Aluminium Powder Forging Process using a Rotating Platen

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1 Proceedings o the 9 th International Conerence on Aluiniu Alloys (004) 1 Edited by J.F. Nie, A.J. Morton and B.C. Muddle Institute o Materials Engineering Australasia Ltd Aluiniu Powder Forging Process using a Rotating Platen S.S. Youn 1, J.H. Park, C.W. Park 1, Y.H. Ki 1, X. Ma 3 1 ERC/NSDM, School o Mechanical Engineering, Pusan National University Jangjeon-Dong, Kujeong-Ku, Pusan , Korea Research Institute o Mechanical Technology, Pusan National University Jangjeon-Dong, Kujeong-Ku, Pusan , Korea 3 School o Engineering and Technology, Deakin University, Geelong, VIC 317, Australia Keywords: Torsional Upset Forging, Sintered Structure, Upper Bound Analysis, Bulging Eect, Densiication Abstract A torsional upset orging process is analysed on the basis o plasticity theory or powder etal orging. Torsional upset orging is a process to be perored by rotating a lower die with a punch travelling along the longitudinal direction o a work-piece. In this study, an upper bound analysis considering bulging eect, inite eleent ethod siulation (DEFORM 3D ), and experiental research have been perored or the process. A siple kineatically adissible velocity ield or a three diensional deoration is presented or the torsional upset orging o a cylindrical billet. Distributions o stress, strain, and orging load in the process have been obtained, and copared with those in conventional upset orging. In the process, an increase in a riction actor and rotation speed results in a decrease in agnitude o upset orce, dead etal zone, and non-hoogeneous deoration. This process can reduce oring load, which leads to iproveent o die lie, and also reduce bulging eect. In addition, the initial sintered-structure and density distribution is iproved by the process and surace deect due to high deoration is decreased. 1. Introduction A powder orging ethod is a cobined process o P/M (Powder Metallurgy) and precision orging, which takes advantage o the. Due to 10-0% o porosity in products by P/M, they have such lower ductility, atigue strength, and ipact strength that their diension and application have a kind o liitation [1]. However, the liitation caused by lower echanical properties can be overcoe by adopting a powder orging process. In a powder orging process, a preor ade by sintering goes through a orging process, so that an increase in density over the whole product and strengthened echanical properties can be achieved. This also leads to a ine icrostructure, a lightweight products and less energy and aterial. Because o these advantages, the process is being studied as an alternative or parts used in vehicle and aircrat. In these days, various researches have been perored aiing at new ethods that are capable o reducing oring load and induce ore unior deoration by positive application o riction at orging. Aong the, torsional upset orging is a cobined process o a vertical oveent o a punch and a rotational oveent o a die. Experients and inite eleent (FE) siulations were carried out on solid aterials or this process by Xue et al [], Park et al [3] and Kein et al [4].

2 13 Upper bound solutions were also obtained by Ki et al [5] and Ma et al [6] in which reductions in load and barrelling were reproduced. In this study, torsional upset orging is used with purpose to achieve unior distribution o density during orging o powder etal. Upper bound analysis including the eect o hydrostatic pressure, laboratory tests, and FE siulation are carried out. Results o torsional upset orging are shown to be superior to those o conventional orging.. Theoretical Background Aong yield criteria o powder etal, Shia-Oyane s criterion is used in this study [7]: {( σ ) + ( σ σ ) + ( σ σ ) } 1 Φ = σ (1) Here, σ 11, σ, σ 33 are principal stresses, σ is the hydrostatic stress, = 1 (.5 1 ρ )( ρ : relative density), and Φ is the plastic potential. Relationship between the stress and strain tensor coponents σ ij and ε ij (i, j =1-3) is expressed as: ' Φ dε ij = dλ = dλ σ ij 1 σ () σ ij 9 Increental voluetric strain is shown as: dρ dv dε v = dε11 + dε + dε 33 = = = dλ σ (3) ρ V 3 σ 3. Upper Bound Analysis Scheatic drawings o analytical odels or torsional upset orging are shown in Figure 1. ho do z θ r punch billet P ω h u Figure 1: Scheatic drawings o analytical odels o torsional upset orging. (a) geoetry o the initial billet (b) scheatic drawing o experiental equipent Kineatically adissible velocity ield is deterined under the ollowing assuptions: (a) the circuerential velocity coponent is not vanishing due to aterial twisting, and the radial velocity varies with the height o the billet; (b) the riction actor,, is constant during deoration; (c) the volue o the billet varies with the densiication; and the aterial yielding conors with the Shia-Oyane s criterion in which the hydrostatic stress aects yielding, but the aterial is perectly plastic and isotropic and (d) the plastic deoration is increental, and the density is unior in the deored body. The deterined velocity ield in the cylindrical coordinate syste (r, z, θ) o Fig. 1(a) is as ollows. h z v r = AB( 1 β z ) r, vθ = arω, v z = Az ( 1 βz / 3) (4) h

3 14 U A = (5) h( 1 βh / 3) Here, A is deterined ro the boundary condition and B is coeicient to indicate plastic low o a speciic aterial according to the relative density (1 or the conventional etals, 0~1 or sintered etal); a is slip actor which is the ratio o rotation speeds between a billet and a die; ω is the angular velocity o the die; U is the copression speed; and β is the bulging coeicient. The rate o total energy dissipation or the torsional upset orging can be expressed as: * J = W& i + W& W& = PU (6) where the rate o internal energy dissipation W & i is calculated as: W & σ & εdv (7) i = v Here, σ is the eective stress, -ε& is the eective strain rate which can be deterined ro the kineatically adissible velocity ield in Eq (4), and V is the volue o testpiece. The rate o rictional shear energy losses W & is shown as: W& σ = V ds (8) S 3 Here, V is the velocity discontinuity at the contact surace S between the billet and the die, which can occur on the upper and lower contact suraces and can be calculated as: z=h: V punch = AB(1 βh ) r (9) z=0: = ( ABr ) + (( 1 a) rw ) V die (10) The rate o oent energy supplied by the rotation o the lower die W & is deterined as: W& = M ω = 3 St ar ω ( ABr) + ( arω) Inserting Eqs. (7), (8) and (11) into Eqs. (6) gives an expression or the copression load P. This expression can be solved nuerically and the corresponding solution obtained will be copared with that ro the tests and FE siulation in Section 5. σds t (11) 4. Experients Aluix13 (Al-4.5 Cu-0.5 Mg-0.7Si) produced by ECKART Co., Gerany, is selected as the test powder etal or the present experients. This powder etal contains 1.5% icro wax as lubricant, and the other properties are shown in Table 1. The powder is then copacted using the die set as shown in Figure. The speciens are inserted into the sintering urnace, aintained at 400 C or 30 inutes to reove wax, and then directly heated up to 580 C or 5 inutes to be sintered. Ater sintering, the pieces are cooled down to 150 C, and then air cooled to 40 C. N gas was used to abricate the speciens as atosphere one. The sintered products are shown in Figure 3. Table 1 Typical physical characteristics o Aluix13 Apparent density [g/c 3 ] Tap density [g/c 3 ] Green Strength [N/ ] >8.0

4 15 Figure : Scheatic drawing and photograph o die set The equation or low stress is assued as a unction o strain rate and deterined by hot copression tests o the sintered speciens using MTS, which is expressed as: σ = 36.5 & ε [ MPa] (1) In the torsional upset orging experient, the punch velocity is 1 /s and the angular velocities o the lower die are varied with the other experiental conditions as shown in Table. Figure 3: Photograph o sintered speciens (h o /d o =0.75, 1.00, 1.5) Table Condition o experient or torsional upset orging Specien size [h o /d o ] Punch velocity [/sec] Friction actor Angular velocity o die [rad/sec] =0.4 0, 0.1, 0.64, 0.446, 0.57, 0.73, 0.836, Ater the orging, the speciens go through solution heat treatent, where they are heated to 510±5 C or 30 inutes, quenched, and then aged at 170±5 C or 18 hours. 5. Results and Discussion Figure 4 shows the results o experients under the condition o 1.0 in h o /d o and 30% in the copression ratio. It can be seen that there is distinct bulging in the case o the conventional upset orging, while bulging is considerably reduced in the case o the torsional upset orging. However, it is diicult to or cylindrical shape exactly because o diiculties in the deterination o the concentricity and direction o lubricants. Figure 4: Photograph o experiental results (h o /d o =1.0, γ=30%, ω=0.64). (a) initial billet (b) conventional upset orging (c) torsional upset orging

5 16 Load (N) 13,000 1,000 11,000 10,000 9,000 8,000 7,000 6,000 Upper Bound ω =0 0 ω =0.64 Experient ω =0.0 ω =0.64 F E M ω =0.0 ω =0.64 5, Stroke [ ] Figure 5: Result o oring load as a unction o stroke (h o /d o =1.0, γ=30%, =0.4) Figure 5 shows the oring loads obtained by the upper bound analysis, the inite eleent siulation (DEFORM 3D ), and experients both or the conventional upset orging (ω=0.0) and the torsional upset orging (ω=0.64rad/s). The oring load during the conventional upset orging is 1.ton, while that o the torsional upset orging is 1.0ton, which eans 16% reduction in the oring load. Foring loads predicted by upper bound analysis are and 1.16ton, which are in good agreeent with experiental results. Figure 6 shows the distribution o relative density by FE siulation. In the case o the conventional upset orging, the densiication is the lowest near the die because o the riction eect. Near the bulging region, the densiication is also low due to the nonvanishing circuerential tensile stress. Thereore, it is ipossible to obtain overall unior deoration because o a large dierence in the relative density. In case o the torsional upset orging, however, there is ore unior distribution o relative density that is the ost essential point in powder orging. (a) conventional upset orging (b) torsional upset orging Figure 6: Distribution o relative density (γ=30%, ω=0.446). Figure 7 copares the distributions o eective strain during the conventional with the torsional upset orging. The eective strain at the central region in the case o the conventional upset orging is higher than in the torsional one and dierence o eective strain over the whole area is lower in the torsional upset orging than in the conventional one, which indicates an iproveent o the dead zone and an unior deoration in the torsional upset orging. In other words, the rotation o the lower die akes dead etal zone between the die and the billet low soothly that results in reduction o the central zone. Figure 8 shows the distribution o the axiu principal stress or the two cases. More unior distribution is shown in the case o the torsional upset orging and it can reduce the dierence o density in the sintered etal.

6 17 (a) conventional upset orging (b) torsional upset orging Figure 7: Distribution o eective strain (γ=30%, ω=0.64). (a) conventional upset orging (b) torsional upset orging Figure 8: Distribution o the axiu principal stress. 6. Conclusions A torsional upset process o powder etal was studied and the ollowing conclusions were obtained by coparing the results ro the theoretical analysis and the FE siulation with those o experients. 1. The torsional upset orging is able to reduce oring load copared with the conventional one.. By using the torsional upset orging, non-unior deoration can be considerably reduced under the inluence o rotation o the lower die. 3. The torsional upset orging leads to a twist shear deoration within the aterial, which is responsible or a relatively unior distribution o the density or an iproved densiication. Reerences [1] R. Duggirala and R. Shivpuri, Eects o Processing Paraeters in P/M Steel Forging on Part Properties: A Review part Ⅱ Forging o Sintered Copact, Journal o Materials Engineering and Perorance, vol.1, , 199. [] K. M. Xue, Y. Lu, H. H. Lin, and X. M. Zhao, Nuerical siulation and experiental research into the process o twist-copression oring, Advanced Technology o Plasticity, , [3] J.H. Park, Y.H. Ki, and Y.E. Jin, "Experiental Investigation o the Foring Paraeters o the Rotational Upset Forging Process", Journal o Materials Processing Technology, vol.111, , 001. [4] X. Kein W. Zhen and L. Yan, FEM Analysis o Cylinder Twist-Copression deoration regularity, Journal o Material Processing Technology vol.69, , 001. [5] Y.H. Ki, J.H. Park, and Y.E. Jin, "An Analysis o Plastic Deoration Processes or Twist-Assisted Upset Forging o Cylindrical Billets", Journal o Engineering Manuacture, Part B, Proceedings o the Institution o Mechanical Engineers, vol.15, no.6, , 001. [6] X.Ma, M.R. Barnett, and Y.H. Ki," Deoration Behaviour o a Cylinder under Siultaneous Copression and Torsion", Key Engineering Materials, vol.33-36, , 00. [7] S. Shia and M. Oyane, Plasticity Theory or Porous Metals, International Journal o Mechanical Science, vol.18, 85-91, 1976.