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4.3.1.1 流體動(dòng)力潤滑
流體動(dòng)力潤滑指通過液體的運(yùn)動(dòng)來實(shí)現(xiàn)潤滑的。就徑向滑動(dòng)軸承而言,它已經(jīng)被用來作為一個(gè)例子,該軸的旋轉(zhuǎn)使?jié)櫥瑒┻M(jìn)入裝載區(qū)。被加載的區(qū)域?qū)⒃谠撦S和軸承表面彼此最接近的那個(gè)點(diǎn)上,進(jìn)入這個(gè)區(qū)域,像一個(gè)彎曲的楔形,是逐漸變細(xì)的。
由于油是被迫進(jìn)入楔形狹窄的部分,其壓力的增加,正是這種流體動(dòng)壓力來支撐軸的載荷。載荷的增加降低了油膜厚度的增加,而增加的流體動(dòng)壓來潤滑油膜厚度。反過來,流體動(dòng)壓力是由原油粘度和在它被壓縮成楔形入口區(qū)的速度來決定的。
類似的壓力楔,它依賴于流體動(dòng)力潤滑,在幾乎所有的系統(tǒng)中都是必要的。舉個(gè)例子,在一個(gè)直線滑動(dòng)軸承中的楔形可以由一個(gè)傾斜的滑塊產(chǎn)生,如fig.4.4所示。
fig.4.4 在直線滑動(dòng)的軸承壓力楔
另外一種小型楔,可以通過倒圓角,倒角或除去滑塊的前沿部分來得到。在某些情況下,一個(gè)楔形塊可能會由一個(gè)表面完全平滑的滑塊而產(chǎn)生,因?yàn)榛瑒?dòng)面中心溫度升高和膨脹,會產(chǎn)生極高的熱量。任意或所有這些類型的楔形都是存在的,比如說在一個(gè)墊式的推力軸承,但某些類型的楔對于流體動(dòng)壓潤滑是必不可少的。
一個(gè)例外是兩個(gè)表面之間的潤滑劑,受到擠壓,被迫從他們之間的空間離開而移向另一個(gè)軸承表面。潤滑劑的粘度,有著防止?jié)櫥捅粩D出的作用。潤滑油的粘度越高,其被擠出需要的承載力是更大的,因此對軸承表面損傷有著更大的保護(hù)作用。這就是所謂的擠壓膜效應(yīng)。
流體動(dòng)力潤滑是對雷諾茲方程的數(shù)學(xué)描述,但對于大多數(shù)使用者來說記住油膜厚度取決于軸承表面的速度和油的粘度是足夠的。粘度是油的唯一性,這在流體動(dòng)力潤滑上是重要的。
流體動(dòng)力潤滑在高速提供了更好的潤滑,在非常低的速度可能會導(dǎo)致潤滑失效。
理想情況下流體動(dòng)力潤滑油膜應(yīng)該是足夠厚的·,以確保在兩個(gè)曲面上的凸起之間沒有聯(lián)系。換句話說,油膜厚度應(yīng)大于表面粗糙高度的總和。在fig.4.3這理想點(diǎn)是B,但由于速度和載荷的軸承,和溫度(由此情況下的粘度)的油,不能保持絕對恒定,通常只是被用來針對那個(gè)點(diǎn)B
這里的h是潤滑油膜厚度,P是壓力,x和z是坐標(biāo),U和V在x和z方向的速度。術(shù)語 和 描述的是該油被擠入楔形塊的速率,而是油的粘度。
這能保證不僅摩擦?xí)浅=咏钚≈档目赡?,而且磨損也將會保持在最低限度。
在實(shí)踐中,新的軸承表面粗糙比通常是可取的,少量的接觸也是可以允許的。這將允許磨損發(fā)生在表面粗糙度減少的地方,或表面運(yùn)行的地方。設(shè)計(jì)軸承系統(tǒng)的目的應(yīng)該是一旦它達(dá)到運(yùn)行條件,油膜厚度大于表面粗糙高度的總和。
一種特殊類型的流體動(dòng)力潤滑可以發(fā)生在特定的負(fù)載很大的接觸中,比如球或滾子軸承和許多類型的齒輪傳動(dòng)中。如果幾何形狀和運(yùn)動(dòng)類型是合適的,潤滑油可以被困在入口區(qū),并變成受到很高的壓力的部分,因?yàn)樗菙D進(jìn)狹小的空間里大多數(shù)高負(fù)載的部分的接觸。
這些壓力有兩個(gè)重要的作用。他們使?jié)櫥瑒┑恼扯却蟠笤黾?,從而提高其承載能力。同時(shí)他們以這樣的方式引起的裝載表面彈性變形來擴(kuò)展到更大的面積的負(fù)載。由于承載能力的控制。fig.4.5給了一個(gè)彈流潤滑的如何發(fā)生的現(xiàn)象。
fig.4.5 彈流潤滑的氣缸在平坦的表面
雖然技術(shù)上流體動(dòng)力潤滑是潤滑液的一種形式,它也可以被應(yīng)用到通過氣體的潤滑,提供那些負(fù)載和速度條件都合適的粘度極低的氣體。
4.3.1.1邊界潤滑
當(dāng)油膜厚度變得太小而不能給表面的流體膜分離時(shí),粗糙的表面開始彼此接觸,潤滑劑的性能除了體積粘性開始變得重要。在fig.4.3,區(qū)域1,油膜已變得非常薄,沒有水動(dòng)力作用而只有邊界潤滑是有效的。
這兒的 ——在入口區(qū)潤滑油的粘度
R——有效半徑
E ——楊氏模量
Q——赫茲接觸壓力
——粘度隨壓力增加的程度
在大多數(shù)正常情況下表面微凸體最初是由涂有薄膜的氧化物,鐵氧化物在鐵或鋼,鋁氧化物(氧化鋁)覆蓋在鋁上等等形成的。當(dāng)這些表面相互摩擦,他們吸附物是比較溫和的。然而,如果氧化物薄膜通過大力摩擦去除,暴露的金屬表面有一個(gè)非常大的傾向去吸附。
因此,如果軸承表面保留有氧化物薄膜,粗糙表面之間的接觸會給予適度的摩擦和磨損。如果他們失去了那層氧化物薄膜,將會有較高的摩擦或嚴(yán)重的磨損。在這兩種情況下的邊界潤滑的目的是減少摩擦和磨損,對此有多種方法可以這樣去做。
(1)吸附作用
所有的固體表面會有一種從他們周圍的環(huán)境吸引一層薄膜物質(zhì)的傾向。這樣的薄膜可能是只有一個(gè)或幾個(gè)分子厚,并且被認(rèn)為是表面上的吸附。較厚或更強(qiáng)的吸附膜的支承表面可以提供更大的保護(hù)。
吸附是一種可逆的過程,并且吸附物是可以解吸的,如果加熱到臨界溫度,或通過某種物質(zhì)被移走,將會受到更強(qiáng)的吸附力,這最明顯的效果是體現(xiàn)在邊界潤滑上,因?yàn)樵跐櫥瑒┐嬖谙拢叫詮?qiáng)的物質(zhì)將被優(yōu)先吸附。在制定潤滑劑時(shí)會有更有效的邊界潤滑添加劑。
吸附的一個(gè)有用的副產(chǎn)物是力學(xué)性能的降低,特別是,在一個(gè)吸附膜的存在下金屬的屈服應(yīng)力,由于這種效果,較低的應(yīng)力是在凹凸碰撞時(shí)產(chǎn)生的。在新的軸承表面運(yùn)行時(shí),去除過度粗糙后效果會更好。
(2)表面上的化學(xué)吸附
吸附到金屬表面后,一些物質(zhì)會與金屬或氧化物表面反應(yīng)生成新的化合物。這種物質(zhì)稱為化學(xué)吸附。
化學(xué)吸附材料比金屬表面的吸附材料能更好結(jié)合起來,而且化學(xué)吸附過程是不可逆的。這種薄膜可以有非常有效的邊界潤滑。
(3)化學(xué)反應(yīng)
在光或適度的摩擦下,吸附和化學(xué)吸附膜在減少摩擦和磨損是非常有效的。他們在劇烈摩擦條件下很容易被機(jī)械去除,因此不能有效地阻止嚴(yán)重的磨損或被抵消。自然氧化層減少嚴(yán)重磨損和抵消,但一旦通過摩擦被去除,表面的再氧化可能太慢而是有效的。
要處理這樣的情況,更多的活性化學(xué)物質(zhì)可以被添加到潤滑油和軸承的表面來產(chǎn)生這種物質(zhì)的反應(yīng),將產(chǎn)生有效的保護(hù)膜。問題是活性化學(xué)物質(zhì)如磷酸是否會繼續(xù)反應(yīng),從而侵蝕金屬表面。
解決的辦法是使用含硫磺的,磷,氯的有機(jī)化合物,這可以在氧化的金屬表面產(chǎn)生吸附或化學(xué)吸附,但將與重新暴露的已被移除氧化膜的金屬表面快速作出反應(yīng)。
對于潤滑和腐蝕的控制,在這種方式下軸承表面的化學(xué)制品的反應(yīng)可以限制到最小程度的必要。盡管如此,一些更強(qiáng)大的極壓添加劑會慢慢地腐蝕某些金屬,應(yīng)該只是被用于在摩擦條件非常嚴(yán)重的地方,比如用在金屬切削上。
4.3.1.4靜壓潤滑
在上面的部分解釋了壓力給全流體膜分離的負(fù)載軸承表面是由表面的運(yùn)動(dòng)產(chǎn)生的。同樣的效果可以通過強(qiáng)制潤滑的軸承外部施加的壓力下得到,這將使全油膜分離來實(shí)現(xiàn)到粘度或速度將不足以支撐負(fù)載的輸出的地方。
外壓的基本理論(有時(shí)稱為靜壓)潤滑是很簡單的。所需的平均壓力等于負(fù)載除以有效承載面積。
在實(shí)踐中,液體靜壓軸承的設(shè)計(jì)還必須考慮到保持穩(wěn)定的需要和控制潤滑油流量。
外部加壓可用于液體潤滑劑或潤滑脂,但也被用來作為常用的氣,它可以抵消那些與粘度非常低的氣體相關(guān)問題的偏移。
4.3.1.5干燥或固體潤滑
一種固體潤滑劑基本上是可被放置在兩個(gè)軸承表面的任何固體物質(zhì),在給定載荷下比軸承材料本身的摩擦將更容易剪切。在干摩擦系數(shù)對剪切力和軸承負(fù)荷相關(guān):=剪切力/負(fù)載
在實(shí)踐中,某些特定的屬性都需要一種優(yōu)良的固體潤滑劑,如化學(xué)穩(wěn)定性,粘附一個(gè)軸承表面的能力,等等。
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e pos 模具工業(yè)現(xiàn)狀Process simulation in stamping – recent
applications for product and process design
Abstract
Process simulation for product and process design is currently being practiced in industry. However, a number of input variables have a significant effect on the accuracy and reliability of computer predictions. A study was conducted to evaluate the capability of FE-simulations for predicting part characteristics and process conditions in forming complex-shaped, industrial parts.
In industrial applications, there are two objectives for conducting FE-simulations of the stamping process; (1) to optimize the product design by analyzing formability at the product design stage and (2) to reduce the tryout time and cost in process design by predicting the deformation process in advance during the die design stage. For each of these objectives, two kinds of FE-simulations are applied. Pam-Stamp, an incremental dynamic-explicit FEM code released by Engineering Systems Int'l, matches the second objective well because it can deal with most of the practical stamping parameters. FAST_FORM3D, a one-step FEM code released by Forming Technologies, matches the first objective because it only requires the part geometry and not the complex process information.
In a previous study, these two FE codes were applied to complex-shaped parts used in manufacturing automobiles and construction machinery. Their capabilities in predicting formability issues in stamping were evaluated. This paper reviews the results of this study and summarizes the recommended procedures for obtaining accurate and reliable results from FE simulations.
In another study, the effect of controlling the blank holder force (BHF) during the deep drawing of hemispherical, dome-bottomed cups was investigated. The standard automotive aluminum-killed, drawing-quality (AKDQ) steel was used as well as high performance materials such as high strength steel, bake hard steel, and aluminum 6111. It was determined that varying the BHF as a function of stroke improved the strain distributions in the domed cups.
Keywords: Stamping; Process ;stimulation; Process design
1. Introduction
The design process of complex shaped sheet metal stampings such as automotive panels, consists of many stages of decision making and is a very expensive and time consuming process. Currently in industry, many engineering decisions are made based on the knowledge of experienced personnel and these decisions are typically validated during the soft tooling and prototyping stage and during hard die tryouts. Very often the soft and hard tools must be reworked or even redesigned and remanufactured to provide parts with acceptable levels of quality.
The best case scenario would consist of the process outlined in Fig. 1. In this design process, the experienced product designer would have immediate feedback using a specially design software called one-step FEM to estimate the formability of their design. This would allow the product designer to make necessary changes up front as opposed to down the line after expensive tooling has been manufactured. One-step FEM is particularly suited for product analysis since it does not require binder, addendum, or even most process conditions. Typically this information is not available during the product design phase. One-step FEM is also easy to use and computationally fast, which allows the designer to play “what if” without much time investment.
Fig. 1. Proposed design process for sheet metal stampings.
Once the product has been designed and validated, the development project would enter the “time zero” phase and be passed onto the die designer. The die designer would validate his/her design with an incremental FEM code and make necessary design changes and perhaps even optimize the process parameters to ensure not just minimum acceptability of part quality, but maximum achievable quality. This increases product quality but also increase process robustness. Incremental FEM is particularly suited for die design analysis since it does require binder, addendum, and process conditions which are either known during die design or desired to be known.
The validated die design would then be manufactured directly into the hard production tooling and be validated with physical tryouts during which the prototype parts would be made. Tryout time should be decreased due to the earlier numerical validations. Redesign and remanufacturing of the tooling due to unforeseen forming problems should be a thing of the past. The decrease in tryout time and elimination of redesign/remanufacturing should more than make up for the time used to numerically validate the part, die, and process.
Optimization of the stamping process is also of great importance to producers of sheet stampings. By modestly increasing one's investment in presses, equipment, and tooling used in sheet forming, one may increase one's control over the stamping process tremendously. It has been well documented that blank holder force is one of the most sensitive process parameters in sheet forming and therefore can be used to precisely control the deformation process.
By controlling the blank holder force as a function of press stroke AND position around the binder periphery, one can improve the strain distribution of the panel providing increased panel strength and stiffness, reduced springback and residual stresses, increased product quality and process robustness. An inexpensive, but industrial quality system is currently being developed at the ERC/NSM using a combination of hydraulics and nitrogen and is shown in Fig. 2. Using BHF control can also allow engineers to design more aggressive panels to take advantage the increased formability window provided by BHF control.
Fig. 2. Blank holder force control system and tooling being developed at the ERC/NSM labs.
Three separate studies were undertaken to study the various stages of the design process. The next section describes a study of the product design phase in which the one-step FEM code FAST_FORM3D (Forming Technologies) was validated with a laboratory and industrial part and used to predict optimal blank shapes. Section 4 summarizes a study of the die design stage in which an actual industrial panel was used to validate the incremental FEM code Pam-Stamp (Engineering Systems Int'l). Section 5 covers a laboratory study of the effect of blank holder force control on the strain distributions in deep drawn, hemispherical, dome-bottomed cups.
2. Product simulation – applications
The objective of this investigation was to validate FAST_FORM3D, to determine FAST_FORM3D's blank shape prediction capability, and to determine how one-step FEM can be implemented into the product design process. Forming Technologies has provided their one-step FEM code FAST_FORM3D and training to the ERC/NSM for the purpose of benchmarking and research. FAST_FORM3D does not simulate the deformation history. Instead it projects the final part geometry onto a flat plane or developable surface and repositions the nodes and elements until a minimum energy state is reached. This process is computationally faster than incremental simulations like Pam-Stamp, but also makes more assumptions. FAST_FORM3D can evaluate formability and estimate optimal blank geometries and is a strong tool for product designers due to its speed and ease of use particularly during the stage when the die geometry is not available.
In order to validate FAST_FORM3D, we compared its blank shape prediction with analytical blank shape prediction methods. The part geometry used was a 5?in. deep 12?in. by 15?in. rectangular pan with a 1?in. flange as shown in Fig. 3. Table 1 lists the process conditions used. Romanovski's empirical blank shape method and the slip line field method was used to predict blank shapes for this part which are shown in Fig. 4.
Fig. 3. Rectangular pan geometry used for FAST_FORM3D validation.
Table 1. Process parameters used for FAST_FORM3D rectangular pan validation
Fig. 4. Blank shape design for rectangular pans using hand calculations.
(a) Romanovski's empirical method; (b) slip line field analytical method.
Fig. 5(a) shows the predicted blank geometries from the Romanovski method, slip line field method, and FAST_FORM3D. The blank shapes agree in the corner area, but differ greatly in the side regions. Fig. 5(b)–(c) show the draw-in pattern after the drawing process of the rectangular pan as simulated by Pam-Stamp for each of the predicted blank shapes. The draw-in patterns for all three rectangular pans matched in the corners regions quite well. The slip line field method, though, did not achieve the objective 1?in. flange in the side region, while the Romanovski and FAST_FORM3D methods achieved the 1?in. flange in the side regions relatively well. Further, only the FAST_FORM3D blank agrees in the corner/side transition regions. Moreover, the FAST_FORM3D blank has a better strain distribution and lower peak strain than Romanovski as can be seen in Fig. 6.
Fig. 5. Various blank shape predictions and Pam-Stamp simulation results for the rectangular pan.
(a) Three predicted blank shapes; (b) deformed slip line field blank; (c) deformed Romanovski blank; (d) deformed FAST_FORM3D blank.
Fig. 6. Comparison of strain distribution of various blank shapes using Pam-Stamp for the rectangular pan.
(a) Deformed Romanovski blank; (b) deformed FAST_FORM3D blank.
To continue this validation study, an industrial part from the Komatsu Ltd. was chosen and is shown in Fig. 7(a). We predicted an optimal blank geometry with FAST_FORM3D and compared it with the experimentally developed blank shape as shown in Fig. 7(b). As seen, the blanks are similar but have some differences.
Fig. 7. FAST_FORM3D simulation results for instrument cover validation.
(a) FAST_FORM3D's formability evaluation; (b) comparison of predicted and experimental blank geometries.
Next we simulated the stamping of the FAST_FORM3D blank and the experimental blank using Pam-Stamp. We compared both predicted geometries to the nominal CAD geometry (Fig. 8) and found that the FAST_FORM3D geometry was much more accurate. A nice feature of FAST_FORM3D is that it can show a “failure” contour plot of the part with respect to a failure limit curve which is shown in Fig. 7(a). In conclusion, FAST_FORM3D was successful at predicting optimal blank shapes for a laboratory and industrial parts. This indicates that FAST_FORM3D can be successfully used to assess formability issues of product designs. In the case of the instrument cover, many hours of trial and error experimentation could have been eliminated by using FAST_FORM3D and a better blank shape could have been developed.
Fig. 8. Comparison of FAST_FORM3D and experimental blank shapes for the instrument cover.
(a) Experimentally developed blank shape and the nominal CAD geometry; (b) FAST_FORM3D optimal blank shape and the nominal CAD geometry.
3. Die and process simulation – applications
In order to study the die design process closely, a cooperative study was conducted by Komatsu Ltd. of Japan and the ERC/NSM. A production panel with forming problems was chosen by Komatsu. This panel was the excavator's cabin, left-hand inner panel shown in Fig. 9. The geometry was simplified into an experimental laboratory die, while maintaining the main features of the panel. Experiments were conducted at Komatsu using the process conditions shown in Table 2. A forming limit diagram (FLD) was developed for the drawing-quality steel using dome tests and a vision strain measurement system and is shown in Fig. 10. Three blank holder forces (10, 30, and 50?ton) were used in the experiments to determine its effect. Incremental simulations of each experimental condition was conducted at the ERC/NSM using Pam-Stamp.
Fig. 9. Actual product – cabin inner panel.
Table 2. Process conditions for the cabin inner investigation
Fig. 10. Forming limit diagram for the drawing-quality steel used in the cabin inner investigation.
At 10?ton, wrinkling occurred in the experimental parts as shown in Fig. 11. At 30?ton, the wrinkling was eliminated as shown in Fig. 12. These experimental observations were predicted with Pam-stamp simulations as shown in Fig. 13. The 30?ton panel was measured to determine the material draw-in pattern. These measurements are compared with the predicted material draw-in in Fig. 14. Agreement was very good, with a maximum error of only 10?mm. A slight neck was observed in the 30?ton panel as shown in Fig. 13. At 50?ton, an obvious fracture occurred in the panel.
Fig. 11. Wrinkling in laboratory cabin inner panel, BHF=10?ton.
Fig. 12. Deformation stages of the laboratory cabin inner and necking, BHF=30?ton.
(a) Experimental blank; (b) experimental panel, 60% formed; (c) experimental panel, fully formed; (d) experimental panel, necking detail.
Fig. 13. Predication and elimination of wrinkling in the laboratory cabin inner.
(a) Predicted geometry, BHF=10?ton; (b) predicted geometry, BHF=30?ton.
Fig. 14. Comparison of predicted and measured material draw-in for lab cabin inner, BHF=30?ton.
Strains were measured with the vision strain measurement system for each panel, and the results are shown in Fig. 15. The predicted strains from FEM simulations for each panel are shown in Fig. 16. The predictions and measurements agree well regarding the strain distributions, but differ slightly on the effect of BHF. Although the trends are represented, the BHF tends to effect the strains in a more localized manner in the simulations when compared to the measurements. Nevertheless, these strain prediction show that Pam-Stamp correctly predicted the necking and fracture which occurs at 30 and 50?ton. The effect of friction on strain distribution was also investigated with simulations and is shown in Fig. 17.
Fig. 15. Experimental strain measurements for the laboratory cabin inner.
(a) measured strain, BHF=10?ton (panel wrinkled); (b) measured strain, BHF=30?ton (panel necked); (c) measured strain, BHF =50?ton (panel fractured).
Fig. 16. FEM strain predictions for the laboratory cabin inner.
(a) Predicted strain, BHF=10?ton; (b) predicted strain, BHF=30?ton; (c) predicted strain, BHF=50?ton.
Fig. 17. Predicted effect of friction for the laboratory cabin inner, BHF=30?ton.
(a) Predicted strain, μ=0.06; (b) predicted strain, μ=0.10.
A summary of the results of the comparisons is included in Table 3. This table shows that the simulations predicted the experimental observations at least as well as the strain measurement system at each of the experimental conditions. This indicates that Pam-Stamp can be used to assess formability issues associated with the die design.
Table 3. Summary results of cabin inner study
4. Blank holder force control – applications
The objective of this investigation was to determine the drawability of various, high performance materials using a hemispherical, dome-bottomed, deep drawn cup (see Fig. 18) and to investigate various time variable blank holder force profiles. The materials that were investigated included AKDQ steel, high strength steel, bake hard steel, and aluminum 6111 (see Table 4). Tensile tests were performed on these materials to determine flow stress and anisotropy characteristics for analysis and for input into the simulations (see Fig. 19 and Table 5).
Fig. 18. Dome cup tooling geometry.
Table 4. Material used for the dome cup study
Fig. 19. Results of tensile tests of aluminum 6111, AKDQ, high strength, and bake hard steels.
(a) Fractured tensile specimens; (b) Stress/strain curves.
Table 5. Tensile test data for aluminum 6111, AKDQ, high strength, and bake hard steels
It is interesting to note that the flow stress curves for bake hard steel and AKDQ steel were very similar except for a 5% reduction in elongation for bake hard. Although, the elongations for high strength steel and aluminum 6111 were similar, the n-value for aluminum 6111 was twice as large. Also, the r-value for AKDQ was much bigger than 1, while bake hard was nearly 1, and aluminum 6111 was much less than 1.
The time variable BHF profiles used in this investigation included constant, linearly decreasing, and pulsating (see Fig. 20). The experimental conditions for AKDQ steel were simulated using the incremental code Pam-Stamp. Examples of wrinkled, fractured, and good laboratory cups are shown in Fig. 21 as well as an image of a simulated wrinkled cup.
Fig. 20. BHF time-profiles used for the dome cup study.
(a) Constant BHF; (b) ramp BHF; (c) pulsating BHF.
Fig. 21. Experimental and simulated dome cups.
(a) Experimental good cup; (b) experimental fractured cup; (c) experimental wrinkled cup; (d) simulated wrinkled cup.
Limits of drawability were experimentally investigated using constant BHF. The results of this study are shown in Table 6. This table indicates that AKDQ had the largest drawability window while aluminum had the smallest and bake hard and high strength steels were in the middle. The strain distributions for constant, ramp, and pulsating BHF are compared experimentally in Fig. 22 and are compared with simulations in Fig. 23 for AKDQ. In both simulations and experiments, it was found that the ramp BHF trajectory improved the strain distribution the best. Not only were peak strains reduced by up to 5% thereby reducing the possibility of fracture, but low strain regions were increased. This improvement in strain distribution can increase product stiffness and strength, decrease springback and residual stresses, increase product quality and process robustness.
Table 6. Limits of drawability for dome cup with constant BHF
Fig. 22. Experimental effect of time variable BHF on engineering strain in an AKDQ steel dome cup.
Fig. 23. Simulated effect of time variable BHF on true strain in an AKDQ steel dome cup.
Pulsating BHF, at the frequency range investigated, was not found to have an effect on strain distribution. This was likely due to the fact the frequency of pulsation that was tested was only 1?Hz. It is known from previous experiments of other researchers that proper frequencies range from 5 to 25?Hz [3]. A comparison of load-stroke curves from simulation and experiments are shown in Fig. 24 for AKDQ. Good agreement was found for the case where μ=0.08. This indicates that FEM simulations can be used to assess the formability improvements that can be obtained by using BHF control techniques.
Fig. 24. Comparison of experimental and simulated load-stroke curves for an AKDQ steel dome cup.
5 Conclusions and future work
In this paper, we evaluated an improved design process for complex stampings which involved eliminating the soft tooling phase and incorporated the validation of product and process using one-step and incremental FEM simulations. Also, process improvements were proposed consisting of the implementation of blank holder force control to increase product quality and process robustness.
Three separate investigations were summarized which analyzed various stages in the design process. First, the product design phase was investigated with a laboratory and industrial validation of the one-step FEM code FAST_FORM3D and its ability to assess formability issues involved in product design. FAST_FORM3D was successful at predicting optimal blank shapes for a rectangular pan and an industrial instrument cover. In the case of the instrument cover, many hours of trial and error experimentation could have been eliminated by using FAST_FORM3D and a better blank shape could have been developed.
Second, the die design