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Forg ing L im i ts fo r an A luminum M at rixCo m posi te : Par t I Exp er im enta l Resu lt s

D.-G.C. SYU and A.K. GHOSH

Forging limits in a discontinuously reinforced aluminum (DRA) matrix composite, 2014 A1/15vol pct A1203, were dete rmined by compressing samples of various cylindrical geometries underdifferent conditions of temperature, strain rate, and lubrication and measuring the limit strainsattained prior to incipient crack formation. In some cases, circumferential grids were machinedon the sample surface to obtain the local fracture strain states. Crack formation was caused bythe secondary tensile stresses; however, crack propagation was relatively slow and somewhatmore severe at 300 ~ than at 400 ~ The forging limit of the composite was found to behigher at 400 ~ than at 300 ~ and also higher at slower strain rates. The plane-strain forginglimit of the composite at 300 ~ and a strain rate of 0.5 s -~ was less than 0.05, while that ofthe matrix was higher than 0.5. It was found that the forging limits can be influenced by thedepth of the circumferential grids and can be lower than those for the smooth surface samples.

I. INTRODUCTION

LL forging processes basically consist of the com-pressive deformation of the workpiece within a pair of

dies. Depending on the geometry of the dies, varyingamounts of lateral constraint may be imposed on theworkpiece so that forging operations are classified intotwo categories: open-die forging and closed-die forging.In open-die forging, the lateral constraint is minimal, andthe amount and distribution of lateral metal flow are con-trolled by factors, such as the geometry of the work-piece, friction and heat transfer between the dies and theworkpiece, and the total reduction in the workpiece di-mension parallel to the compression direction. In open-die forging, formability is primarily determined by themateria l s intrinsic properties and the forging conditions.A mate rial s intrinsic properties include the grain struc-

ture, the presence or absence of second phase (precipi-tates or reinforcements), homogeneity (distribution andshape of second phases or texture), and crystal structure.The heat transfer induced by thermal conductivity andspecific-heat capacity is also important in hot forging.Other forging conditions that are important include heat-ing rates, forging temperatures (which influence graingrowth, recrystallization, phase transformation, the oc-currence of a liquid phase, and dissolution or growth ofa second phase, e t c . ) , as well as strain rates and loadinghistories (including variation of strain rates and strainstates).

During the early stages of closed-die forging, the lat-eral constraint is minimal, and the process is similar tothe open-die forging operation. Cracks and defects ofconcern can often occur during this stage. As the con-straint increases, material begins to fill the shape of thedie, and the ease of the metal to flow becomes the keyfactor for attaining the final shape, i . e . , to fill in thecorner of the die. Hence, the two important parameters

D.-G .C. SY U former ly Graduate Student the Univers i ty ofMichigan is Senior Engineer Taipei Municipal Gov ernm ent HsintienTa i p e i Ta i wa n Re pub l i c o f Ch i na . A .K. GH OSH Pr o f e ss o r o fMaterials Scien ce and Engineering is with the Universi ty o f MichiganAnn Arbor MI 48109-2136.

Man uscript submit ted Augu st i 1 1992.

that determine a successful closed-die-forging operationare the strain states that avoid cracking and the abilityof plastic flow to fill the shape of the die.

The focus of this investigation is on the experimentaldetermination of the limit strains or f o rg i n g - l i m i t d i a -g r a m that can be used by the die designer in actual forg-ing operations. The forging limit is defined by acombination of compressive strain (E ) and tensile strain(e2) on the surface of a forging at the onset of an incip-ient crack. Forging limit refers to strain limit reached ina localized region of a forging but does not provide asense of the material s capability for die filling duringforging process. To test the capacity for plastic flow, aseparate forgeability test was designed, which is dis-cussed in the Appendix and not directly dealt with in thetext.

The forging-limit diagram is divided into three re-gions-the safe, the marginal, and the failure regions.Several investigators have determined the forging-limitdiagrams of aluminum alloys ~,21 and steels. 134~s] The mostcommon method for determining the forging-limit dia-gram is to use the upset test in which the strain path i . e . ,e2/e~) can be varied by using different sample geome-tries and lubrication conditions. The upset test is an axial-compression test o f a right-circular cylinder between twoflat, parallel dies. The advantage of the upset test is thatthe strain state on the cylindrical surface can be easilymeasured and controlled by varying the frictional bound-ary conditions, the ratio of height to diameter, and thegeometry of the cylinder. Figure 1 shows the possiblestrain paths in diffe rent upset tests. The strain-path vari-ations are caused mainly by the different degree of bar-reling on the surface of upset samples. At a givencircumferential strain, e2, which is tensile, the well-lubricated upset sample produces a more negative com-pressive axial strain, el, than the poorly lubricatedone. t2-8~ Samples with higher ratios o f height to diameteralso produce more negative et s than those with lowerratios. Samples with a raised collar on the surface (de-scribed in detail in the Section II) provide strain statesthat are close to positive plane strain (El = 0) .

Experimental data from several previous works ~ show

METALLURGICALAND MATERIALS TRANSA CTIONS A VOLUM E 25A SEPTEMBER 1994--20

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