavorian

Fig. 2—Sheet metal motion during deep drawing.
requiring a deep draw, including cor- ners where the steel experiences cir- cumferential compression on the binder. This is one reason why higher- strength AHSS grades should have open-ended part-design developments.
Of note: The rm value of many alu- minum alloys also is close to or less than 1.
Impact on Formability
Unlike n-value when testing steels, changes in r-value do not directly affect the forming-limit curve itself. The key impact is in the uniaxial tension strain path, where higher r-value moves the slope of the strain path farther away from plane strain on the vertical axis. This means that parts now can tolerate greater major strain before necking, translating to deeper and more com- plex draws.
A line in true strain space charac- terizes the uniaxial tension strain path:
slope = – (1+r)/r
Fig. 1 shows how r-value and the associated strain-path slope impacts the achievable major strain. When forming a material with r = 1 and FLC0 = 0.29, the forming strains in the uniaxial tensile strain path reach a true major strain of 0.58 (equating to an
engineering strain of 78 percent) before necking. Deforming a material with r = 2 may reach a true major strain of 0.90 (an engineering strain of 145 per- cent) in this strain path before necking occurs.
R-Value Influence on Cup Drawing
Cup drawing occurs when a cylin- drical punch contacts and then pushes a circular blank into a die. Material in the blank flows over the die radius into the cup wall. The sheet metal move- ment from the flat blank to the vertical sidewalls (Fig. 2) is the only movement; no sheet metal flows within the flat bottom region. The flange of the cir- cular blank undergoes radial tension and circumferential compression as the flange moves in a radial direction toward the circular die radius in response to the pull generated by a flat-bottom punch. Blankholder pres- sure should be set to prevent buckles in the blank, but no higher as that would restrict material flow.
Limiting draw ratio (LDR) represents the ratio of largest blank diameter (dBmax) to punch diameter (dP) that can successfully be drawn into a cup:
Factors that increase LDR include higher r-value, reduced friction coef- ficient (through use of a better lubri- cant) and a larger die-corner radius with an associated increase in the radius/thickness ratio. The biggest impact in LDR occurs with r-value; grades with high r-value deform more in the plane of the sheet and less through the thickness, which allows more deformation before thinning and tearing.
The higher r-value of mild steel results in a greater LDR compared with dual-phase steels. Transformation induced plasticity ( TRIP) and 3rd Gen steels have improved LDR, which ben- efits deep drawability. Both steel types rely on the TRIP effect, where austenite in the microstructure transforms to martensite as a flat blank deforms into the targeted part shape. The magnitude of this transformation from plane strain deformation in the cup wall is greater than transformation from shrink flang- ing in the flange area, making the wall area stronger than the flange area, thereby increasing the measured LDR.
Remember that the four corners of a rectangular pan each behave as a quarter of a cup draw, so similar trou- bleshooting strategies and constraints apply. MF
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MetalForming/March 2023 41
LDR = d
Bmax
/d
P
Metal Matters
Circular-blank diameter Circular-blank diameter
Drawn-cup diameter