In a growing number of industrial applications, lasers have now been considered as an alternative solution for directly welding plastic / composite materials to metals. This non-contact machining approach provides the highest process flexibility Sex.
Currently, one of the drivers of the automotive industry is how to make a lighter-weight car without adding cost or damaging or affecting performance, quality and safety, etc. For the seat structure, this mainly includes the use of Thinner and stronger steels, hybrid material structures have also been studied extensively in recent years, and this also applies to metal-only hybrid structures and component structures built of metal / plastic composites.
The use of these hybrid material structures poses many different challenges, most notably how to bond the constituent materials with different chemical, mechanical and thermal properties.The most current technologies currently used for the welding of plastics and metals are sticky Adhesive bonding, mechanical welding, overmolding, or a combination of these processes involve a large number of assembly operations and create design limitations.
Lasers have now been considered as an alternative solution for directly welding plastic / composite materials to metals in an increasing number of industrial applications, without the need for additional liquid / solid binders or Assembled components and lasers offer high process flexibility compared to mechanical joints and complex and expensive molds To evaluate the possibilities of this new laser technology, Faurecia Automotive Seating in Germany and the European Community co-financed a Project called PMjoin.
Laser process steps
This direct, contactless laser process consists of two steps: First, a grooved microstructure is formed on the metal surface by scanning the laser in this area and ablating the material locally.The grooves can be as small as a few microns wide, While the depth can be varied by scanning the laser multiple times on the same area.Figure 1 shows two different groove geometries: the top is a pattern created using a continuous wave (CW) single mode fiber laser characterized by a Small, recalculated irregular cross-sectional grooves, while the bottom is a style built using a nanosecond pulsed laser featuring a regular groove shape with a large recast structure on the top.
Figure 1: Detail of a structured surface (top) made with a continuous wave laser (center row position) and a nanosecond pulse laser (bottom row).
In the second step of the method, the plastic is placed over the structured metal and heated to the melting temperature.For a plastic that is transparent to the laser wavelength, the laser beam can be applied from the plastic side end - the laser energy is transmitted through the plastic to the bonding interface The energy at this interface is absorbed by the metal, the metal gradually warms up, and the low thermal conductivity of the plastic ensures local hot spots and melts the plastic.
For plastics that are opaque to the laser wavelength, including most automotive structural plastics / composites, the metal must be heated from the metal side by conductive heating of the metal.When sufficient heat is locally generated at the interface, the plastic melts In both cases, good temperature control is crucial to avoid plastic overheating (causing pores) or burning.
Although conductive heating solutions are not energy efficient, they are as effective as a transmissive heating solution that creates a stable joint in which continuous wave direct diode lasers are used to conduct conductive heating from the metal side. In conductive heating and transmission In the heating process, pressure must be applied to ensure that heat is effectively conducted into the plastic.As soon as the plastic reaches the melting temperature, it flows under pressure into the microstructure and, upon cooling, fixes itself to the metal structure, thereby forming a mechanical interlock .
Car seat design concept
As part of the PMjoin project, Faurecia devised a conceptual seat back structure based on its versatile steel seat structure (Figure 2), where two high strength steel side members of the seat back are made of PAGF30 composite Instead, the upper and lower cross members and reclining components of the backrest, as well as the seat cushion and adjustment rail structure, remain unchanged.
In the first part, the effect of microstructure parameters on the mechanical properties of the joint was first studied.Therefore, a set of experimental designs were arranged to produce simple specimens with various groove patterns and then tensile shear , Tensile force and peel test.The parameters studied include the number of repetitions (laser) operation, the structural density (spacing between grooves), the angle of the groove with respect to the material surface, the orientation of the groove structure with respect to the load direction, the type of laser, And laser power For example, a shear strength value of 17 N / mm2 can be achieved using a simple groove geometry that is more than twice the value achieved by a roughened (sandblasted) surface, and 4 times the surface value of untreated metal.
As mentioned earlier, it has been found that different groove geometries can be achieved with different (structured) lasers.The irregular shape of the groove and the size (and shape) of the re-cast layer protruding from the surface of the material help Plastic anchors in the connector.
In the second step of the study, the results of the small-scale tests were transferred to the conceptual seat back structure.The mechanical load at each weld point, the weld between the upper and lower beam members and the composite side members, and the steel recline assembly And the composite side members are determined by finite element (FE) analysis of a representative steel structure Based on the results of the small-scale tests, the FE-calculated load is determined at each joint Pass the required joint area.
Due to the use of the composite side members of the previous project, a slight redesign was required at the weld points to ensure adequate weld area with the new laser-based technology. In addition, a new steel bracket was designed and manufactured To weld the existing reclining assembly to the composite side members together with a suitable positioning and clamping fixture.
Test Results
A small number of conceptual seat back structures were fabricated using the parameters identified in the pilot tests and were subjected to quasi-static front and back impact tests as well as dynamic front impact tests. Both types of tests helped to understand the How failure manifests itself, and dynamic shock tests show what actually happens in real life. Although the latter gives only one qualified or unqualified report, the quasi-static impact test returns a quantitative result - that is, The structure in what torque or power state will fail.
While still positive as the first test result, the observed failure modes also indicate that the concept of design is still too rigid.In addition, the steel and composite parts have been redesigned to maximize their impact on the seatback structure The potential for performance to realize more advanced designs remains to be seen, however, and the hybrid structure passes the dynamic impact test anyway.
Figure 2: General car seat structure.
This conceptual study clearly shows that the use of lasers to weld plastics directly to metals is the ideal solution to replace traditional approaches such as adhesive bonding, mechanical welding or overmolding, and as a result semi-structured hybrid components are now highly efficient Take advantage of this laser welding technology.
However, there is still some way to go before it is used in mass production of hybrid modules, which requires at least one other design iteration to maximize the potential of a mechanical strength process for both materials and structures, Humidity and temperature, etc. In addition, other alternative technologies for conducting heat may also be considered.