Development and Modelling of Installation a Fastener in Sheet Metals | FASTENER EURASIA MAGAZINE
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Installation a Fastener in Sheet Metals Without Preliminary Process

 

 

by Süleyman Kahraman

Fecon Arge ve Mühendislik Hizmetleri Ltd. Şti. Bursa, Türkiye

www.fe-con.com.tr, suleyman.kahraman@fe-con.com.tr

 

ABSTRACT

Many industries develop innovative designs due to effects of increasing competition conditions, developing materials and manufacturing technologies, changing of regulations etc… Therefore, parts manufactured from plastic, composite, light alloys and steels are increasingly used together in a final product especially in automotive, aviation and white goods industries. As a result of these conditions, joining of these parts made of various materials become a problem.

In this study, a new method was investigated for installation of a bolt made of medium carbon steel to a sheet metal made of aluminium alloys. In order provide an industrial solution, certain design criterion was determined, and designed process was improved by means of finite element analysis. At the same time, data obtained from finite element analysis was supported by experimental studies.

Keywords: Self-clinching fasteners, weight reduction, leak-proof fasteners, cost-effective installation

 

INTRODUCTION

Fasteners or fastening techniques are existing in almost all products which we daily observe. Fastener or fastening techniques are used in a product for permanently or demountable joining of one or more part in the product. Function of the fasteners or fastening techniques are determined by product designer or developer depending on various criteria such a load bearing capability, production process methods, cost etc…

When the issue mentioned above is taken into the consideration for permanently installation of a bolt manufactured from steel to sheet metal made of aluminium alloys, generally two methods are commonly used: conventional welding method and self-clinching techniques.

Conventional welding methods are generally used for installation of fastener to sheet metals made of similar materials and it is commonly utilized in serial productions like automotive industry. But the cases for dissimilar material installation like bolt made of steel and sheet metal made of aluminium alloys, welding methods cannot provide a solution since difference between melting point of both materials are very high for suitable welding process [1-2]. Therefore, bolt material also should be chosen as aluminium in order to provide installation by welding method. But, unit price of the bolt will be extremely increased, and this will not be a cost-effective solution. Additionally, aluminium oxide existing on material surface and other welding parameters also increases complexity process [3]. Besides that, after welding operation heat affected zone occurs and some mechanical problems can be observed in this area [4].

Self-clinching method had been developed for installation of dissimilar materials. Therefore, the case discussed above can be solved by means of this method instead of welding methods. In self-clinching method, a fastener is installed into sheet metal, which is preliminary drilled for a hole or formed a certain shape, by force implementation. During force implementation either fastener or sheet metal is subjected to plastic deformation. As a result of the deformation, sheet metal and fastener are clinched together and shape bonding between this couple is formed so that an installation that resist to axial removal force and moment reveals [5]. No heat input or additional material is not required for this method. Therefore, it provides cost-effective solution in terms of first investment and operation cost when it is compared with welding methods [6]. On the other hand, in this method no harmful gases during process is not formed so self-clinching technique is an environmentally friendly method.

In this study, it is aimed to develop a new method based on self-clinching technique. Main differences between new method and current self-clinching method are : firstly, no preliminary hole or shape preparation will not be required for new method so that process efficiency will be increased. Secondly, certain wall thickness of the sheet metal will stay even after installation of fastener so that additional material usage such as gasket or foam for leak proof application will not be required any more so that cost-effective solution for especially leak-proof area is created. In this sense, developing and improved stage of this patented process is reviewed in the scope of this paper. During the developing stage computer aided engineering tools were used and also study was supported by means of experimental studies. As a result, it was tried to be created a method that can be utilized in current industries.

 

MATERIAL AND METHOD

The main design criterias of the developed method in the scope of this study are: bolt will be directly installed into sheet metal without any preliminary operation requirement. After installation, bolt and sheet metal couple will provide leak proof application. Sheet metal thickness is 2mm and material of sheet metal is AA1050A H14 aluminium alloys. Screw size is M6 and it is made of AISI1050 medium carbon steel and the material manufactured by cold drawing.

Conceptual design details belonging to developed self-clinching bolt are given in Figure-1. The bottom side of the bolt is spherically designed for decreasing of contact surface between bolt and sheet metal. A clearance in bolt geometry is created. Material during the deformation will flow in this clearance and shape bonding form between sheet metal and bolt is arised so that installed couple will resist to axial removal forces. Similarly, half-hole geometry is created in head side of bold and materials after deformation of sheet metal will flow from this area so that resisting of axial removal force will be provided. Other function of this head is that installation force will be transmitted from this geometry so that deformation of sheet metal will start and further step of deformation, sheet metal will be oriented by means of this geometry toward clearance created for axial removal force. After installation, screw side of the bolt will be a demountable point i.e.: a part will be settled to screw and it can be tighted by standard nut so that the relevant part can be demounted by dismantling of nut, as well.

Figure-1: Conceptual design details of relevant self-clinching bolt

Component of all process belonging to developed method are given in Figure-2. These are: bolt, sheet metal, punch, blank holder and anvil. Functions of the punch is transmitting of the force coming from press to bolt and it orients to material to head of the bolt. Function of the blank holder is blocking of spring back of sheet metal. All components are settled on the anvil.

The installation process starts by implementation of force to direction of -Z that is indicated in Figure-2. Force created by press is transmitted by means of punch to head side of bolt so that sheet metal is subjected to plastic deformation firstly by bolt itself. Deformed sheet metal accumulates to direction of inner diameter of blank holder. At the certain stage of the deformation and stroke of the press, punch participates to deformation together with bolt, as well. At this stage, material flow is oriented to head of the bolt by participation of punch. By this orientation, clearances for axial removal force and moment are squeezed by deformed sheet metal (Figure-3). The entire process is carried out directly to sheet metal when it is virgin state i.e.: no process is to be managed before the installation. Additionally, certain size of wall thickness of sheet metals stays for leak proof application.


Figure-2: Components of the developed process

Figure-3: Conceptual view of bolt and sheet metal after installation

After conceptual design and generation of 3D CAD data of components of the developed process, it was carried out validation of the process by means of finite element analysis (FEA). MSC Simufact-Forming software was used for this validation. As mentioned before, bolt material was chosen as AISI 1050 medium carbon steel. In order to define material data in FEA, tensile test for AISI 1050 steel was carried out. Dimensional details of test specimen are given in Figure-4. Test specimens were manufactured by means of universal CNC lathe machine and tensile test was carried out 250 kN capacity by universal electro-mechanic tensile test bench in room temperature. Strain rate was determined as 10-3 1/s. Values obtained from the tests were transferred in to FEA model.

Figure-4: Tensile test specimen of AISI 1050 medium carbon steel

The material data of AA1050A H14 chosen for sheet metal was obtained from MSC Simufact-Forming material library. Material data is defined for 20°C room temperature and 10-2 1/s strain rate according to GMT equation (Table-1). After conceptual design, FEA model was created by utilized from MSC Simufact-forming software. In Figure-5 components of the process are given as they were created in FEA model. Accordingly, process was modelled as 3D and 90° section cut. Screw side of the bolt was not taken into the consideration in FEA model. Sheet metal and bolt were meshed with hexahedral solid elements. Sheet metal was subjected to mesh refinement especially for area that is exposed to highly plastic deformation (Figure-5.a). Adaptive mesh algorithm was chosen for sheet metal in order to avoid any mesh distortion during deformation stage. Punch, anvil and blank holder were determined as rigid die. Friction between components were determined as 0.1 and friction coefficient tolerance chosen as 0,04. Friction effects between components are detected as per Bilinear Coulomb approach. All analysis was performed by means of computer having Intel Core i7 8750 Hz processor, 16 GB DDR4 Ram.


Table-1: AA1050A H14 Aluminium material equations and parameters (GMT)

Figure-5: Finite element analysis models of process: a. sheet metal, b: bolt, c: all components

 

Created FEA model was solved by means of MSC Marc software. Linear press was determined in FEA mode and similarly, in experimental study, too. Ram speed was chosen as 1 mm/s for both cases. Experimental studies are carried out 250kN electro-mechanic test bench that is also used for material tests. During the experimental studies, force and stroke data were recorded. Obtained data were compared with data obtained from FEA model. Besides that, installation states were compared by cutting the samples obtained from experimental studies. As a result, it is aimed to be reflected of FEA model to physical conditions. Besides that, material of other components (punch, anvil and blank holder) was chosen as DIN 1.2379 steel. These components were also manufactured by means of universal CNC lathe.

 

RESULTS AND DISCUSSION

Stress-strain curves belonging to material of bolt (AISI 1050) and sheet metal (AA1050A H14) are respectively given in Figure-6 and Figure-7. Mechanical properties of these materials are summarized in Table-2. Accordingly, when the mechanical properties of bolt material are investigated, it is understood that the yield strength is 595 MPa, tensile strength is 610 MPa and elongation is 15%. Obtained values from the tensile test shows that the relevant material is capable to provide sufficient mechanical performance relevant to functional properties of the bolt. Additionally, mechanical properties of AISI 1050 shows that it allows to manufacturing technique such as machining and bulk metal forming. Besides that, obtained mechanical properties almost comply to mechanical properties of grade 8.8 material indicated in specification ISO 898. Material of sheet metal (AA1050A H14) was obtained from data existing in Simufact-Forming material library. This material was transferred to FEA model for 20°C temperature (same temperature that experimental studies is carried out) and 10-2 1/s strain rate according to GMT equation as indicated in Figure-7.

Figure-6: Stress strain curve belonging AISI 1050 medium carbon steel

Figure-7: Stress strain curve of AA1050 H14 according to GMT equation for 20°C temperature.

Components of the developed process indicated in Figure-2 were manufactured by machining methods. Manufactured components were measured and CAD data prepared for FEA model were revised according to dimensional measurements so that any deviation possibility between experimental study and FEA model stemming from this deviation was avoided. Revised CAD data is transferred to FEA model indicated in Figure-5. Experimental setup was prepared as indicated in Figure-5. Experimental studies were carried out with similar conditions determined for FEA model. Obtained force and stroke data were recorded and resemblance of FEA model to experimental study was managed by optimizing of mesh size, friction coefficient between components etc…

Table-2: Mechanical properties of AISI1050 and AA1050A materials

Graphics are given in Figure-8 obtained from installation operation belonging FEA model and belonging experimental studies. When the relevant graphics are investigated, it is observed that the developed process shows main 3 different slopes. These can be separated for strokes between 0,0-0,5 mm, 0,5-1,0 mm and bigger than 1,0 mm. Especially graph belonging to experimental study shows some discontinuity between 0-0,5 mm. This issue can be stemed from clearance between compenents of the process or elastic deformation of sheet metal. Load increasing ratio reduces for second area of the graph i.e.:  stroke between 0,5-1,0 mm. Slope of the relevant graph for strokes over 1,0 mm increases. Additionally, slope differences between FEA model and experimental study also increases.

Figure-8: Force and stroke graph obtained from experimental study and FEA model

Samples were sectioned after installation of bolt to sheet metal and views obtained from both studies were compared under digital microscope. The comparison is given in Figure-9. Conditions of sheet metal and bolt after installation were controlled. Accordingly, material flow direction and last condition of the deformed sheet metal inside the clearance for axial removal force seems to be very similar for both experimental and FEA model. Additionally, deformation on head of bolt during installation is observed for both studies and area inside the relevant clearance corresponding last filling point of sheet metal affects from the load for also both cases.

Figure-9: Section cut of samples belonging to experimental study and FEA model after installation.

The developed process was validated by both FEA model and experimental studies. Aim of design criterias, which are leak proof application and without preliminary operation requirement, were achieved after all these studies. Additionally, repetitive installation operation was carried out in one sheet metal as given Figure-10. In these trials, spring back effects were not observed on sheet metal. These trials show that the developed process is suitable for serial production lines existing in automotive or white good applications by dedicated installation tools.

Figure-10: Repetitive installation trials

DISCUSSION AND RECOMMENDATIONS

In this study, a fastener that can be installed in a sheet metal by self-clinching technique was developed. The main advantages of the developed fastener and installation process are that it is not required any preliminary operation such as drilling hole or pre-form and after installation of fastener it provides leak-proof implementation. The relevant process was developed by utilizing from computer aided engineering tools as well as experimental studies.

When obtained force and stroke graphics are investigated that is given in Figure-8, both graphics obtained from both FEA model and experiments show main three characteristic. Force increasing ratio in last zone of the relevant graphs (stroke over 1,0 mm), it is observed that this ratio increases higher than previous zones. It is understood from this characteristic that the last stroke and force values of installation operation is very important for proper process. Because, no extra volume for deformed sheet metal is existing or it is very restricted for material flow. Therefore, hydrostatic stress can drastically increase and share stress diminish. In case of continuing to stroke or force implementation, it is possible to lead to death metal zone inside deformed materials that may negatively affect of load bearing capability of sheet metal and bolt couples after installation. 

Slope of the graphs especially in last zone differs from data obtained from FEA model and experimental studies. It is thought that strain hardening factor by increasing deformation in especially sheet metal material may not be sufficiently reflected for FEA models.  Therefore, it can be investigated by carrying out tensile test for sheet metals and FEA models can be re-driven with this new material data.

 

REFERANCES

[1] Jiang T., Liu Z., Wang P. “Effect of aluminium pre-straining on strength of clinched galvanized SAE1004 steel-to-AA6111-T4 aluminium” (2014- Journal of Materials Processing Technology)
[2] Meschut G., Janzen V. and Offerman T. “Innovative and highly productive joining Technologies for multi-material lightweight car body structure” Journal of Materials Engineering and Performance, Volume 23(5) May 2014-1515.
[3] Liu L., Ren D. and Liu F.é"A Review of Dissimilar Welding Techniques for Magnesium Alloys to Aluminium Alloys” materials, Materials 2014, 7, 3735-3757; doi:10.3390/ma7053735
[4] Israel M., Mauermann R., Schellnock J. “Thick Sheet Clinching Joining up to 20 mm Total Thickness” (2013-Advanced Shipping and Ocean Engineering)
[5] Kahraman S., Aygen M., Saray O., “Investigation of effect of dimensional properties on the load bearing capability of self-clinch fasteners” 9th International Automotive Technologies Congress, OTEKON, pp. 918-926, 2018
[6] Balon P., Świątoniowski, A. “Forming of automotive parts with nuts clinch process in comparison to welding of nuts” Key Engineering Materials Vols 611-612 (2014) pp 1503-1510