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Punch design for floating based micro-tube hydroforming die assembly
Accepted Manuscript Title: Punch Design for Floating Based Micro-Tube Hydroforming Die Assembly Authors: Gracious Ngaile, James Lowrie PII: DOI: Reference:
S0924-0136(17)30498-3 https://doi.org/10.1016/j.jmatprotec.2017.10.049 PROTEC 15469
To appear in:
Journal of Materials Processing Technology
Received date: Accepted date:
7-9-2016 28-10-2017
Please cite this article as: Ngaile, Gracious, Lowrie, James, Punch Design for Floating Based Micro-Tube Hydroforming Die Assembly.Journal of Materials Processing Technology https://doi.org/10.1016/j.jmatprotec.2017.10.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Punch Design for Floating Based Micro-Tube Hydroforming Die Assembly Gracious Ngaile and James Lowrie Department of Mechanical and Aerospace Engineering North Carolina State University Raleigh NC, USA
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Phone: 1-919-515-5222: email: [email protected] Abstract
Conventional punches used in tube hydroforming (THF) are hollow to facilitate supply of pressurized fluid to the die cavity. The fabrication of hollow micro-punches for micro-THF which can sustain punch loads presents a challenge. This study proposes different types of micro punch designs that can be used in conjunction with floating based micro-tube hydroforming (THF) die set-up. Finite element analysis was carried out to determine the feasibility of the proposed punch design variants followed by Micro-THF
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experiments. The experiments were carried out to hydroform Y, T, and bulge shaped parts from SS 304 1mm
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and 2mm tubular blanks. The study has demonstrated that notched punches can effectively be used in micro-
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THF. The major benefit of using notched punches is that, longer micro-punches with the desired strength can
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be fabricated with ease using electrical discharge machining (EDM). Keywords: micro tubes, hydroforming, micro punch design
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1. Introduction
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Micro-Tube Hydroforming (THF) is a material forming process that utilizes pressurized fluid in place of a hard tool to plastically deform a micro tubular material into a desired shape. The process has the potential to
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produce complex micro-tubular shapes for applications in various fields such as electronics devices, medical devices, microfluidics devices, and micro-electrical mechanical systems. This technology is still it its infancy stage with numerous technical challenges that need to be investigated. The major challenges confronting
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micro-THF are briefly summarized below: Severe tribological performance as the surface to volume ratio increase in micro forming: Tiesler et al (1999) carried out double cup backward extrusion experiments to study the effect of miniaturization. They found that
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the friction factor can increase by a factor of 20 when micro samples were used as compared to macro samples. Engel, et al (2006) pointed out that the severity of tribological performance in micro forming is largely attributed by the high ratio of open to closed lubricant pockets on micro parts. Bunget and Ngaile (2011) carried out finite element simulation on backward cup extrusion at different cup wall thickness and found that as the wall thickness reduces to a micro level, the ratio of friction energy to the total energy
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expended during backward extrusion increase significantly. The increase in the frictional energy was associated with drastic increase in the surface-to-volume ratio when forming micro parts.
Anisotropic behavior of metals at micro-scale and formability: Because on the micro scale a part can no longer be considered a homogeneous, the deformation behavior of a micro blank may significantly differ from
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its macro scale counterpart. For example, the ability of the part to be formed can be adversely affected by just one grain being in an unfavorable position, whereas on the macro level other grains would be present to make up for the lack of formability in one grain. Parasiz et al. (2007) investigated the influence of size effect in micro-extrusion process and found that the extruded pins are curved and the distribution of plastic strain and hardness along the section of the extruded pins are not uniform when the material with coarse grain size is used. A similar study on forward backward extrusion of brass with different grain sizes was carried out by Chan et al (2011) who found that coarse grain samples lead to inhomogeneous deformation. In another study,
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Fu et al, (2011) conducted tensile test with annealed pure copper foils with different thicknesses and grain
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sizes to study the size effects on fracture behavior. They observed noticeable decrease in the fracture strain which was associated with the fact that the number of slip systems that are active in the part are significantly
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decreased. Zhuang, et al (2012) investigated the deformation features in hydroforming of micro-tubes with
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OD=800 μm and wall thickness of 40 μm. They found that premature failure takes place at random locations for materials having between 1 and 2 grains through the tube wall thickness.
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Handling of micro billets: Micro handling and positioning of micro parts is another challenge that may
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necessitate a change in the overall design of a micro-manufacturing system. In a review paper, Salmeron et al (2005) pointed out that the key problems limiting micro-handling technologies is the lack of flexible and
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high-precision –micro-handling machinery as well as lack of standardization. In an attempt to reduce the complexity of handling micro billets, Merklein et al (2012), introduced a new manufacturing method where pins are manufactured from a sheet metal similar to progressive forming operation commonly used in
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stamping. Mahshid et al (2014) introduced a methodology for determining the precision of handling system of a micro transfer press by combining analysis of positing accuracy, dynamic behavior, and gripping
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characteristics. The analysis of the actuator showed static positioning accuracies of the order of 3µm.
Developing micro THF die system with capability for tube expansion and material feed: One of the major challenges in Micro-THF is the difficulty in developing die systems that allows feeding of the materials to the die cavity while ensuring that sealing from high fluid pressure is achieved. Most researchers working on micro-THF have focused on micro tube expansion studies that do not require feeding. Hartl (2010) and Jorn (2012) were involved in the development of a micro-THF press system with a maximum pressure of 400MPa.
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They were able to hydroform micro camera gear shaft with OD=0.8mm and tube wall thickness of 0.04mm. This system, however, did not incorporate a material feed. Micro-THF tooling with feeding capabilities is crucial to enable hydroforming of complex parts. To address this challenge, Ngaile (2013) proposed a floating based micro-THF die assembly. The system is based on decoupling the actions occurring at the punch-tubedie junction of a conventional THF system. In a typical macro-THF system the punch-tube-die junction has to
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fulfill several requirements namely; material feed, sealing, and high pressure fluid supply. All these requirements are mutually depended and thus presenting a huge challenge in scaling down the conventional THF to a micro level. Figure 1 shows a schematic diagram of the floating based THF system. The 1st level contains a high pressure chamber and the 2nd level contains a floating die assembly. The system is called “floating” because the die assembly is submerged in a pressurized fluid chamber. The unique feature of this THF system is that the functions of the tool-tube interfaces (punch-tube and die-tube interfaces) are decoupled to better suit the needs of a micro-scale process. In the conventional macro THF system, the punch is
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responsible for supplying and maintaining the high pressure in the working fluid as well as feeding the tube
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into the die cavity. In order for the punch-tube interface to function at all, a reliable seal must be created that moves with the tube material so that both expansion and feeding can take place simultaneously. In the floating
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dies assembly, the function of sealing is decoupled from the punch-tube interface and added to the die-tube
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interface [Fig. 2]. The sealing could be achieved by wrapping the tube with a Teflon sheet or the tube could be coated by a solid film which would act both as a seal and a lubricant. The design should be such that fluid is prevented from entering the die cavities. As pointed out by Ngaile and Lowrie (2014), this decoupling gives
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more freedom in the punch design and avoids many of the issues involved with creating a reliable seal with
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the punch.
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The objective of this paper is to explore design alternatives for punches that can be used in floating based micro-THF die systems. We will first discuss potential punch design geometries that can be used in the floating based micro-THF tooling followed by finite element (FE) simulation of the THF process with focus
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on representative micro-THF parts. These simulations will provide punch loads which will then be used as input in the FE structural analysis. The results from the FE structural analysis will be studied to determine if the induced stress levels in the micro punch are in the acceptable range, assuming hardened tool (HRC 60)
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with a maximum compressive yield strength of 2,000MPa. The study is concluded by experimental validations.
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Figure 1. Floating based micro-THF dies assembly [13]
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Figure 2. Floating die assembly surrounded by pressurized fluid
2. Strategies for Designing Micro Punches for Material Feed Using a micro-THF Floating Die Setup The experimental setup for floating based micro-THF die assembly is shown in Figure 3a [Ngaile and Lowrie, 2014]. The system consists of the top and bottom pressure housing chambers, floating micro die assembly, punch housing, and cams for material feed. The high fluid pressure is supplied to the die assembly though a computer controlled pressure intensifier. The pressure can be varied from 0 to 140MPa using a control unit
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shown in Figure 3b. The 150 ton press, not shown, is used for closing the top and bottom pressure housing chambers. Only the bottom pressure housing chamber is shown in Figure 3a.
Figure 4 shows tube hydroforming dies for 1mm OD and 2mm OD tubes. The dies used in the set up were fabricated from A2 tool steel and hardened to 60 HRC. As seen in Figure 3a, the punches are pushed towards
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the die cavity via a cam mechanism. In the development of the micro-THF tooling set up, the punch effective length (see figure 3a) has to be chosen based on the punch buckling analysis discussed later in section 3.3. For the setup given in Fig 4, the effective length is set to 5mm and cams are sized to accommodate material feed
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of 3mm.
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Figure 3. (a) Micro THF Floating die assembly setup, (b) high fluid pressure system and control unit
Figure 4. Dies for 1mm and 2mm OD tubular specimens
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Manufacturing of micro-punches that can withstand the material feed forces without failure is a huge challenge. Conventional punches used in macro-THF are hollow to facilitate passage of high fluid pressure. Although hollow punches are capable of exerting uniform pressure on the tube ends and facilitate supply of pressurized fluid into the tube, a hollow micro-punch with OD=1mm will require a very small hole of the order of 0.1-0.2mm to have sufficient rigidity. Furthermore, drilling such a small hole along the length of the
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punch can be very challenging, particularly if the punch is long. With the current micro-machining technology, drilling of holes of the order of 0.2mm at 4mm depth is feasible. However, a 4mm long punch may be too short for practical implementation in a THF system.
In this study, punch design variants for material feed using floating die assembly are proposed. The punch design variants include notched type and micro-holed punches [Fig.5]. In contrast to the conventional hollow punch, notched punch variants retains the rigidity of the punch since a very small amount of material can be
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taken out from a 1mm dia. rod. Furthermore, micro notches could easily be fabricated by electrical discharge
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machining (EDM) or through a micro-milling operation. Since the purpose of the notch is to facilitate fluid transfer to the tube, a very small notch could be used. Similarly, micro-holed punches shown in Figure 5e will
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retain punch rigidity as micro holes for fluid transfer are drilled close to the punch nose. One of the
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drawbacks of notched micro-punches is that the uniform pressure is not applied on the tube ends. Therefore, non-uniform deformation will be exhibited at the tube ends. To determine the suitability of these punches
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finite element (FE) analysis were carried out as discussed in the next section.
(c) Partially Notched Punch Nose Design
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(a) Hollow Punch Design (Conventional)
(d) Fully Notched Punch Nose Design
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(b) Fully Notched Punch Design
(e) Micro-Holed Punch Design Figure 5. Hollow, notched, and micro-holed punch variants
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3. Finite Element Simulations 3.1 Micro-THF of T, Y, and bulge shapes In order to establish the axial force exerted on punches, micro tube hydroforming of a Y, T and bulge shaped parts were simulated using 1mm and 2mm dia. These geometries were chosen as they represent the majority of deformation modes encountered in THF. Stainless steel tubes (SS 304) with wall thicknesses of 0.1mm and
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0.2mm for 1mm and 2mm dia. respectively were used. For all simulations the length of the tube were kept to 12mm. The simulations were carried out using ANSYS explicit. In the simulations two friction levels, µ=0.05 and µ=0.1were used and the stress-strain relation of the tube materials was assumed to follow the power law with strength coefficient K=1275MPa and strain hardening exponent n=0.45. The tubular blanks were modeled with 3,600 shell elements. In all the simulations solid punch was used and was assumed to be rigid. Figure 6 shows the loading paths and wall thinning distribution for Y, T and bulge shapes for 1mm OD tubular parts. During hydroforming T and Y shapes, the punch feed was varied lineally to a maximum of
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2.3mm at a maximum pressure of 180MPa. For the bulge part a maximum feed of 1.7mm was used. To
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ensure accuracy all simulation were carried out such that the energy error is below 1% of the total energy. As seen in Figure 6 the maximum thinning exhibited for Y, T, Bulge shapes are 16%, 15% and 19% respectively.
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Figure 6 also shows that in the course of feeding, significant thickening was observed. Tube thickening was in
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the order of 60% for Y and T shaped parts. One of the reasons for this level of thickening is that a higher aspect ratio (tube length/diameter, L/D=12) was used. Thus, to feed the tube material to the guiding zone, a significant friction stress had to be overcome. The aspect ratio (L/D) of 12 was deliberately chosen to ease
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tube handling in the actual micro-tube hydroforming.
Figure 7 shows the maximum loads that were exerted on the punches when 1mm OD and 2mm OD tubular
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blanks were hydroformed at friction coefficient values of µ=0.05 and µ=0.1. The simulation shows that in order to hydroform the T-shaped part from 1mm OD tube, a maximum punch load of 750N is needed. The Y and bulge shape exhibited a slightly lower punch loads. When the tube size was increased to 2mm OD, the
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maximum punch increased to 2,500N for µ=0.1. Again, Y and bulge parts exhibited lower punch loads. These
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loads were later used for punch stress analysis as discussed in section 3.2.
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(a) Loading path for Y-shape
(d ) Thickness distribution
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(c )Loading path for T-Shape
(f) Thickness distribution
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(e) Loading path for bulge shape
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Figure 6. Loading paths and wall thinning distribution for Y, T and bulge shapes for 1mm tube , µ=0.1, and initial wall thickness t=0.1mm.
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Figure 7. Influence of friction and tube size on punch load 3.2 Punch stress analysis
Micro-THF simulations for Y, T and Bulge shapes discussed in section 2 have shown that a maximum force
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of 750N is required to axially feed the material to the die cavity for a 1mm OD, 12mm long SS304 tube. To hydroform similar geometries using a 2mm OD tube sample, a maximum force of 2,500N is required. Thus a
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punch load of 750N was used for stress analysis of all 1.0mm OD punches and a load of 2,500N was used for
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all 2.0mm OD punch variants. The punch stress analyses were carried out using ANSYS structural FE software. For all simulations the punch length was set to 25mm. In the FE model, force was distributed over
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an area consistent with the tube thickening simulated in the previous section. I.e., a 1mm OD tube with initial wall thickness of 0.1mm at a punch feed of 2.4mm (see figure 6a), resulted in wall thickening of 0.16mm.
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Thus, uniform pressure (750N/area) was applied to an annulus area corresponding to this thickening. Figure 8
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shows the effective stress distribution for conventional punch with a hole in the middle for supplying high pressure fluid to the deforming tube. Figures 8a & b show simulations for 1mm OD and 2mm OD punches respectively. Both punches have a 0.35mm hole drilled at the middle. These punches exhibited maximum
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effective stress in the range of 1,490MPa. Most tool steel materials will be able to handle such loading. The challenge, however, is the difficulty in drilling 0.35mm-hole at a depth of 25mm. By increasing the hole to
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0.5mm, a 1mm OD punch exhibited maximum effective stress of over 2000MPa. This stress exceeds the yield stress of most tool steel materials available today. Furthermore, by increasing the size of the middle hole, the
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rigidity of the punch drops significantly. Figures 9a & b show effective stress distribution for fully notched 1mm and 2mm OD punches. The punch were modelled with a notch width of 0.35mm. The notch depth for 1mm OD punch and 2mm OD punch were 0.5mm and 1mm respectively. The maximum stress exhibited by both punches is in the order of 2,100MPa. By closely looking at the punch noses, it can be observed that an average stress level of 1,500MPa is exhibited. Only at the notch radii the stress is higher. This stress level suggests that these punches will operate without failure. It should be noted that the stress level could further be lowered by reducing the notch size.
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Notches of different sizes as small as 25μm wide could easily be produced using wire Electro Discharge Machining (wire-EDM). As discussed earlier longer notched punch can easily be produced using wire-EDM.
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With this variant, the challenge for fabrication is eliminated as compared to hollow punches.
(a) 2mm OD; 0.35mm ID
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(a) 1mm OD; 0.35mm ID
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Figure 8. Stress distribution on conventional punch tips, 1mm OD and 2mm OD
(a) 1mm OD; 0.35mm notch width
(b) 2mm OD; 0.35mm notch width
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Figure 9. Stress distribution on fully notched punch; 1mm OD and 2mm OD; notch width=0.35mm Figures 10a & b show punch stress evolution for partially notched punch nose variants for 1mm OD and 2mm OD punches respectively. A maximum effective stress in the order of 1,900MPa is observed for both punches. This stress level is in the border of most hardened tool steels. This stress level could slightly be reduced by altering the notch size. Note that unlike the punch with a notch all the way through the punch length, one of the advantages of the partially notched punch nose variant is that it exhibits the highest rigidity because the
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rod has only been machined at the end of the punch. Sufficient clearance between the punch and the die is,
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however, needed to ensure pressurized fluid is supplied to the notched zone.
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(a) 1mm OD, 0.35mm notch width, max depth 0.5mm (b)2mm OD, 0.35mm notch width, max depth 1mm
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Figure 10. Stress distribution on partially notched punch nose; 1mm OD and 2mm OD; notch width=0.35mm
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Figures 11a & b show effective stress maps for punches that are fully notched at the punch nose. The stress level for the 1mm OD punch exceeds the limit of most hardened tools steel materials. However, for the 2mm
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small notch will have to be adopted.
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punches a maximum stress of 1,700MPa was exhibited. To avoid yielding of the 1mm OD punches, a very
(a) 1mm OD; 0.35mm notch width, 0.5mm long
(b) 2mm OD; 0.35mm notch width,1.5mm long
Figure 11. Stress distribution on fully notched punch nose; 1mm OD and 2mm OD; notch width=0.35mm
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The FE simulation results for all punch variants are summarized in Table 1. These results show the potential to design robust micro punches that can withstand material feeding loads for micro-THF using the floating die assembly setup. Conventional hollowed punches could only be used for floating based micro-THF if it is possible to drill very small holes of the order of 0.35mm on a sufficiently longer punch. It should be noted that while very short hollow punches could be produced, practical implementation of very short punches on
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the floating die assembly is difficult, or may not be feasible. With the exception of fully notched punch nose, all other punch variants exhibited acceptable stress levels and can withstand higher flexural loading compared to hollow micro punches. Furthermore, notches of different sizes, as small as 25μm wide, could easily be produced using wire Electro Discharge Machining (EDM), or micro milling operation.
750
1,499
750
2,100
750
Conventional Punch Fully notched punch
Below yield stress of tool steels Difficult to drill 0.3mm hole x 25 mm depth
2,500
1,479
Below yield stress of tool steels Potential for use
2,500
2,398
Partially notched punch nose
2,500
1,905
Stress induced at the limit of tool steel Potential for use as the average stress at the punch nose is 1700MPa Below yield stress of tool steels Potential for use
Fully notched punch nose
2,500
1,786
Below yield stress of tool steels Potential for use
1,936
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Fully notched punch
750
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Conventional Punch
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2,650
Stress induced at the limit of tool steel Potential for use as the average stress at the punch nose is 1700MPa Just below yield stress of tool steels Potential for use Stress induced is above most tool steels Punch geometry not suitable
Partially notched punch nose Fully notched punch nose
2mm OD
Remarks
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Maximum Punch effective stress [MPa]
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1mm OD
Input punch load [N]
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Punch type
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Table 1: FEA result summary
3.3 Punch Buckling Analysis In addition to the punch stress analysis discussed in section 3.2, linear buckling analysis was carried out to establish effective lengths of micro punches that can withstand punch forces without buckling. ANSYS Workbench 14.5 was used for these simulations. In the simulations, a tool steel with Young’s modulus
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E=200 GPa, and poison ratio of ν=0.3 were assumed. A force of 1.0 N was applied in the model so that the load multiplier given in the ANSYS result will represent the critical buckling load. Of the punch variants discussed in section 3.2, buckling simulation results for axially full notched punches, OD=1mm and Leff = 10mm are presented. Note that among the three notched micro punch variants, fully notched punch would require the least buckling load, thus, the analysis is only carried out for this micro punch variant. Figure 12
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shows buckling critical loads that were obtained for different notch sizes. The buckling load for a 1mm OD punch without notch is also presented. For this punch geometry the critical buckling load can also be obtained analytically. This simulation is included to provide a base for maximum critical load that could be attained for a micro punch with an infinitesimally small notch.
The buckling simulation shows that critical buckling loads varies from 631N to 916N depending on the notch size. For a small notch of 100µm x 100µm, a maximum punch load of 916N can be used, whereas a micro
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punch with a 350µm x 500µm notch could withstand a maximum punch load of 631N before the punch
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buckles. It should be noted that these results are applicable for a punch with 10mm effective length. In other words if the micro-THF system allows for shortening the effective punch length, higher punch loads than
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those given in Figure 12 could be used. The influence of notch radius on buckling was also studied for notch
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radius of R=100µm, 150µm, and 175µm. Figure 13 shows that the change in buckling load is insignificant.
200µm x 200µm notch
300 µm x 300µm notch
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100µm x 100µm notch
Figure 12. Buckling loads for fully notched punches
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350 µm x 500µm notch
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Figure 13. Influence of notch radius on buckling load
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3.4 Effect of notched punch on hydroformed part
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To investigate the effect of notched punch on micro-THF, simulations were conducted for T-shape part. A punch with a notch width of 0.35mm was used as shown in Figure 14. The FE simulation conditions used in
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The simulations results show that there is some
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this analysis were similar to the FE conditions discussed in section 3.2
material extruded through the notch and result in
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the formation of ‘tab’ on the tube ends. This means
that the tubes will have to be trimmed after the
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forming process has taken place. The presence of the notches do not appear to prevent the formation
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of the bulge. Note that, the process of trimming tube ends after hydroforming is a common practice even in conventional THF. Notched punches for
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1mm OD and 2mm OD were fabricated and used in micro-meso tube hydroforming as discussed in the
Figure 14. Influence of notched punch on deformed part
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next section.
4. Experimental Verification, and Discussion 4.1 Fabrication of notched punches and experimental procedures Among the four micro punch variants discussed in section 3, namely; conventional punch, full notched punch, partially notched punch nose, and fully notched punch nose, the later three were fabricated. All the punched were fabricated using wire EDM and hardened to 60HRC. Figures 16a and 17a show fully notched punches
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placed on die blocks used for hydroforming 1mm OD and 2mm OD tubular specimen respectively. Figure 18a shows punches with fully notched punch nose placed on a die block for 2mm OD samples, whereas Figure 19a shows partially notched punch nose type punch for 2mm OD samples. The notched width for all punch types fabricated was 0.350mm. This width was dictated by the size of wire used in the EDM machine. Micro tube hydroforming experiments were carried out to test the effectiveness of these punches using the
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floating based micro THF test setup (Fig 3) discussed in section 2. Tubular specimens used in the experiment were made from SS304. The 1mm OD tube samples had a wall thickness of 0.1mm while the 2mm OD samples had wall thickness of 0.2mm. The lengths of the tube samples used in the experiments was 12mm. Teflon sheet was used as a lubricant for all experiments carried out. Fluid pressure was applied linearly to a maximum of 130MPa in 100s. Feeding of material was carried out through the feed actuators connected to the cam. The feed was actuated when the hydroforming pressure reached 35MPa and the feed was stopped when the pressure reached 80MPa.
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4.2 Experimental results and discussion
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Figure 16b shows micro hydroformed geometries from 1mm OD tube samples when fully notched punch was used. A fractured sample, was hydroformed without axial feeding. For the middle part, a feed of 0.25mm was
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used during hydroforming. The buckled part (bottom) underwent a feed of 1mm. Excessive feeding was used
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intentionally to check if the punch could withstand the punch load. Fully notched punches were also used to hydroform Y and T parts from 2mm OD samples as shown in Figures 17. Figure 17b shows the Y and T hydroformed parts which were subjected to a feed of 1 mm from both ends. These parts were not fully formed
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to their final geometries due to the limitation on the fluid pressure in the current system. As discussed in the
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hydroforming simulations, to acquire fully formed Y and T shapes, a fluid pressure of 180MPa is needed. Figure 17c shows bulged parts which were formed using the middle die cavities. No material feed was applied
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for the bulged parts.
(b) Figure 16. 1mm OD hydroformed parts; feeding via fully notched micro punches. (Tube length L=12 mm, wall thickness = 0.1mm, P=100MPa)
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(b) Feed 2mm, pressure 100MPa
(c) No feed, Pressure 80MPa
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Figure 17. 2mm OD hydroformed parts; feeding via fully notched micro punches. (Tube length L=12 mm, wall thickness = 0.2mm)
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Figure 18 shows Y, T, and box shaped parts that were hydroformed using a fully notched punch nose at a
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maximum pressure of 130MPa. The total material feed for the Y and T shape was 1.4mm. The box shaped part shown in Fig 18ci fractured when material feed of 0.5 was used. When the feed was increased to 1.0mm
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no fracture was observed (Fig 18cii). Figure 19 shows hydroformed results when a partially notched punch
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nose was use. In this case a total feed of 0.7 was used.
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(b) Feed 1.4mm, Pressure 130MPa
(c) i 0.5mm feed bust, ii 1.0mm feed Figure 18. 2mm OD hydroformed parts; feeding via fully notched micro punch nose. (Tube length L=12 mm, wall thickness = 0.2mm, P=130MPa)
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(b)
Feed 0.7mm, P=130MPa
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Figure 19. 2mm OD hydroformed parts; feeding via Partially notched micro punch noses. (Tube length L=12 mm, wall thickness = 0.2mm, P=130MPa)
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The above experimental results have demonstrated that all three variants of micro notched punches can be
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used in micro tube hydroforming set up. Although notched punches have shown to be suitable for micro-THF processes in terms of acceptable stress level, manufacturability, and higher flexural strength, they do have a
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drawback that pressure exerted at the ends of the tube is not uniform. This leads to non-uniform deformation at the tube ends. A very small indentation was observed on the ends of the hydrofomed samples due to none
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uniform pressure distribution caused by the punch notch. In practice, however, the ends of macro tube hydroformed parts are usually trimmed out. In the case of micro THF parts, grinding or polishing of the tube
5. Conclusions
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end may be required.
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This paper has proposed design variants for micro punches that can be used with the floating based microTHF die assembly. In addition to the finite element analysis to determine the suitability of punches, experimental verification for three punch types namely; fully notched punch, partially notched punch nose
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and full notched punch nose variants were successfully carried out. The drawn conclusions from this study are:
The finite element analysis conducted to investigate the strength of proposed micro punches has shown that fully notched punches and partially notched punch nose type induce von Mises stresses that are below the yield stress of tool steel.
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Notched punches have the potential to withstand material feed load required to form Y, T, and Bulge shapes from SS304 micro tubes. Since fabrication of notched punches takes away a very small amount of material from a 1mm dia. rod, the rigidity of the punch can be retained. In order to be able to use the proposed punches for micro tube hydroforming of SS 304 or materials with
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equivalent strength, the micro-THF set-up should be designed such that the effective punch length does not exceed 10mm as the buckling load for a punch with a small notch is in the order of 700N-800N.
Hydroforming experiments carried out using different notched punches for 1mm and 2mm OD SS304 tubes, have demonstrated the potential of using these types of punches for micro-tube hydroforming on a floating based micro-THF die assembly.
The non-uniform deformation occurring at the ends of the tubes caused by non-uniform pressure exerted
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by the notched punches has shown to have little effect on the feeding. As long as the tube ends are
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polished or ground, the non-uniform deformed ends will not affect the integrity of the hydroformed part.
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Acknowledgement
The author would like to acknowledge the National Science Foundation, through which this work was
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funded under Project No. CMMI # 0900148. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National
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Science Foundation.
References
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Bunget, C. Ngaile, G., 2011. Influence of ultrasonic vibration on micro-extrusion, Ultrasonics (51) 2011 pp.
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Chan, W.L. Fu, M.W. Yang, B., 2011. Study of size effect in micro-extrusion process of pure copper, Materials and Design 32, pp 3772-3782
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Engel, U., 2006 Tribology in microforming”, Wear, 260, p. 265-273 Fu, M.W. Chan, W.L., 2011. Geometry and grain size effects on the fracture behavior of sheet metal in micro scale plastic deformation, Material and Design, 32 pp. 4738-4746
Hartl, C., 2010. Micro-Hydroforming; Micro-manufacturing Engineering and Technology, Elsevier science Jörn, L., 2012. “Fundamental Studies On Tube Hydroforming And Laser Assistance In The Manufacture Of Micro-Parts.” Doctoral Dissertation, The University of Strathclyde, Glasgow.
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Mahshid, R., Nørgaard, H., Hansen, Arentoft, M., 2014. Characterization of precision of a handling system in high performance transfer press for micro forming, CIRP Annals - Manufacturing Technology, Volume 63, pp. 497-500
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Merklein M, Stellin T, Engel, U. 2012. Experimental Study of a Full Forward Extrusion Process From Metal Strip. Key Engineering Materials 504–506:587–592.
Ngaile, G., 2013. Development of a micro-Tube Hydroforming Die System, International conference on micro manufacturing (ICOMM2013), Victoria Canada.
Ngaile, G., Lowrie, J., 2014. New Micro Tube Hydroforming System Based on Floating Die Assembly
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Concept”, Journal of Micro- and Nano-Manufacturing, December 2014, 10.1115/1.4028320
Parasiz SA, Kinsey B, Krishnan N, Cao J, Li M., 2007. Investigation of deformation size effects during
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micro extrusion. ASME Journal of Manufacturing Science, 129: p. 690–7
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Sanchez-Salmeron, A.J. Lopez-Tarazon, R. Guzman-Diana R., Ricolfe-Viala, C., 2005. Recent development
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Vo. 167, Issues 2–3, 30, pp. 499-507
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in micro-handling systems for micro-manufacturing, Journal of Materials Processing Technology,
Tiesler, N., Engel, U., Geiger, M, 1999. Forming of microparts—effects of miniaturization on friction, in:
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M. Geiger (Ed.), Advanced Technology of Plasticity, Proceedings of the Sixth International Conference on Technology of Plasticity ICTP 1999, vol. II, 19–24 September 1999, Nuremberg,
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Germany, Springer, Berlin, pp. 889–894. Zhuang, W., Wang S., Lin J., Balint D., Hartl, Ch., 2012. Experimental and numerical investigation of
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S0924-0136(17)30498-3 https://doi.org/10.1016/j.jmatprotec.2017.10.049 PROTEC 15469
To appear in:
Journal of Materials Processing Technology
Received date: Accepted date:
7-9-2016 28-10-2017
Please cite this article as: Ngaile, Gracious, Lowrie, James, Punch Design for Floating Based Micro-Tube Hydroforming Die Assembly.Journal of Materials Processing Technology https://doi.org/10.1016/j.jmatprotec.2017.10.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Punch Design for Floating Based Micro-Tube Hydroforming Die Assembly Gracious Ngaile and James Lowrie Department of Mechanical and Aerospace Engineering North Carolina State University Raleigh NC, USA
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Phone: 1-919-515-5222: email: [email protected] Abstract
Conventional punches used in tube hydroforming (THF) are hollow to facilitate supply of pressurized fluid to the die cavity. The fabrication of hollow micro-punches for micro-THF which can sustain punch loads presents a challenge. This study proposes different types of micro punch designs that can be used in conjunction with floating based micro-tube hydroforming (THF) die set-up. Finite element analysis was carried out to determine the feasibility of the proposed punch design variants followed by Micro-THF
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experiments. The experiments were carried out to hydroform Y, T, and bulge shaped parts from SS 304 1mm
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and 2mm tubular blanks. The study has demonstrated that notched punches can effectively be used in micro-
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THF. The major benefit of using notched punches is that, longer micro-punches with the desired strength can
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be fabricated with ease using electrical discharge machining (EDM). Keywords: micro tubes, hydroforming, micro punch design
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1. Introduction
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Micro-Tube Hydroforming (THF) is a material forming process that utilizes pressurized fluid in place of a hard tool to plastically deform a micro tubular material into a desired shape. The process has the potential to
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produce complex micro-tubular shapes for applications in various fields such as electronics devices, medical devices, microfluidics devices, and micro-electrical mechanical systems. This technology is still it its infancy stage with numerous technical challenges that need to be investigated. The major challenges confronting
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micro-THF are briefly summarized below: Severe tribological performance as the surface to volume ratio increase in micro forming: Tiesler et al (1999) carried out double cup backward extrusion experiments to study the effect of miniaturization. They found that
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the friction factor can increase by a factor of 20 when micro samples were used as compared to macro samples. Engel, et al (2006) pointed out that the severity of tribological performance in micro forming is largely attributed by the high ratio of open to closed lubricant pockets on micro parts. Bunget and Ngaile (2011) carried out finite element simulation on backward cup extrusion at different cup wall thickness and found that as the wall thickness reduces to a micro level, the ratio of friction energy to the total energy
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expended during backward extrusion increase significantly. The increase in the frictional energy was associated with drastic increase in the surface-to-volume ratio when forming micro parts.
Anisotropic behavior of metals at micro-scale and formability: Because on the micro scale a part can no longer be considered a homogeneous, the deformation behavior of a micro blank may significantly differ from
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its macro scale counterpart. For example, the ability of the part to be formed can be adversely affected by just one grain being in an unfavorable position, whereas on the macro level other grains would be present to make up for the lack of formability in one grain. Parasiz et al. (2007) investigated the influence of size effect in micro-extrusion process and found that the extruded pins are curved and the distribution of plastic strain and hardness along the section of the extruded pins are not uniform when the material with coarse grain size is used. A similar study on forward backward extrusion of brass with different grain sizes was carried out by Chan et al (2011) who found that coarse grain samples lead to inhomogeneous deformation. In another study,
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Fu et al, (2011) conducted tensile test with annealed pure copper foils with different thicknesses and grain
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sizes to study the size effects on fracture behavior. They observed noticeable decrease in the fracture strain which was associated with the fact that the number of slip systems that are active in the part are significantly
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decreased. Zhuang, et al (2012) investigated the deformation features in hydroforming of micro-tubes with
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OD=800 μm and wall thickness of 40 μm. They found that premature failure takes place at random locations for materials having between 1 and 2 grains through the tube wall thickness.
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Handling of micro billets: Micro handling and positioning of micro parts is another challenge that may
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necessitate a change in the overall design of a micro-manufacturing system. In a review paper, Salmeron et al (2005) pointed out that the key problems limiting micro-handling technologies is the lack of flexible and
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high-precision –micro-handling machinery as well as lack of standardization. In an attempt to reduce the complexity of handling micro billets, Merklein et al (2012), introduced a new manufacturing method where pins are manufactured from a sheet metal similar to progressive forming operation commonly used in
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stamping. Mahshid et al (2014) introduced a methodology for determining the precision of handling system of a micro transfer press by combining analysis of positing accuracy, dynamic behavior, and gripping
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characteristics. The analysis of the actuator showed static positioning accuracies of the order of 3µm.
Developing micro THF die system with capability for tube expansion and material feed: One of the major challenges in Micro-THF is the difficulty in developing die systems that allows feeding of the materials to the die cavity while ensuring that sealing from high fluid pressure is achieved. Most researchers working on micro-THF have focused on micro tube expansion studies that do not require feeding. Hartl (2010) and Jorn (2012) were involved in the development of a micro-THF press system with a maximum pressure of 400MPa.
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They were able to hydroform micro camera gear shaft with OD=0.8mm and tube wall thickness of 0.04mm. This system, however, did not incorporate a material feed. Micro-THF tooling with feeding capabilities is crucial to enable hydroforming of complex parts. To address this challenge, Ngaile (2013) proposed a floating based micro-THF die assembly. The system is based on decoupling the actions occurring at the punch-tubedie junction of a conventional THF system. In a typical macro-THF system the punch-tube-die junction has to
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fulfill several requirements namely; material feed, sealing, and high pressure fluid supply. All these requirements are mutually depended and thus presenting a huge challenge in scaling down the conventional THF to a micro level. Figure 1 shows a schematic diagram of the floating based THF system. The 1st level contains a high pressure chamber and the 2nd level contains a floating die assembly. The system is called “floating” because the die assembly is submerged in a pressurized fluid chamber. The unique feature of this THF system is that the functions of the tool-tube interfaces (punch-tube and die-tube interfaces) are decoupled to better suit the needs of a micro-scale process. In the conventional macro THF system, the punch is
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responsible for supplying and maintaining the high pressure in the working fluid as well as feeding the tube
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into the die cavity. In order for the punch-tube interface to function at all, a reliable seal must be created that moves with the tube material so that both expansion and feeding can take place simultaneously. In the floating
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dies assembly, the function of sealing is decoupled from the punch-tube interface and added to the die-tube
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interface [Fig. 2]. The sealing could be achieved by wrapping the tube with a Teflon sheet or the tube could be coated by a solid film which would act both as a seal and a lubricant. The design should be such that fluid is prevented from entering the die cavities. As pointed out by Ngaile and Lowrie (2014), this decoupling gives
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more freedom in the punch design and avoids many of the issues involved with creating a reliable seal with
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the punch.
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The objective of this paper is to explore design alternatives for punches that can be used in floating based micro-THF die systems. We will first discuss potential punch design geometries that can be used in the floating based micro-THF tooling followed by finite element (FE) simulation of the THF process with focus
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on representative micro-THF parts. These simulations will provide punch loads which will then be used as input in the FE structural analysis. The results from the FE structural analysis will be studied to determine if the induced stress levels in the micro punch are in the acceptable range, assuming hardened tool (HRC 60)
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with a maximum compressive yield strength of 2,000MPa. The study is concluded by experimental validations.
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Figure 1. Floating based micro-THF dies assembly [13]
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Figure 2. Floating die assembly surrounded by pressurized fluid
2. Strategies for Designing Micro Punches for Material Feed Using a micro-THF Floating Die Setup The experimental setup for floating based micro-THF die assembly is shown in Figure 3a [Ngaile and Lowrie, 2014]. The system consists of the top and bottom pressure housing chambers, floating micro die assembly, punch housing, and cams for material feed. The high fluid pressure is supplied to the die assembly though a computer controlled pressure intensifier. The pressure can be varied from 0 to 140MPa using a control unit
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shown in Figure 3b. The 150 ton press, not shown, is used for closing the top and bottom pressure housing chambers. Only the bottom pressure housing chamber is shown in Figure 3a.
Figure 4 shows tube hydroforming dies for 1mm OD and 2mm OD tubes. The dies used in the set up were fabricated from A2 tool steel and hardened to 60 HRC. As seen in Figure 3a, the punches are pushed towards
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the die cavity via a cam mechanism. In the development of the micro-THF tooling set up, the punch effective length (see figure 3a) has to be chosen based on the punch buckling analysis discussed later in section 3.3. For the setup given in Fig 4, the effective length is set to 5mm and cams are sized to accommodate material feed
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of 3mm.
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Figure 3. (a) Micro THF Floating die assembly setup, (b) high fluid pressure system and control unit
Figure 4. Dies for 1mm and 2mm OD tubular specimens
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Manufacturing of micro-punches that can withstand the material feed forces without failure is a huge challenge. Conventional punches used in macro-THF are hollow to facilitate passage of high fluid pressure. Although hollow punches are capable of exerting uniform pressure on the tube ends and facilitate supply of pressurized fluid into the tube, a hollow micro-punch with OD=1mm will require a very small hole of the order of 0.1-0.2mm to have sufficient rigidity. Furthermore, drilling such a small hole along the length of the
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punch can be very challenging, particularly if the punch is long. With the current micro-machining technology, drilling of holes of the order of 0.2mm at 4mm depth is feasible. However, a 4mm long punch may be too short for practical implementation in a THF system.
In this study, punch design variants for material feed using floating die assembly are proposed. The punch design variants include notched type and micro-holed punches [Fig.5]. In contrast to the conventional hollow punch, notched punch variants retains the rigidity of the punch since a very small amount of material can be
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taken out from a 1mm dia. rod. Furthermore, micro notches could easily be fabricated by electrical discharge
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machining (EDM) or through a micro-milling operation. Since the purpose of the notch is to facilitate fluid transfer to the tube, a very small notch could be used. Similarly, micro-holed punches shown in Figure 5e will
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retain punch rigidity as micro holes for fluid transfer are drilled close to the punch nose. One of the
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drawbacks of notched micro-punches is that the uniform pressure is not applied on the tube ends. Therefore, non-uniform deformation will be exhibited at the tube ends. To determine the suitability of these punches
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finite element (FE) analysis were carried out as discussed in the next section.
(c) Partially Notched Punch Nose Design
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(a) Hollow Punch Design (Conventional)
(d) Fully Notched Punch Nose Design
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(b) Fully Notched Punch Design
(e) Micro-Holed Punch Design Figure 5. Hollow, notched, and micro-holed punch variants
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3. Finite Element Simulations 3.1 Micro-THF of T, Y, and bulge shapes In order to establish the axial force exerted on punches, micro tube hydroforming of a Y, T and bulge shaped parts were simulated using 1mm and 2mm dia. These geometries were chosen as they represent the majority of deformation modes encountered in THF. Stainless steel tubes (SS 304) with wall thicknesses of 0.1mm and
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0.2mm for 1mm and 2mm dia. respectively were used. For all simulations the length of the tube were kept to 12mm. The simulations were carried out using ANSYS explicit. In the simulations two friction levels, µ=0.05 and µ=0.1were used and the stress-strain relation of the tube materials was assumed to follow the power law with strength coefficient K=1275MPa and strain hardening exponent n=0.45. The tubular blanks were modeled with 3,600 shell elements. In all the simulations solid punch was used and was assumed to be rigid. Figure 6 shows the loading paths and wall thinning distribution for Y, T and bulge shapes for 1mm OD tubular parts. During hydroforming T and Y shapes, the punch feed was varied lineally to a maximum of
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2.3mm at a maximum pressure of 180MPa. For the bulge part a maximum feed of 1.7mm was used. To
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ensure accuracy all simulation were carried out such that the energy error is below 1% of the total energy. As seen in Figure 6 the maximum thinning exhibited for Y, T, Bulge shapes are 16%, 15% and 19% respectively.
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Figure 6 also shows that in the course of feeding, significant thickening was observed. Tube thickening was in
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the order of 60% for Y and T shaped parts. One of the reasons for this level of thickening is that a higher aspect ratio (tube length/diameter, L/D=12) was used. Thus, to feed the tube material to the guiding zone, a significant friction stress had to be overcome. The aspect ratio (L/D) of 12 was deliberately chosen to ease
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tube handling in the actual micro-tube hydroforming.
Figure 7 shows the maximum loads that were exerted on the punches when 1mm OD and 2mm OD tubular
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blanks were hydroformed at friction coefficient values of µ=0.05 and µ=0.1. The simulation shows that in order to hydroform the T-shaped part from 1mm OD tube, a maximum punch load of 750N is needed. The Y and bulge shape exhibited a slightly lower punch loads. When the tube size was increased to 2mm OD, the
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maximum punch increased to 2,500N for µ=0.1. Again, Y and bulge parts exhibited lower punch loads. These
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loads were later used for punch stress analysis as discussed in section 3.2.
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(a) Loading path for Y-shape
(d ) Thickness distribution
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(c )Loading path for T-Shape
(f) Thickness distribution
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(e) Loading path for bulge shape
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Figure 6. Loading paths and wall thinning distribution for Y, T and bulge shapes for 1mm tube , µ=0.1, and initial wall thickness t=0.1mm.
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Figure 7. Influence of friction and tube size on punch load 3.2 Punch stress analysis
Micro-THF simulations for Y, T and Bulge shapes discussed in section 2 have shown that a maximum force
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of 750N is required to axially feed the material to the die cavity for a 1mm OD, 12mm long SS304 tube. To hydroform similar geometries using a 2mm OD tube sample, a maximum force of 2,500N is required. Thus a
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punch load of 750N was used for stress analysis of all 1.0mm OD punches and a load of 2,500N was used for
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all 2.0mm OD punch variants. The punch stress analyses were carried out using ANSYS structural FE software. For all simulations the punch length was set to 25mm. In the FE model, force was distributed over
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an area consistent with the tube thickening simulated in the previous section. I.e., a 1mm OD tube with initial wall thickness of 0.1mm at a punch feed of 2.4mm (see figure 6a), resulted in wall thickening of 0.16mm.
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Thus, uniform pressure (750N/area) was applied to an annulus area corresponding to this thickening. Figure 8
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shows the effective stress distribution for conventional punch with a hole in the middle for supplying high pressure fluid to the deforming tube. Figures 8a & b show simulations for 1mm OD and 2mm OD punches respectively. Both punches have a 0.35mm hole drilled at the middle. These punches exhibited maximum
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effective stress in the range of 1,490MPa. Most tool steel materials will be able to handle such loading. The challenge, however, is the difficulty in drilling 0.35mm-hole at a depth of 25mm. By increasing the hole to
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0.5mm, a 1mm OD punch exhibited maximum effective stress of over 2000MPa. This stress exceeds the yield stress of most tool steel materials available today. Furthermore, by increasing the size of the middle hole, the
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rigidity of the punch drops significantly. Figures 9a & b show effective stress distribution for fully notched 1mm and 2mm OD punches. The punch were modelled with a notch width of 0.35mm. The notch depth for 1mm OD punch and 2mm OD punch were 0.5mm and 1mm respectively. The maximum stress exhibited by both punches is in the order of 2,100MPa. By closely looking at the punch noses, it can be observed that an average stress level of 1,500MPa is exhibited. Only at the notch radii the stress is higher. This stress level suggests that these punches will operate without failure. It should be noted that the stress level could further be lowered by reducing the notch size.
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Notches of different sizes as small as 25μm wide could easily be produced using wire Electro Discharge Machining (wire-EDM). As discussed earlier longer notched punch can easily be produced using wire-EDM.
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With this variant, the challenge for fabrication is eliminated as compared to hollow punches.
(a) 2mm OD; 0.35mm ID
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(a) 1mm OD; 0.35mm ID
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Figure 8. Stress distribution on conventional punch tips, 1mm OD and 2mm OD
(a) 1mm OD; 0.35mm notch width
(b) 2mm OD; 0.35mm notch width
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Figure 9. Stress distribution on fully notched punch; 1mm OD and 2mm OD; notch width=0.35mm Figures 10a & b show punch stress evolution for partially notched punch nose variants for 1mm OD and 2mm OD punches respectively. A maximum effective stress in the order of 1,900MPa is observed for both punches. This stress level is in the border of most hardened tool steels. This stress level could slightly be reduced by altering the notch size. Note that unlike the punch with a notch all the way through the punch length, one of the advantages of the partially notched punch nose variant is that it exhibits the highest rigidity because the
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rod has only been machined at the end of the punch. Sufficient clearance between the punch and the die is,
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however, needed to ensure pressurized fluid is supplied to the notched zone.
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(a) 1mm OD, 0.35mm notch width, max depth 0.5mm (b)2mm OD, 0.35mm notch width, max depth 1mm
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Figure 10. Stress distribution on partially notched punch nose; 1mm OD and 2mm OD; notch width=0.35mm
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Figures 11a & b show effective stress maps for punches that are fully notched at the punch nose. The stress level for the 1mm OD punch exceeds the limit of most hardened tools steel materials. However, for the 2mm
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small notch will have to be adopted.
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punches a maximum stress of 1,700MPa was exhibited. To avoid yielding of the 1mm OD punches, a very
(a) 1mm OD; 0.35mm notch width, 0.5mm long
(b) 2mm OD; 0.35mm notch width,1.5mm long
Figure 11. Stress distribution on fully notched punch nose; 1mm OD and 2mm OD; notch width=0.35mm
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The FE simulation results for all punch variants are summarized in Table 1. These results show the potential to design robust micro punches that can withstand material feeding loads for micro-THF using the floating die assembly setup. Conventional hollowed punches could only be used for floating based micro-THF if it is possible to drill very small holes of the order of 0.35mm on a sufficiently longer punch. It should be noted that while very short hollow punches could be produced, practical implementation of very short punches on
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the floating die assembly is difficult, or may not be feasible. With the exception of fully notched punch nose, all other punch variants exhibited acceptable stress levels and can withstand higher flexural loading compared to hollow micro punches. Furthermore, notches of different sizes, as small as 25μm wide, could easily be produced using wire Electro Discharge Machining (EDM), or micro milling operation.
750
1,499
750
2,100
750
Conventional Punch Fully notched punch
Below yield stress of tool steels Difficult to drill 0.3mm hole x 25 mm depth
2,500
1,479
Below yield stress of tool steels Potential for use
2,500
2,398
Partially notched punch nose
2,500
1,905
Stress induced at the limit of tool steel Potential for use as the average stress at the punch nose is 1700MPa Below yield stress of tool steels Potential for use
Fully notched punch nose
2,500
1,786
Below yield stress of tool steels Potential for use
1,936
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Fully notched punch
750
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Conventional Punch
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2,650
Stress induced at the limit of tool steel Potential for use as the average stress at the punch nose is 1700MPa Just below yield stress of tool steels Potential for use Stress induced is above most tool steels Punch geometry not suitable
Partially notched punch nose Fully notched punch nose
2mm OD
Remarks
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Maximum Punch effective stress [MPa]
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1mm OD
Input punch load [N]
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Punch type
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Table 1: FEA result summary
3.3 Punch Buckling Analysis In addition to the punch stress analysis discussed in section 3.2, linear buckling analysis was carried out to establish effective lengths of micro punches that can withstand punch forces without buckling. ANSYS Workbench 14.5 was used for these simulations. In the simulations, a tool steel with Young’s modulus
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E=200 GPa, and poison ratio of ν=0.3 were assumed. A force of 1.0 N was applied in the model so that the load multiplier given in the ANSYS result will represent the critical buckling load. Of the punch variants discussed in section 3.2, buckling simulation results for axially full notched punches, OD=1mm and Leff = 10mm are presented. Note that among the three notched micro punch variants, fully notched punch would require the least buckling load, thus, the analysis is only carried out for this micro punch variant. Figure 12
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shows buckling critical loads that were obtained for different notch sizes. The buckling load for a 1mm OD punch without notch is also presented. For this punch geometry the critical buckling load can also be obtained analytically. This simulation is included to provide a base for maximum critical load that could be attained for a micro punch with an infinitesimally small notch.
The buckling simulation shows that critical buckling loads varies from 631N to 916N depending on the notch size. For a small notch of 100µm x 100µm, a maximum punch load of 916N can be used, whereas a micro
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punch with a 350µm x 500µm notch could withstand a maximum punch load of 631N before the punch
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buckles. It should be noted that these results are applicable for a punch with 10mm effective length. In other words if the micro-THF system allows for shortening the effective punch length, higher punch loads than
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those given in Figure 12 could be used. The influence of notch radius on buckling was also studied for notch
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radius of R=100µm, 150µm, and 175µm. Figure 13 shows that the change in buckling load is insignificant.
200µm x 200µm notch
300 µm x 300µm notch
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100µm x 100µm notch
Figure 12. Buckling loads for fully notched punches
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350 µm x 500µm notch
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Figure 13. Influence of notch radius on buckling load
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3.4 Effect of notched punch on hydroformed part
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To investigate the effect of notched punch on micro-THF, simulations were conducted for T-shape part. A punch with a notch width of 0.35mm was used as shown in Figure 14. The FE simulation conditions used in
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The simulations results show that there is some
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this analysis were similar to the FE conditions discussed in section 3.2
material extruded through the notch and result in
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the formation of ‘tab’ on the tube ends. This means
that the tubes will have to be trimmed after the
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forming process has taken place. The presence of the notches do not appear to prevent the formation
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of the bulge. Note that, the process of trimming tube ends after hydroforming is a common practice even in conventional THF. Notched punches for
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1mm OD and 2mm OD were fabricated and used in micro-meso tube hydroforming as discussed in the
Figure 14. Influence of notched punch on deformed part
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next section.
4. Experimental Verification, and Discussion 4.1 Fabrication of notched punches and experimental procedures Among the four micro punch variants discussed in section 3, namely; conventional punch, full notched punch, partially notched punch nose, and fully notched punch nose, the later three were fabricated. All the punched were fabricated using wire EDM and hardened to 60HRC. Figures 16a and 17a show fully notched punches
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placed on die blocks used for hydroforming 1mm OD and 2mm OD tubular specimen respectively. Figure 18a shows punches with fully notched punch nose placed on a die block for 2mm OD samples, whereas Figure 19a shows partially notched punch nose type punch for 2mm OD samples. The notched width for all punch types fabricated was 0.350mm. This width was dictated by the size of wire used in the EDM machine. Micro tube hydroforming experiments were carried out to test the effectiveness of these punches using the
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floating based micro THF test setup (Fig 3) discussed in section 2. Tubular specimens used in the experiment were made from SS304. The 1mm OD tube samples had a wall thickness of 0.1mm while the 2mm OD samples had wall thickness of 0.2mm. The lengths of the tube samples used in the experiments was 12mm. Teflon sheet was used as a lubricant for all experiments carried out. Fluid pressure was applied linearly to a maximum of 130MPa in 100s. Feeding of material was carried out through the feed actuators connected to the cam. The feed was actuated when the hydroforming pressure reached 35MPa and the feed was stopped when the pressure reached 80MPa.
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4.2 Experimental results and discussion
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Figure 16b shows micro hydroformed geometries from 1mm OD tube samples when fully notched punch was used. A fractured sample, was hydroformed without axial feeding. For the middle part, a feed of 0.25mm was
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used during hydroforming. The buckled part (bottom) underwent a feed of 1mm. Excessive feeding was used
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intentionally to check if the punch could withstand the punch load. Fully notched punches were also used to hydroform Y and T parts from 2mm OD samples as shown in Figures 17. Figure 17b shows the Y and T hydroformed parts which were subjected to a feed of 1 mm from both ends. These parts were not fully formed
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to their final geometries due to the limitation on the fluid pressure in the current system. As discussed in the
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hydroforming simulations, to acquire fully formed Y and T shapes, a fluid pressure of 180MPa is needed. Figure 17c shows bulged parts which were formed using the middle die cavities. No material feed was applied
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for the bulged parts.
(b) Figure 16. 1mm OD hydroformed parts; feeding via fully notched micro punches. (Tube length L=12 mm, wall thickness = 0.1mm, P=100MPa)
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(b) Feed 2mm, pressure 100MPa
(c) No feed, Pressure 80MPa
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Figure 17. 2mm OD hydroformed parts; feeding via fully notched micro punches. (Tube length L=12 mm, wall thickness = 0.2mm)
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Figure 18 shows Y, T, and box shaped parts that were hydroformed using a fully notched punch nose at a
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maximum pressure of 130MPa. The total material feed for the Y and T shape was 1.4mm. The box shaped part shown in Fig 18ci fractured when material feed of 0.5 was used. When the feed was increased to 1.0mm
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no fracture was observed (Fig 18cii). Figure 19 shows hydroformed results when a partially notched punch
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nose was use. In this case a total feed of 0.7 was used.
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(b) Feed 1.4mm, Pressure 130MPa
(c) i 0.5mm feed bust, ii 1.0mm feed Figure 18. 2mm OD hydroformed parts; feeding via fully notched micro punch nose. (Tube length L=12 mm, wall thickness = 0.2mm, P=130MPa)
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(b)
Feed 0.7mm, P=130MPa
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Figure 19. 2mm OD hydroformed parts; feeding via Partially notched micro punch noses. (Tube length L=12 mm, wall thickness = 0.2mm, P=130MPa)
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The above experimental results have demonstrated that all three variants of micro notched punches can be
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used in micro tube hydroforming set up. Although notched punches have shown to be suitable for micro-THF processes in terms of acceptable stress level, manufacturability, and higher flexural strength, they do have a
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drawback that pressure exerted at the ends of the tube is not uniform. This leads to non-uniform deformation at the tube ends. A very small indentation was observed on the ends of the hydrofomed samples due to none
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uniform pressure distribution caused by the punch notch. In practice, however, the ends of macro tube hydroformed parts are usually trimmed out. In the case of micro THF parts, grinding or polishing of the tube
5. Conclusions
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end may be required.
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This paper has proposed design variants for micro punches that can be used with the floating based microTHF die assembly. In addition to the finite element analysis to determine the suitability of punches, experimental verification for three punch types namely; fully notched punch, partially notched punch nose
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and full notched punch nose variants were successfully carried out. The drawn conclusions from this study are:
The finite element analysis conducted to investigate the strength of proposed micro punches has shown that fully notched punches and partially notched punch nose type induce von Mises stresses that are below the yield stress of tool steel.
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Notched punches have the potential to withstand material feed load required to form Y, T, and Bulge shapes from SS304 micro tubes. Since fabrication of notched punches takes away a very small amount of material from a 1mm dia. rod, the rigidity of the punch can be retained. In order to be able to use the proposed punches for micro tube hydroforming of SS 304 or materials with
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equivalent strength, the micro-THF set-up should be designed such that the effective punch length does not exceed 10mm as the buckling load for a punch with a small notch is in the order of 700N-800N.
Hydroforming experiments carried out using different notched punches for 1mm and 2mm OD SS304 tubes, have demonstrated the potential of using these types of punches for micro-tube hydroforming on a floating based micro-THF die assembly.
The non-uniform deformation occurring at the ends of the tubes caused by non-uniform pressure exerted
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by the notched punches has shown to have little effect on the feeding. As long as the tube ends are
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polished or ground, the non-uniform deformed ends will not affect the integrity of the hydroformed part.
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Acknowledgement
The author would like to acknowledge the National Science Foundation, through which this work was
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funded under Project No. CMMI # 0900148. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National
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Science Foundation.
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