By Piet Marchal and Roel Boesenkool.

31/8/99

Introduction

To initiate and facilitate strong links between technical establishments and in and between supply chains the European Commission is supporting thematic networks, one of which is the thematic network on design optimisation and guidelines for multimaterial applications (DOGMA). The activities in the DOGMA network are organized around themes that are addressed by partners forming a cluster. One of these clusters is the materials development cluster. The DOGMA materials development cluster was assigned the task of making an inventory of the process of materials development and to set up a formal procedure addressing the points that must receive attention during this process. The procedure will be tuned making use of case studies. The DOGMA case study that was done on materials development at Hoogovens is presented in this paper.

Design environment

The functions of products are influenced and sometimes directly defined by customers and producers, and also by local, national and international authorities. On the basis of information communicated by these parties designers generate product options which are expected to fulfil the needs perceived. In between boundaries set by the availability of resources these options specify form and function of the product and determine aspects as production methods, costs and recyclability. A schematic representation of the relations between parties and aspects influencing the design of products is given in figure 1.

One of the resources that has to be taken into account in the design process and of which a development example will be addressed further in this paper, is the material or the combination of materials that is going to be used for the product.

On the one hand the material often sets limits to the functionality that can be realized and on the other hand in the past new materials have made it possible to develop new activities and new technologies. The latter may be illustrated by the example of aviation as we know it today for which the application of aluminium, which came available towards the end of the nineteenth century, is a "conditio sine qua non". But it may also be illustrated by other examples such as the computer and semi conducting materials, concrete and building, glassfiber and telecommunication, celluloid and cinema, plastics and sealings and last but not least paper and writing.

Aside from the breakthroughs, materials development is an evolutionary proces through stepwise improvements in existing materials, often driven

by cost reduction and improved functional performance. This is schematized in figure 2.

Looking at how these developments are brought about, breakthroughs are expected to result from scientific research which need not be specifically aimed at defined objects but which may be primarily driven by curiosity and ambition and which is departing from existing knowledge. Improvements on existing materials often may originate from research and development done by materials suppliers, the driving force for this being the wish to reduce costs and to improve performance in order to maintain and extend market share. New applications of existing materials may be the domain of the creative mind, be it an inventor, a designer or an engineer.

In all these different development modes communication and cooperation play a vital role. Strong links in the supply chains and between supply chains and the scientific world are needed to make materials development possible. Examples thereof are given in this paper that presents the DOGMA network case study on the development of a multimaterial at Hoogovens, the aluminium polypropylene sandwich sheet Hylite.

Eureka project Carmat

In the automotive industry low cost and customer appeal are very important aspects of competetiveness. Also major efforts are made to reduce the contribution of cars to the environmental load and to improve on active and passive safety. In the course of 1985 PSA, the consortium of Peugeot and Citroën, adressed its materials suppliers in this context with the challenge to realize a significant weight reduction of side panels, roof and bonnet, other properties such as strength, stiffness, impact resistance and appearance being up to standard. Different suppliers, among them DSM, Bayer, Basf, Sollac and Hoogovens, took up this challenge and the Eureka project Carmat was started in 1987. The Citroën AX was chosen as the car in which the Carmat developments were to be implemented.

Automotive hang on parts today are usually made out of steel. Therefore different potential materials solutions for weight reduction can be ranked according to their position in a matrix giving weight and costs of the parts, taking the steel parts as a reference (for the sandwich data an estimation had to be made). This matrix is given in figure 3.

The matrix gives a first orientation on the possibilities to realize the goal of weight reduction. The alternative materials that arise from the matrix for this application are steel, aluminium, plastics and metal-polymer sandwiches. The materials in this matrix must fulfil minimum requirements for panel stiffness, dent resistance, energy absorption, fire resistance, cuttability, formability, joinability, available dimensions, paintability and customer appeal. As a consequence of this last requirement plywood, that could fulfil all other requirements including fire resistance, is not included in the matrix because of insufficient customer appeal.

Of the alternatives given in the matrix Hoogovens decided to pursue two lightweight alternatives: aluminium and an aluminium-polymer sandwich material. The development of the aluminium-polymer alternative, which is named Hylite, is the subject of the following description.

Development of Hylite

Aluminium polymer sandwich materials were known at the time the development in the Carmat project started. These sandwiches were mainly applied in building and their characteristics had to be extended to lower thicknesses and better formability to make them suitable for automotive applications.

The first orientation into the knowledge and skills needed to perform this task was done through literature study and contacts with knowledge institutes and suppliers of polymers. One of the early contacts was the Norwegian Center for Industrial Research (SI) which was particularly knowledgeable in the area of adhesion of polymers to metals [1]. Also in Norway presses for the production of test panels of the dimensions needed for automotive applications were available at the firm Raufoss. The materials for the sandwich panel were selected bearing in mind the Carmat functional needs and requirements stemming from the production technique that was applied, for instance applicable dimensions, allowable loads and the processing temperatures. As the skin material the aluminium grade AA5182 was chosen because it combines good corrosion resistance with high strength and good formability. For the core polymer Acrylonitril Butadieen Styrene was chosen because of its formability, low price, and high E-modulus in the service temperature range from -35 ºC upto +85 ºC which was defined by PSA [2]. To facilitate the adhesion of the ABS core to the aluminium skins a chromium phosphate conversion layer was applied to the aluminium. After preliminary tests at facilities of Hoogovens Research and Development (HR&D) for the orientation into production aspects as pretreatment and time-temperature profiles the first panels of 165x185 mm² were produced in March 1988 at Raufoss and in October panels of 700x1000 mm² were realized. On the basis of the results of these experiments at Raufoss it was decided to invest in lamination tooling for the 4000 kN experimental press at HR&D. Using standard methods extensive testing of the 1200x1500 mm² panels produced on this press followed to assess their compliance to the demands. Aspects which were tested included dynamic dent resistance, free form cold bending, warm bending, bending stiffness, peeling, pressing, deepdrawing and resistance to humid environments. Simultaneously with the standard laboratory tests, bonnets for the Citroën AX were pressed and tested. The outcome of this extensive test period was proof that it was possible to produce a sandwich material that meets the goal of reduced weight with other properties being up to standard.

In the beginning the motivation of Hoogovens to participate in the Carmat project may have been to maintain and to improve the customer-supplier relationship with PSA and to learn about the developments in materials applications in the automotive industry. To go beyond the activities done so far a perspective of commercial exploitation of this materials development was needed, because of the cost involved in the subsequent development route. Considering the technological possibilities for production and application of the sandwich material and the expected market opportunities it was decided to go on with the development, the next step being the development of the production method from the batch method of pressing that had been used sofar to the continuous method of rolling. This was because it was perceived that mass production was a prerequisite for a competitive price level of the new material and because Hoogovens has its core competence in this type of continuous materials production.

This development started in 1990. Again the first tests for the new production method were done with experimental equipment available at Hoogovens Research and Development, in this case being a rolling stand. A key question was if sufficient adhesion of the core and the skin material could be realized in the much shorter process time of the continuous processing. This proved to be possible and based on these findings it was decided to follow two parallel paths. The first path was to build a small scale experimental production line to acquire hands-on experience with the production proces together with experimental knowledge. The second path was to consider the possibilities for commercial production.

In the course of 1991 it became clear that one of the functional requirements for the new sandwich material originating from the Carmat project became more severe. The temperature up to which form stability after pressing had to be garanteed was raised from 90 ºC to 145 ºC. This was because the paint hardening temperature of

90 ºC in car production, which was originally aimed at, could not be realized in the given circumstances. For the sandwich material this meant that the ABS core had to be replaced with a polymer with form stability up to the higher temperature of 145 ºC. The possibly applicable materials Poly-Ethelene-Teraftalate (PET) and Poly Propylene (PP) were tested extensively and eventually PP was chosen as the new core material.

Another technical problem that had to be overcome was the forming of so called Lüders lines during pressing. The lines, that become visible in side-light, are not acceptable for automotive applications. A solution for the problem was found in an optimized rolling and annealing technology. The experimental production line proved to be a very valuable tool in overcoming the technical problems.

The possibilities for commercial production were perceived to be such that a new business was established, Hoogovens Hylite BV. This affiliate of Koninklijke Hoogovens developed the production methods to a point where the lightweight sandwich sheet Hylite could be added to the market in 1994. In August 1997 Hylite sandwich sheet succesfully completed the final phases of pre-validation for use in the mass production of automotive body panels. The pre-validation project was a joint undertaking of Hoogovens Hylite, Volkswagen and Grau WerkzeugSysteme (Germany) in which Hoogovens Hylite was responsible for project management, material and application know-how, Volkswagen was responsible for bonnet design and the assesment of material processing and also provided car-manufacturing know-how, and Grau WerkzeugSysteme looked after engineering and tooling. The successfull completion of this project demonstrated that Hylite had become a mature material, suitable and ready for the mass production of automotive body outer panels.

Evaluation

The driving force for the development of Hylite has been a market pull. Several market parties, first of all PSA, have indicated their interest in a material for automotive outer parts lighter than steel, other properties being the same. This description, "lighter than steel with other properties being the same", can be seen as the essence of the functional specification for which the materials designers had to find a technical solution. From this global functional specification a detailed technical specification can be derived because steel is well known in the indicated automotive applications. A sandwich material with a metal skin and a lightweight polymer core was chosen in a very early stage as the solution to address the functional specification. With the choice for a metal polymer sandwich and the detailed technical specification the way to proceed was established. Different aluminium alloys and different core materials were evaluated with respect to producebility of the sandwich and its compliance to functional and technical specification. This evaluation was done in different phases in which costs were carefully controlled. At the end of each phase a decision on the continuation of the development towards a commercial product was made based on technical achievements, costs and market expectations [3]. In contact with the market parties the sandwich material evolved to its present form were the material competes with steel and aluminium in price, weight and performance.

References and acknowledgement

This case study is based on internal Hoogovens files and documents on research in the field of sandwich material for automotive applications and the development of Hylite.

[1]. Rapportage van de eerste uitgebreide proefneming voor het samenstellen van een kunststof-aluminium laminaat.

Boesenkool, 18 mei 1988. Arch. lab.-nr: 62839.

[2]. Haalbaarheidsonderzoek continu lamineren.

Boesenkool en Bottema. 26 maart 1990. Arch. lab.-nr 68025.

[3]. Hylite production, quality, costs and future.

Boesenkool, Buters, Eger, Ritter, Roukema. January 29^th 1996.

[4]. Other internal documents.
 
 

^{1) Koninklijke Hoogovens.}

Koninklijke Hoogovens is an international producer of steel and aluminium that has branches in Europe and North America and that produces annually more than six million tonnes of steel and approximately four hundred thousand tonnes of aluminium. The total sales are more than 10 billion Dutch guilders per year. Koninklijke Hoogovens is involved in research and development in close contact with knowledge institutes and buyers for a long time now. This has resulted in a broad spectrum of high quality products. Examples of improvements on existing materials are the new steel grades that are developed over the last decades which have lower inclusion levels due vacuum treatment of the liquid steel and new steel grades that have better formability, higher strength and better weldability due to changes in chemistry and process control during hot and cold rolling. Other important product developments in steel were the galvanised, galvannealed and prepainted steels and packaging steel with improved deepdrawability and thinner gauges. Aluminium research lead to aluminium brazing sheet and to aluminum alloys with improved strength, corrosion resistance and weldability.

At this moment, summer 1999, Koninklijke Hoogovens and British Steel are in the process of realizing their envisaged merger.
 
 
 
 
 
 
 

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