Basic rules for rheologic forging process of semisolid alloy

Journal of University of Science and Technology BeijingMaterialsVolume I1, Number 6, December 2004, Page 566Basic rules for rheologic forging process of semisolid alloyShuming Xing, Lizhong Zhang, Jianbo Tan, Chuanlin Zheng, Hanwu Liu, Peng Zhang, and Yunhui DuSemisolid Processing Research Center, Beijing Jiaotong University, Beijng 100044, China(Received 2004-01-28)Abstract: Semisolid mold forging is a major type of semisolid processing, which is different from neither traditional mold forgingnor traditional permanent casting. However, processing defects are often seen in work pieces because of lacking available rules forthe process design and control. Some basic rules for the process design and control, simply named the shortest flowing length, pres-sure flling and the minimum uplifting mold pressure, are advanced in the paper based on amount of researches and experiments. Theequations to determine the major process parameters are given out such as the flling pressure, forming pressure and locking moldpressure for the process design and control. The rules and equations are experimentally proved available and applicable by severalactual work pieces produced by the semisolid forging process.Key words: semisolid alloy; theologic forming; mold-forge; process design; rule1 Introductionsolid forging process or the liquid casting process. Ifthe process design and control is not reasonable, someSemisolid forging, a new process to make metalsdefects will be seen in the products.and alloys shaped, is a major type of semisolid form-ing process. Its obvious advantages are easier to pro-Although some suggestions for the process designduce complex work pieces because of its excellentand control can be seen in some papers [5, 6], the .forming ability, more flexible to shape, and morespecific and complete rules are still necessary to ex-compact in the inner quality for its high pressure thanplore in detail. Three basic rules for the semisolidthe semisolid die casting process. Although there areforging process design and control and three basicmany examples to successfully produce work piecesequations used to determine the process parametersby the process in the world [1, 2], the process defectsare experimentally and theoretically advanced in thesuch as shrinkages, cracks, cold shut, shut run and sopaper according to previous researches and recent ex-on are often seen in the products due to lacking valu-perimental results.able design rules [3, 4]. So it is necessary and urgentto build basic rules for the design and control of the2 Three Basic rulessemisolid rheologic forging process.To determine the process project is a key task in theIt is known that a semisolid process is generally di-process design, especially in the selction of formingvided into two essential types, one is called rheologicequipment and mold designs. The major contents of aforming process, the other is named thixologic form-process project include determining the forming situa-ing process. Similarly, the semisolid forging processtion of a wok piece, selecting the dividing mold planes,can also be divided into two types: semisolidcalculating the key process parameters such as therheologic forging process and semisolid thixologicfilling pressure, locking mold pressure and formingprocess. In the later process the billets used to formpressure, and deciding the number and shapes of the:an keep a certain shape even at the semisolid tem-core used to form inner holes. Although a processperature, therefore its process design and control isproject is strongly relative to the shape and demandssimilar to the traditional forging process of solid mate-IL ovist some common princi-rials. However, in the rheologic forge process, the中国煤化工by tereicall and ex-charge used to form is semisolid slurry that can flow:YHC N M H Ghe common basic princi-like common liquid without any special shapes. So theples or rules are: (1) the shortest flowing length, i.e.process design and control for the rheologic forginghe length to be filled must be shorter than the limitprocess is a special problem differing from either theflowing length of the alloy; (2) pressure filling, whichCorresponding author: Shuming Xing, E-mail: smxing@ center.nju.cdu.cnS.M. Xing et al, Basic rules for rheologic forging process of semisolid alloy567demands the most parts of the mold forcefully flledlIn fact the limit flowing length of semisolid slurryby a pressure, ie. the mold is filled with pressure andis relative to the dimensions of flowing channels. Theavoiding filling without pressure as much as possible;larger the section area of flowing channels, the smaller(3) the minimum uplifing mold pressure, i.e. keep thethe loss of pressure and temperature during the flow-pressure to uplift mold less than that to lock mold.ing process, and therefore the longer the limit flowinglength. As a result, the SFL rule is an equivalence ofIf the first rule is violated in the process design orthe maximum flowing section area rule.control, the work piece cannot be completely formedat all. If the second one is breached, it is difficult toIn usage of the SFL rule, it is important to know theobtain a dense work piece. If the third one is violated,effects of process factors and slurry factors on thethe process cannot carry out because of safety or eco-limit flowing length. An equation to determine thenomics. Only if the three basic principles or rules arelimit flowing length Lmax of slurry with a criticalabided well, a fine product may be produced out.stress Tc flowing through a circle pipe with radius Racted by the pressure p; has been theoretically and ex-2.1 The shortest flowing length (SFL) ruleperimentally deduced by S.M. Xing and L.Z. ZhangThe essence of the semisolid rheologic forging is a[5], shown as:process to make semisolid slurry flow into molds, beLmax=PLR(1)solidified into solid and compensate the shrinkage un-Tcder the action of a pressure. However, semisolid slurryFrom equation (1), it is seen that the limit flowingcan flow only under some conditions. The condition islength is relative to the illing section dimension whi-experimentally proved that the pressure or sheeringch is controlled mainly by forming situations for astress acted on the slurry must be larger than the criti-work piece. So, to a work piece, both of the length tocal stress of the slurry [7, 8]. However, the criticalfill and the limit flowing length are different with thestress is varied with the microstructure and tempera-forming situations. For a simple example, when a Cu-ture of the slurry, which may change during the flow-boid work piece of 500 mm long, 200 mm wide, anding process even the outer pressure keeping constant.50 mm thick, is indirectly pressed to form, the lengthIn fact the longer the length flowed of the slurry, theto fill and the area of the flowing channel section at alager the loss of pressure and temperature along thhorizontal forming place is respectively 250 mm andlength filled. As a result, the critical stress of the slurry250 cm, which is 1 time smaller and 2.5 times largerincreases with the flowing length because of the de-respectively than those at a vertical forming placecrease of its temperature during the flowing process [5,shown as the figure 2. As a result, it is confirmed that7]. When the flowing length is larger than some value,the work piece formed at a horizontal forming place isnamed limit flowing length, the slurry becomes a solidmore fine and complete than at vertical forming place.and can not flow at all. Only if the length to be filledis shorter than the limit length, a perfect work pieceMoreover, from equation (1) it is seen that the limitcan be obtained. So the first basic principle to be abid-flowing length is relative to the critical stress of theed in the rheologic forging process project design isslurry. Because the critical stress is controlled by stainthat the maximum length to fill is shorter than therate, which is controlled by the flling speed of thelimit flowing length of the slurry, simply named theslurry, a fast flling speed is benefit to obtain a com-shortest flowing length rule (SFL rule for short), asplete work piece. So high pressing velocity of theshown in figure 1.punch in the semisolid rheologic forging process ispopular in industrial production.2.2 Pressure flling (PF) ruleMost advantages of the semisolid processing are re-sulted from the action of pressure on the slurry [6, 7].However, there are two different periods in the actualprocess, one is named pressure filling, the other isnamed naturallv filling or filling without pressure. Theform中国煤化工ime when the punchtouclCNMHGe when the slurryLm Flowing lengthsolidn 1心s conprctcly, i WuICl1 une slurry filling proc-Figure 1 Infuence of the flowing length on efficient pres-ess is continually acted by a pressure. The later, fllingsure and critical stress.without pressure, exists in the beginning stage from568J. Univr. Sci. Technol. Beijing, Vol.11, No.6, Dec 2004the time when the slurry is just poured into mold tocasting process. However, in the pressure flling stage,that time when the punch is contacted with the slurry.the slurry is filled into the mold acted by a high pres-In the naturally flling process, the force making slurrysure all the time so as to form a complete and perfectflow is only the gravity which is not large enough towork piece. Therefore, in a semisolid forge process, tomake the slurry flow and to fill the mold well. As ashorten the naturally filling stage as much as possibleresult, the macrostructure of the work parts iris a basic rule, namely the pressure flling rule (PFnaturally filling is the same as that in the traditionalrule).(ab)PunchWorkpieceFigure 2 Comparison of different forming situations: (a) horizontally forming; (b) vertically forming.There are two kinds of pressing modes, directlywithout pressure. However, in the indirectly pressingpressing mode and indirectly pressing mode as shownmode, the slurry is poured into a special cavity first,in figure 3, in the semisolid rheologic forging process.and then is filled into the mold by a pressure, thereforeIn order to avoid naturally filling mold, it is necessarythe whole work piece is formed by a pressure. It is forto select a rational press mode. In the directly pressingthis reason that the indirectly pressing mode is alwaysmode, the semisolid slurry is first poured into theto be used to produce a complex work piece in thebottom of the mold cavity and solidified and formedprocess.(a)(b)Core. Pressing flling- Natural flingFigure 3 Pressure flling and natural flling: (由) directly pressing; (b) indirectly pressing.If a work piece has to be formed by the directlybasic principle of the semisolid forging process.pressing mode, the parts with complex shapes of the2.3 Rule of the minimum uplifting mold pressurework piece are filled by pressure as much as possibleMUMP rule)through rationally selecting the forming place andThe pressure to uplift mold is a force to make thepressing mode. For example, fine work pieces can be中国煤化工s and slurry in the mold.obtained with the forming place as shown in figure:YHCN M H Ge locking mold presure3(a), otherwise the slat of the work pieces cannot beor uc sunung mauint, it unold will be separated incompletely formed with the forming place shown asthe flling mold process and result in some accidentsfigure 3(b). It is confirmed that the PF rule is anotheror produce some excrescent constructions. The smal-S.M. Xing et al, Basic rules for rheologic forging process of semisolid alloy569ler the uplifing mold pressure in the process is, thelarger than the critical stress at the whole filling period.larger the power of the forming machine needed is.From equation (1), an additional rule to determine theTherefore, making the uplifing mold pressure mini-filling pressure is obtained and shown as:mum is the third basic rule, namely the rule of mini-2Lmaxmum uplifting mold pressure (MUMP rule).(3)There are many ways to abide the rule such asIf the actual flling pressure breaches equation (3),making the minimum projection area of the work pie-the mold cannot be completely flled, and completece on the plane vertical to the pressing direction,work pieces cannot be obtained.strengthening the exhaust effects and decreasing theflowing velocity of the slurry. In fact, the minimum3.2 Rule to determine the forming pressureuplifting mold pressure is proportional to the projec-The forming pressure, a necessary pressure to formtion area on the plane vertical to the pressing directiona work piece， is used to compensate the volumeand the pressure acted on the slurry, shown as the fol-shrinkage in the solidification process to prevent thelowing formula:work piece from shrinkage defects. However, the val-F= pA(2ues of the forming pressure are mainly relative to thedimensions of flowing channels. The longer the flow-where p is the forming pressure, A the projectioning channels are, or the smaller the flowing channelsarea of the work piece and other additional parts onare，the larger the forming pressure needed is.the plane vertical to the pressing direction.Moreover the pressure must be large enough to makeActually, the projection area of the work piece andthe solidified shell deform and has to be kept enoughother assistant parts on the plane vertical to the press-time to the end of solidification. Consequently, theing direction is also controlled by the forming situa-forming pressure needed p, must be larger than nottion to a set of work piece. For example, as shown inonly the critical stress of the slurry, but also the com-figure 2, the projection area is 1000 cm2 when thepressing strength Ts of the solidified shell at theforming situation of the work piece is horizontal, andforming temperature. Referring equation (3)， theit is decreased 10 times and into only 100 cm2 whenmathematical forms of the rule is obtained as:the forming situation of the work piece is vertical.p。2RTs; 2Lmax(4)The significance of the rule is not only supplying afoundation upon which one can select the formingIt is experimentally proved that if the pressure des-machine, but also increasing the forming pressureigned breaches equation (4), some shrinkage defectseven without increasing the power of the forming ma-are seen in the inner of the work piece. So equation (4)chine and finally improving the quality of the workis a usable rule to determine the forming pressure ofiece.the semisolid forging process.3 Additional rules3.3 Rule to determine the locking mold pressureBased on the above basic rules, there are some ad-In order to prevent the mold from opening in theditional rules of the process to be abide such as theforming process, an outer pressure has to provide byrules to determine the forming pressure, flling pres-forming machine to the mold. This pressure is nameda locking pressure. Obviously, the locking pressuresure and locking pressures.has to be larger than the uplifting mold pressure in the3.1 Rule to determine the flling pressurefilling mold process. However, the uplifting moldThe pressure for flling mold, flling pressure forpressure is changed in the filling mold process asshort, plays a very important role in the semisolidshown in figure 4. In the beginning stage (0 to t) inforging process. When the flling pressure acted on thewhich the slurry do not fill full the mold, and the air inslurry is smaller than the critical stress, the slurry can-the mold is heated by the slurry with high temperaturenot flow at all. Consequently, in order to make theand compressed because of the volume decreasingslurry into the mold, a pressure larger than the criticalstep, by step with the flling, the pressure acted on thestress must be acted on the slurry. However, the criti-mol中国煤化工ever, at some time 1cal stress of the semisolid slurry is different with thetheMYHd the pressure is sud-kind of alloy, the micro-construction of the slurry andlen!,CNMHGpme Bnd thren s1owthe temperature of the slury which is changed in thedown to the forming pressure Po. After the time t3, theflowing process [9,10]. So the filling pressure must bepressure acted on the mold is continuously fall down570J. Unir. Sci. Technol. Bejing, Vol.1I, No.6, Dec 2004to zero with the volume decreasing of the work piecemade of C grade steel or A356 aluminum alloy. Thebecause of the solidification. From the analysis, it isforming machine was a THP200 press with a maxi-seen that the minimum locking pressure has to bemum locking force of 2000 kN and a maximumlarger than the maximum uplifting mold pressure Pmax，pressing force of 630 kN. The slurry used to form wasinstead of the forming pressure Pb.prepared by a SSC2-30 slurry machine with a maxi-mum content of 30 kg steel. The macrostructure andmicrostructure of the work piece obtained werechecked strictly. The experiments include sevengroups are shown in table 1. There were 5-7 tests un-der different conditions in every group. Only one ruleof the above six rules was breached in each group.The results obtained are shown in table 1. It is seenfrom table 1 that all of the experiments has proved theavailability of the rules obtained above. If the SFLrule is breached shown as the test group 1, the work4 121pieces obtained are not complete with some deformi-Timeties, mainly the shut run defects; if the MUMP rule orFigure4 Variation of uplifting pressure with time.equation (5) is breached as the tests group 3 and 6, aActually, the maximum uplifting pressure Pmax, dif-large unwanted parts on the work piece surface,ferent from the forming pressure Pb, shown in figure 4,named fins or veining, is seen in the dividing moldis difficult to be accurately determined because it isplane which not only increases the material consump-controlled by many uncertain factors such as the ex-ion, but also increases the succedent machining cast;haust effects, shape of the mold cavity, flling velocityand if the PF rule or the SFL rule or equation (3) isand fluidity of the slurry. However, the locking moldbreached shown as the test group 1, 2 and 4, some sur-pressure can be determined experientially, which is atface defects, such as fins, cold shuts and veinings areleast 1.2-1.5 times as large as the forming pressure Pb,seen in the work pieces obtained; if equation (4) is noti.e.fitted, some inner shrink defects are seen in the workpieces obtained. Only if the three basic rules and thePlock 2(1.2-1.5)p.(5)three equations (3)-(5) are true in the process designnd production, satisfying work pieces are obtained4 Experiments and discussionshown as group 7 in table 1.In order to prove the availability of the rules ob-However, the wide applicability of the rules fortained above, a series of experiments was carried on inwork pieces with different shapes and of differentthe Semisolid Forming Laboratory of Beijing Jiaotongmaterials is still necessary to be proved in the futureUniversity. The work piece selected was the crookresearches and production.tongue, an accessory to connect the carriages of a train,Table 1 Examples of experimental resultsTestBasic rulesEquationsDefects of work piecesgroup No.SFLrule PF rule MUMP rule_ Eq.(3)Eq.(4)Eq.(5)BreachedTrueTrue .rueMisruns, shut runs or cold shutsVeinings or shut runTrDeformities or thick finsTrue :Cold shuts or shut runsShrinkage cavities or holesSatisfying5 Conclusions中国煤化工- misolid forging processnen all of the rules and(1) There are three essential rules respectivelyYHC N M H Gcomplete work piece cannamed as the shortest flowing length (SFL), the pres-be obtained.sure flling (PF) and the minimum uplifting moldpressure (MUMP), and three equations (3)-(5) neces-(2) The basic rules and equations obtained are ex-S.M. Xing e al, Basic rules for rheologie forging poess of semisolid aly571perimentally proved available. 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