45°斜三通管注塑模具設(shè)計(jì)-抽芯塑料注射模含SW三維及17張CAD圖帶開(kāi)題,45,三通,注塑,模具設(shè)計(jì),塑料,注射,sw,三維,17,cad,開(kāi)題 Journal of Materials Processing Technology 180 (2006) 18 Experimental characterisation in sheet forming processes by using Vickers micro-hardness Ali Mkaddem , Riadh Bahloul, Philippe u Roncer accepted Abstract to e of inspection, changes, ution vir heterogeneous examined beha in ratio observ zone ge gradient phenomenon, good K 1. Introduction Sheet metal forming industry has become one of the major manufacturing centres of the automobile industry. The popularity weight process, in mentioned bobbing, in characteristics. forming decrease during In pioneering studies 37, the main investigation of material strength, resistance to thinning, damage and ability of material to forming are extensively performed for one material state or for separate steps. 0924-0136/$ doi: of sheet metal products is attributable to their light and their higher formability. Sheet working consists in a more complex plane straining designed with high ratio of thickness reductions volving a considerable amount of texture evolution as by Tang and Tai 1. Diversity of sheet metal manufacturing sequences as bobbing-off, straightening and bending, as shown Fig. 1, leads to a level and progressive change of material At several time, mechanical and environmental conditions of are considered among the main causes that induce a in steel strength designed with microstructure changes forming cycle 2. Corresponding author. E-mail address: ali.mkaddemangers.ensam.fr (A. Mkaddem). Indentation micro-hardness testing at low loads is a well- accepted tool for assessing various mechanical properties such as flow stress, fracture stress, Youngs modulus and fracture toughness of rolled material 8. Thus, a proposed method for measuring micro-hardness is used to follow the properties vari- ation with the evolving of manufacturing sequences of 0.09% HSLA sheet metal carbon steel. Curves deduced from the mea- sured values within the sheet thickness are discussed for all considered steps of the forming processes. 2. Rolling process 2.1. Rolling cycle The rolling process consists of several successive steps as shown in Fig. 2, which govern the final state of matrix material. The level of thickness reduction leads to a high-hardened metal by introducing an important change to the crystallographic texture 9. see front matter 2006 Published by Elsevier B.V. 10.1016/j.jmatprotec.2006.04.006 LPMI-ERT-ENSAM/CER, 2 Boulevard d Received 23 April 2004; In this paper, an experimental micro-hardness procedure is proposed sheet forming process. As micro-hardness technique offers a reliable which may happen during manufacturing progress. This contrib gin sheet, unreeled sheet, straightened sheet and bent sheet. Measurement within the sheet thickness. The micro-hardness profiles viour to straightening operation that is widely adopted in steel working between virgin material and straightened material has been clearly for displaying the mechanical properties modifications under a lar which are generally activated simultaneously for elastoplastic idea about the interaction of processmaterial during manufacturing. 2006 Published by Elsevier B.V. eywords: Micro-hardness; Sheet metal; Bobbing; Straightening; Bending; Damage technique Dal Santo, Alain Potiron ay, BP3525-49035 Angers, France 10 April 2006 valuate the evolution of HSLA steel behaviour during each sequence it was retained here to follow the mechanical characteristic consists in characterisation of sheet material at different steps: performed on virgin HSLA steel showed that material is highly after the bobbing-off step showed a high sensitivity of sheet order to make sheet sufficiently flat for forming. A level of hardening ed. Moreover, micro-hardness is investigated on bent parts at the fold deformation. In this way, hardening phenomenon and damage steel, are quantified accurately. Results compared into them gave a 2 A. Mkaddem et al. / Journal of Materials Processing Technology 180 (2006) 18 Fig. 1. Main sequences for manufacturing cycle. Fig. 2. Main steps for sheet rolling processes. ature precipitations, tion. material adapted of must heterogeneous The the cooling ical bobbing reheating precipitates 10,11 function secondly great rolling 2.2. Reference material Generally, rolled materials are delivered in bobbin form. Before bobbing, the matrix material is considered “virgin”. This state of sheet would be considered the reference metal. It was kept from the inner extremity of the bobbin that is sufficiently flat as can be seen in Fig. 3a. Reference sheet allows for quan- tifying the microstructure hardening for the following state of straightening and bending. small associated ture material entation elaboration microstructure. 3. 3.1. 3.1.1. the sheet grain w are obligatory through In the general manufacturing practice, the holding temper- of the slab must be enough high to make once again the formed at the end of coiling, in the austenitic solu- It must be controlled accurately in order to lead to good properties. At the end of hot rolling step, the temperature would also be precisely to make certain the successful precipitation as small a grain size as possible. During this sequence, it be assured that the used temperature cannot conduce to a microstructure. After rolling, the sheet is cooled by soaking, and then bobbed. grain size of material microstructure depends strongly on adopted cooling speed. The grain size is as small as the speed is high. The bobbing step has a specific influence on the final mechan- and microstructure characteristics of rolled steels. The temperature consists in an own thermal treatment of the sheet. It has a particular effect on grain size and that can be developed during process progression . The distribution of matrix material contents is firstly a of present elements quantities in the HSLA steel and it is a function of the rolling conditions, which play actions in microstructure evolution during each step of the cycle. ent. outer Fig. 3. (a) Reference sheet and (b) based matrix The thermo-mechanical rolling control process leads to a grain size that provides high mechanical characteristics with a high formability. Fig. 3b shows the microstruc- of the 0.09% C sheet steel. Ferrite grains for the considered are generally smaller than 10H9262m and preferred ori- caused by rolling are clearly marked. The previous steps, essentially, govern these final properties of Behaviour characterisation Virgin sheet Micro-hardness test During the rolling step, when the final temperature is reached, precipitates density increases rapidly and firstly close to the surfaces where cooling rate is the highest. Consequently, development stops and a sufficiently small grain size ould be obtained. Moreover, the microstructure characteristics not homogeneous within the sheet thickness on HSLA steel. The difference of microstructure specificities induces changes in the subsequent mechanical behaviour the thickness as a consequence of granular gradi- Micro-hardness that is performed from inner surface to surface of the considered steel with 200 g load, offers material for HSLA rolled steel. A. Mkaddem et al. / Journal of Materials Processing Technology 180 (2006) 18 3 precisions microstructure. where coated se the attrib sheet surf in belatedly e ment step. and between ment as mation. serv sequent 3.1.2. mean design rial outside neous hardness dif the between ratio Fig. 5. Mean width X-ray diffraction evolution within the virgin sheet thickness. High values of mean width, as has been noted by the micro- hardness investigation, are located at the surfaces of the sheet. The curve shows minima for an angle of 1.27 . There is no representative difference between the states of the two surfaces. 3.1.3. Correlation results in based-e dict measurement combination can v V high 3.2. the Fig. 4. Micro-hardness evolution within the virgin sheet thickness. to understand the material behaviour of the virgin A specimen has been taken from the reference zone (Fig. 3) material is considered with less deformation. Then, it was and polished to improve accuracy of measurement. Later, veral indentation tests were carried in the cutting plan where surface is polished. Fig. 4 shows that the relative micro-hardness (values uted to the mean value H v ) is not uniform within the thickness. In particular, highest values are located close to aces where grain is smaller. Hardening ratio seems to be low the neutral zone where precipitates are generally developed compared to near surface zones. The micro-hardness volution confirms the great importance of an accurate adjust- of temperature for holding step, rolling step and cooling The difference between the relative values in neutral zone surfaces reaches 8%, whereas there is no clear variation the two surfaces of the sheet. Micro-hardness measure- performed on virgin microstructure would be considered the reference behaviour of the sheet before any large defor- In a practical way, micro-hardness characterisation must e to identify the material behaviour progress with the sub- manufacturing sequences. Mean width X-ray test Typical analysis has been carried out on the virgin sheet by X-ray diffraction technique. The measured values of the mean width X-ray peaks which the variation in terms of plastic strain in the matrix mate- are plotted within the thickness from inside surface to the surface of the sheet as given in Fig. 5. It is worth noting that mean width distribution is inhomoge- which confirms once again the results obtained by micro- measurement. This typical evolution indicates that the ferent steps of elaboration process have a direct effect on following material behaviour and induce a marked deviation surfaces and neutral zone designed by the hardening variation within the thickness. Fig. The typical distribution of micro-hardness and mean width vestigated in the sheet thickness lets us to think of an evident xperimental relation that can lead proportionally to pre- micro-hardness values from the mean width X-ray peaks for a fixed indention loads. In this way, a simple between Figs. 4 and 5 indicates that a linear relation be found for the considered steel. As expected in Fig. 6, there is a marked fitness of the measured alues to the linear law that is, essentially, based on the fact that ickers number and X-ray measured values have a relatively fluctuation. Straightened sheet After rolling, the sheet metal is bobbed. During this operation, microstructure undergoes some mechanical modifications 6. Variation in relative Vickers micro-hardness with the mean width X-ray. 4 A. Mkaddem et al. / Journal of Materials Processing Technology 180 (2006) 18 Fig. 7. Principal of roller levelling for straightening process. such as the introduction of residual stress. To produce high quality parts, the bobbin must be bobbed-off and straightened. Roller levelling is a method widely used in steel working in order to operations. to roll fe ifications bending are This trolled the tial sheet residual of material sequence sheet surf straightened etration. the third sarily bobbin, follo rial the dif second by combinations Fig. 8. Relative micro-hardness results obtained for straightened sheet. T Dif First Second Third straighten steel plates after rolling, heat treatment or cooling Roller levelling is carried out by subjecting the plate or bobbin multiple back and forth bending sequences with decreasing penetration as shown in Fig. 7. It is a complex process and w details are known about the plate material behaviour mod- during this process. The sheet is exposed to reversed effect during straightening and the strains in the sheet controlled by the device geometry of the levelling machine. means that the sheet is subjected to reversed strain con- cyclic loading. This operation consists in applying a variable pressure to sheet by acting different rollers in the machine. It is essen- for steel manufacturing that, after straightening 12, the would be sufficiently flat with required roughness and low stresses level which can increase the forming success parts by using any subsequent processing step. Compulsorily, undergoes plastic deformation during the straightening which causes an amount of hardening ratio within the designed by a micro-hardness level observed specially at aces. Several micro-hardness tests have been performed on sheets with different combinations of roller pen- From the three pre-existing rollers, only the position of first one will be changed. The positions of the second and rollers are kept constant. In the industrial practice, the highest penetration is neces- performed on the first roller which is placed behind the whereas the less penetration is performed on the roller wing the second one or the third one. The considered mate- is of 4 mm thickness. The roller position will be designed by triple combination t, t, t when there is no penetration. For ferent penetrations a, b and c, respectively, for the first, the and the third rollers, the roller position will be designed the combination t a, t b, t c. The three investigated are reported in Table 1. able 1 ferent straightening adjustment for characterisation Roller 1 (mm) Roller 2 (mm) Roller 3 (mm) Combination adjustment 0 3 4 034 adjustment 1 3 4 134 adjustment 2 3 4 234 A. Mkaddem et al. / Journal of Materials Processing Technology 180 (2006) 18 5 Table 2 Bending parameters used for the investigated part Punch radius (mm) Die radius (mm) Blank-holder radius (mm) Stroke (mm) Clearance (mm) Thickness (mm) Bending angle ( ) 444 290490 trated in of steel, identified. ened In e micro-hardness more when highest which dif sheet tions straightening bobbin cation within pre mation induced the similar hardness it v penetration of rial material leads tallographic follo highly te of in depth, depth, the The remains Straightening conditions would have a direct effect on the mechanical behaviour during the following forming step. Strengthening caused by the roller levelling increases the prop- erties gap between the middle thickness zone and closed surface zones of the sheet. The straightening process would lead to a homogeneous distribution of residual stress but the introduced hardening level would modify the material formability for the subsequent sheet processing sequences. 3.3. Bent sheet During bending, material undergoes an important straining ratio, especially in the fold zone, which is characterised by the hardening encla high generation that zone means gated zone the compressi outer The mens through 200 profiles is Fig. The measurement results are designed by the curves illus- in Fig. 8, with virgin micro-hardness profile for each vestigated case. With comparison between virgin state and straightened state sheet, variation of mechanical properties of the considered resulting from the straightening sequence, can be clearly Referring to Fig. 8, it can be noted in all cases that straight- material becomes more hardened than the reference one. addition, the behaviour seems to keep a similar non-linear volution within the sheet thickness as it has been observed for evolution performed on virgin sheet. In the three cases, microstructure modifications seem to be localised near the surface than in middle zone of thickness the measured relative values are less. In particular, the hardening ratio has been measured at the outer surface, undergoes a positive stress field at bobbing sequence. The ference of micro-hardness level between the two sides of the can be attributed to the superposition of plastic deforma- in the external zones of the bobbin during bobbing and operations. In an aforementioned work 13, it has been laid down that the s outer side is more sensitive to micro-hardness quantifi- than the inner zone, that residual stress field is not similar the sheet thickness and that it depends greatly on the vious manufacturing steps. At this stage, the sheet is subjected to a low plastic defor- but it is sufficient to identify the only hardening level during straightening sequence. As can be seen in Fig. 8, hardening level especially at the outer zone of the sheet is not for the three investigated cases. Referring to the micro- profile, which is carried out in the reference material, can be noted that the degree of hardening designed by the ariation of Vickers micro-hardness values is as marked as roller is high. This observation is associated with the effect the plastic strain quantity that is induced in the matrix mate- and which depends directly on the roller penetration. When flow occurs, a local shearing process is activated which to a fast amount of dislocation number. A high level of crys- texture density designs this phenomenon, which is wed by a local hardening process into grains and leads to a strengthened matrix material. The high crystallographic xture density of matrix material causes a relative local motion shearing plans to which the variation of micro-hardness values the middle thickness zone is attributed. In detail, 1% hardening level has been computed for 0.5 mm whereas the hardening level reaches 4.5% for 4.5 mm respectively, for inner side and outer side of the sheet in more severe straightening case of 034 roller combination. variation between inner side and the middle thickness zone relatively constant referring to the virgin state of sheet. phenomenon 14. Depending on the process parameters, plastic strain in the ve of fold zone can take a severe gradient leading to a level of hardening ratio and damage that is designed by and development of micro-defects 15. In an aforementioned investigation, Kurt 16 has deduced micro-hardness is higher in the outer zone than in the inner of parts fold. In this investigation, HSLA sheet steel parts have been bent by of wiping die-bending process. Vickers number is investi- particularly in the fold zone of parts from the compression to the tensile zone. Tests are performed in such a way that inner sheet surface corresponds to the inner fold zone where ve hydrostatic stress state is applied. In this case, the sheet surface would correspond to the tension fibres zone. main bending parameters used for the investigated speci- are reported in Table 2. The influence of the bending operation has been analyzed the micro-hardness testing profiles measured by using g-indention load. The treatment of measurement data leads to micro-hardness of virgin, straightened and bent sheets plotted in Fig. 9. It is found from the plotted data that the micro-hardness law also non-linear for the bent specimen. The deviation of values 9. Relative micro-hardness profiles for virgin, straightened and bent sheets. 6 A. Mkaddem et al. / Journal of Materials Processing Technology 180 (2006) 18 between the neutral zone and the closed surface zones becomes more marked with bending. Certainly, during this operation the induced plastic deformation is higher than the one developed by the roller levelling machine during straightening step. The microstructure seems to be more sensitive to the bending oper- ation than to the straightening one. ber of material fibres, ment the an fold can hardness reliable of designed damage e in 4. 4.1. eral for a H where can where considered be where be H H material e aged For a power law, the hardening evolution would be written as: R = Q R ( Pl ) n R (5) Allowing that the micro-hardness is linearly proportional to the hardening law, the following relation can be deduced: H v = Q H ( Pl ) n H (6) where Q R and Q H are, respectively, the hardening and the micro- hardness modulus. n R and n H are, respectively, the hardening and the micro-hardness components that must be equal if the linearity between H v and R is admitted. For the subsequent development, it would be retained that n R = n H = n. Tensile test conduces easily to identify n and Q R . Micro- hardness tests lead to finding the H v -law for damaged material. In addition, admitting that for a so small plastic strain P