Sheet metal forming and joining

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One man is at a computer the other man is working with machine.

Sheet metal forming and joining

Sheet metal forming and joining

The research on sheet metal forming at Sheet Metal Centre of HAMK concentrates on formability of materials in different forming processes. Our automated strain analysis systems (GOM/Aramis, GOM Argus and ASAME) as well as our friction testing equipment offer unique technology for the Finnish steel industry requirements. With those systems it is easy to study strains during forming processes, as well as strains in formed sheet steel products and evaluate the success of the forming process. Those systems are also used for determining forming limit curves (FLC).

We use bending under tension test (BUT) to study friction in sheet steel forming; this test is well suited for various types of sheet steel and coatings. Furthermore, it is possible to vary the parameters of the forming process, such as the level of lubrication, the shape of the tool and the forming speed. With computer-assisted forming simulations (FEM) it is possible to numerically analyse the manufacturability of sheet metal products. Simulation will ensure that the forming process, the technical design and the forming tools will produce a faultless end product.

Numerical analysis

Manufacturing products out of sheet metal requires forming. If the product is complex its manufacturability is difficult to analyse, and empirical design principles are not always sufficient. On one hand, sheet metal product manufacturing is very susceptible to changes in tool geometry, in sheet material properties, and in other parameters related to the forming process. On the other hand, tools used for serial production of sheet metal products are costly, and developing a functional working method can require a number of trials and experimentations. To replace at least some of this development work, a faster and less expensive computer-assisted analysis can be used already in the design phase to virtually test the functionality and manufacturability of sheet metal products.
With simulation, following information can be obtained about the product itself and the forming process:

  • strains in the part during and after forming
  • surface pressure between tool and material
  • final form of the part, considering elastic recovery
  • forming forces required
  • material thickness in different sections of the part
  • potential wrinkling and rupture
  • estimation of the strength and stiffness of end product

Formability of sheet metal

Formability of sheet steel is illustrated with so called forming limit curves (FLC). These diagrams are utilized when analysing the manufacturability of sheet metal products, and as a comparison tool for numerical calculations. At Sheet Metal Centre, the forming limit curve is defined according to the Nakazima method, using formed cup-like test samples. Based on the displacements on the stochactic pattern or a regular grid on the surface of the samples, it is possible to calculate strain values of sheet steel by using strain measurement systems based on cameras. Our GOM/Aramis 5M -equipment can be utilized for continuous measurement of the forming process and GOM/Argus and ASAME –systems for measurement of formed pieces.

In addition to forming limit determination, the strain measurement systems at SMC can also be used for researching sheet steel products. The results from the measurements can be utilized in verifying calculation results. Strain measurement systems are also used to with other formability tests and tensile tests.

Stenhøj two-cylinder FPS 200/100M deep-drawing press:

  • Upper cylinder compressive force, max 2000 kN
  • Retaining force, max 1000 kN
  • Press bed size 650 mm x 500 mm
  • Speed control 1,5 mm/s…10 mm/s
  • Outer dimensions: width 1915 mm, depth 1040 mm, height 3000 mm

Tools

  • tools based on Nakajima-method for sheet thicknesses 0,2 mm…5 mm
  • deep drawing tool: cylinder Ø75mm, rectangle 50 x 100 mm

Friction testing

Friction between sheet bar and tools can have a significant effect on the success of the manufacturing process of formed sheet steel products. Thus it is important to have knowledge on frictional properties when sheet steel materials and forming processes are developed. Typically, friction is described by only parameter, Coulomb’s friction coefficient. This coefficient is required e.g. as basic data for forming simulations, as well as for comparison between sheet and coating. The Sheet Metal Centre at HAMK is one of the few research units in Finland who can provide friction testing services.

Friction is affected by the materials of the tools and the formed sheet steel, their coatings, lubrication, speed of forming as well as the tool geometry. Studying the friction phenomena in practical processes is difficult and expensive. Several laboratory tests have been developed for studying friction, and one of them is the BUT-test (Bending Under Tension). In BUT, a sheet steel strip under tension is being pulled around a cylindrical jig. This testing arrangement represents the behaviour of sheet steel in deep drawing at the point of the rounding of the drawing die. The test will measure pulling forces applied on different sides of the tool and the drawing speed of the strip. The results are used to calculate the friction coefficient between the sheet and the tool.

Sheet metal joining

Common joining techniques for sheet metal products are resistance welding, screws, riveting, clamping, glueing and combinations of these methods. SMC is following up on the development of these techniques and transferring the results of its R&D work into practice, to be utilized in both industry and education. Typically, research services in this area are determining weldability of materials in resistance welding, evaluation of the life of spot welding electrodes, and defining joint strengths (tension and shear resistance).

SMC is equipped with following machinery: TOX -clamping system, self-piercing riveter Henrob LP, spot and projection welding machine KRW 81 P (electronic steering RCU 101), seam welding machine LSK-75 A (electronic steering RCU 101), and medium frequency resistance welding equipment (MFRW).

Laser welding

In addition to being useful in cutting sheet metal, laser is also well suited for joining of sheet metal with several benefits. Laser welding enables a well-aligned, small and fast heat input, therefore decreasing the amount of deformations in the joined parts. The weld seam can be made from the other side of the joint, thus the coating of a coil coated sheet metal normally suffers only minor damages in a small area. It is possible even to avoid damaging the coating on the opposite side of the dovetail lap joint completely.

At the Riihimäki unit of HAMK there are strong traditions on research related to laser machining with modern equipment. Riihimäki has made several research projects funded by companies and TEKES. The main emphasis is on laser welding. Other research areas are laser cutting, laser surface hardening  and laser-assisted machining.

Research areas related to laser welding:

  • Diode laser welding
  • Fibre laser welding
  • Laser-laser-hybrid welding (diode + fibre)
  • Laser welding with filler wire

Equipment:

  • Fibre laser 1 kW (IPG)
  • Diode laser 2 kW (Rofin-Sinar)
  • Laser-laser-hybrid (IPG+Rofin-Sinar)
  • Diode laser 1 kW (Laserline)
  • Wire feeding device
  • Articulated arm robot (Fanuc)

Research projects:
Possibilities and practical applications of diode laser welding  (TEKES 2000-2002)
On-line Process Control of Thin Sheet Hybrid Laser Welding (CRAFT 2002-2003)
Applications of joint fibre and diode laser welding for machining metal (TEKES 2005-2007)
Utilizing laser beam in machine tools (TEKES 2007-2009)

Services:

  • R&D work related to laser welding
  • R&D work related to other methods of laser machining
  • Coordinating research projects

More information:
Tapio Väisänen
tapio.vaisanen@hamk.fi

HÄME UNIVERSITY OF APPLIED SCIENCES