AMS looks at production techniques and material innovation in the press shop
As director of the Centre for Precision Forming (CPF) at Ohio State University in the US, Professor Taylan Altan is in a prime position from which to comment on developments in metal forming technology and applications. Set up in the 1980s, the centre is currently backed by the likes of Honda, Hyundai, GM and Chrysler in its execution of joint academic-industrial research, with pressing and the requirements of the automotive industry at its forefront.
When it comes to the motives for the take-up of advancements in this area, Professor Altan confirms that the continuing drive towards ‘lightweighting’ of vehicles is provoking increasing interest in newer and more appropriate material formulations – in particular advanced high-strength steels (AHSS) and light alloys based on aluminium and magnesium.
But the professor also notes that exploiting these new materials can be problematical due to the auto industry’s reliance on and familiarity with traditional materials.
As newer materials have increasingly made their presence felt, companies have had to acknowledge that their depth of knowledge of some of their basic properties – for instance their potential for ‘springback’ after forming – is comparatively limited.
As far as actual production technologies are concerned Professor Altan identifies one specific development as the most significant recent in the area. This is the advent of servo-press technology – in which the actuation is effected directly by an electric motor – to replace older ‘mechanical’ or hydraulic machines.
The great advantage the newer technique brings is control and variability of the speed of operation. “You can do the actual pressing at very slow speed if necessary and then accelerate the machine’s movements very rapidly so that overall cycle time is still shorter than with hydraulic presses,” states Professor Altan. Typically, he says, a servo press might manage 15 cycles in the time in which its hydraulic counterpart might manage only ten. That ability to carry out the actual pressing at low speed can also mean that part quality can be better even though precisely why is, as he admits, not entirely explicable.
Professor Altan dates the advent of servo-press technology back about 15 years and though the first machines exerted relatively low forces, machines exerting forces in the range of 2,500-3,000 tonnes have arrived in the last five years.
The other major innovation in production processes the professor identifies is the hot forming of steel materials though a combination of very high temperatures followed by rapid cooling (see pages 46-48). He regards Volkswagen as the leading in-house OEM implementer of this technique with Fiat close behind, but at the moment it is still a method that many car makers prefer to leave to specialist suppliers.
Other techniques that the CPF is monitoring – which therefore can be taken to indicate genuine interest among its industrial funders – include hydroforming of parts using a combination of hard mould tools and pressurised fluids, as well as warm-forming of aluminium at temperatures of around 300°C. Of the two, comments Professor Altan, the former is now an established technology, while the latter is still at the experimental stage.
Professor Altan reveals that the CPF has some research underway that aims to establish just how far development of warm-forming has advanced around the world. As with any metal forming process, he says, the development of hot and warm counterparts to the cold techniques that still predominate involve a series of trade-offs which potential users have to consider. Essentially, heating the metal takes time and energy and therefore increases cost, but will enhance its formability and hopefully product quality.
Three other current research projects at the CPF give further insight into the potential problems and opportunities that industrial companies regard as crucial for the near future.
The first is investigating a means to establish the formability of metals as soon as they are delivered to the factory. The second is looking at the possibility of creating a software tool that can accurately predict the likelihood of fracturing of parts during the manufacturing process. The third, rather more specific, investigation is into the feasibility of using servo-press technology to form aluminium alloys in addition to steels.
One professional out in the commercial world who confirms Professor Altan’s observations about the increasing importance of servo-press technology in automotive metal forming is Frank Viola, sales director, automotive press technology with German-based supplier of press equipment Schuler. The company only introduced its first servopresses as recently as 2007, but he says that servo and other mechanical presses already constitute more than 90% of the company’s sales to the automotive sector.
The difference between the basic technologies in the different types of machine – the transmission of force by hydraulic pressure in the one category and by the direct action of electric motors in the other – does have implications for the type of application for which they are most suited. Viola explains that generally, the physical construction of the equipment means hydraulic machines are more flexible as they lend themselves to working with different die sets more easily than mechanical counterparts.
For that reason they have tended to be more common among the supplier than the OEM community simply because suppliers are more likely to want to switch between the production of different parts with greater frequency. But there is also a major division among mechanical machines that probably goes a long way to explaining the increasing popularity of servo presses. As Viola explains, non-servo machines incorporate a flywheel drive and clutch and brake systems, whereas servo counterparts do not.
As far as the market for mechanical presses is concerned, therefore, Viola says that Schuler’s expects demand for “flywheel” machines to be reduced to nothing within “two to three years”.
The company’s existing ServoDirect technology, says Viola, already provides a basis for his confidence in that claim. For instance, its use of torque motors enables flexible programming of the slide motion for each die set. Consequently, output can be double that of a non-servo mechanical counterpart. Moreover, another important attribute that follows on from that mix of precise control and variability of the strike length is the ability to “integrate further processes within the die.”
Viola believes that the increasing prevalence of servo machines will be further reinforced by a new drive concept that the company is introducing to the market. This is TwinServo Technology (TST), in which the machine is driven by two independent servo motors in the press bed.
Previously, explains Viola, the drive system for servo presses has invariably been located in the upper part of the machines above the actual die equipment. TST means that the slide is pulled rather than pushed downwards toward the die. The result, says Viola, is that the whole machine is more rigid and there is, according to the company, 30% less vertical deflection in the system.
Furthermore, the fact that the force application points on the slide are located closer to the edge enables the use of the entire clamping surface. The rigidity also means that slide tilting can be actively influenced by regulating the two mechanically separate, but electronically linked drives. In practice this should mean shorter tryout times and reduced die wear.
While Viola expects servo technology to increasingly push into the domain of hydraulic pressing, he agrees the latter is likely to remain valid in certain areas. In technical terms it is, he acknowledges, the better option for parts that require ‘deep drawing’. It also continues to be competitive where flexibility – specifically the ability to switch quickly between the production of different parts – is a priority.
That is very much the case at French commercial vehicle manufacturer Renault Trucks, in Lyon, for instance, where Schuler has recently installed a hydraulic pressing line to support the production of a new range of trucks launched by Renault in June this year. One of the major attributes of the line is that it can use the dies of an older press line as well as those developed for the manufacture of the cabins of the new truck range, with changeover times for any of the 80 die sets involved of less than five minutes. An important consequence is that batch size is no longer a factor in determining the economics of production.
A similar account of the significance of servo-press technology is provided by Klaus Rothenhagen, vicepresident, international sales, for the Italian arm of multinational provider of pressing and stamping equipment AIDA. The company produced its first 1,000-tonne servo press in 2004 but its machines now have much greater capabilities. Rothenhagen says the biggest machine AIDA has shipped is a single 3,000-tonne force transfer machine – a standalone machine with multiple variable dies – used by component manufacturer Kamtek, in Alabama, US, largely to make parts for the German-owned OEMs in the US.
However, in the tandem line configuration it is more common to have an initial large machine and then several subsequent machines with a lower press force, precisely the format of two press lines from AIDA that are scheduled to go into operation at Jaguar Land Rover’s (JLR) Halewood and Castle Bromwich plants next year. In each case, says Rothehangen, the lead press will have a press force of 25,000kN, the second of 18,000kN and the third to fifth machines 12,000kN. The lines will be able to run steel blanks at 22spm (strokes per minute) and aluminium at 18spm. With a bed size of 5,000mm x 2,500mm they will be among the largest such lines in the world. Installation of the Halewood line is due to start in September this year with production set to get underway in the second quarter of 2014. The Castle Bromwich installation will follow in roughly six months.
Interestingly Rothenahgen says the relatively short lifespan of the technology in the automotive industry means it is still possible to discern corporate preferences in its application.
He observes it has been enthusiastically adopted by BMW, Daimler and Volkswagen as well as JLR, but has not yet made serious inroads at Renault, Peugeot, Fiat, Ford, GM or Chrysler.
Another metal forming technique, once enthusiastically received, but then with limited take-up, is hydroforming, which uses pressurised liquid to help form the metal.
Over the last five to ten years automotive interest in this method has started to pick up again, according to Dean Gericke, engineering manager for new business at Canada’s Vari-Form, which is almost entirely devoted to providing hydroformed parts for automotive customers, specifically parts hydroformed from tubing rather than flat sheet. The two techniques, explains Gericke, pose distinct challenges and are not a natural fit for one single company.
The hydroforming of tubing – the more common as far as automotive parts are concerned – is mainly used to produce parts that will be highly loaded, explains Geicke.
Vari-Form itself uses water as an “internal mandrel” that can both push metals into shape against hard dies and act as a back-support for integrated hole-punching operations. The firm makes parts in both aluminium and steel – the highest grade of the latter currently being DP780 grade, though Gericke says the company will soon be manufacturing in the tougher DP980 grade. One current part made by Vari-Form in DP780 is the roof rail for the Ford F150 pickup truck.
In general terms, says Gericke, hydroformed parts offer distinct advantages in terms of structural efficiency over stamped and welded counterparts. He explains that such parts will be “closed box sections” and that fabricating them in the latter manners involves creating flanges on the parts to act as connection points. Occasionally, he concedes, those extra elements can be made to serve a purpose in the finished part, but usually they are superfluous. When tubing is used that issue simply does not arise as all the material usage is relevant to the required shape and performance.
Nor are comparative cycle times an issue. Gericke says that a typical cycle time for a box-type part within Vari- Form would “20 to 27 seconds”, which though obviously longer than that required for a single stamping operation is comparable to the overall cycle time for producing a complete non-hydroformed counterpart given the supplementary operations that will also be involved. The same considerations apply to final cost-per-part, meaning that hydroformed parts are, he says, “very competitive” in economic terms.
Nevertheless Gericke concedes that misconceptions persist about the usefulness of hydroformed parts in the industry, inhibiting their adoption. “Some people think it is expensive and that it is not adaptable to existing parts,” he states. It is often also thought that hydroformed parts are difficult to integrate within wider body structures because the flanges that occur in stamped and welded parts can be made to serve as attachment points. Gericke says this is not an issue as if necessary, appropriate “pinch flanges” can be created in the hydroforming process. There is, he states emphatically, “no need to tear up the body shop” in any sort of major reconfiguration exercise in order to start using hydroformed parts in an existing body-in-white.
Gericke was closely involved in a detailed analysis of the comparative weight and cost benefits of hydroformed versus stamped and welded parts. The study, which though commissioned by Vari-Form was researched independently by the EDAG Group, took as its starting point the theoretical Future Steel Vehicle (FSV) developed by the US AutoSteel Partnership and which has already demonstrated a potential mass reduction of over 35% compared with a real benchmark. It took the exercise further by proposing the replacement of several conventionally produced elements of the vehicle with hydroformed counterparts.
The results of the exercise, says Gericke, confirmed the message that hydroforming can produce real benefits in weight and cost terms. The weight of the vehicle was reduced by a little under 15kg and the cost of the parts involved cut by just over 10%. (See box, right) Given that Gericke is confident those benefits could be realised in existing as well as future vehicles, he believes the real barrier is psychological rather than practical. Many people in the industry are “very comfortable with stamping and welding,” a comfort he concedes has been amplified by continuous improvements in materials formulations which have made it possible to generate incremental weight reductions without addressing fundamental issues of manufacturing technology.
However, Gericke is adamant that stamping is “coming to the end of its life” as a means of enabling continued significant weight reduction. There are signs that attitudes are beginning to change, with the first mass production passenger vehicle using significant hydroformed elements already on the market – the Ford Fusion has both a hydroformed roof rail and B-pillar. In addition, Gericke says a number of other OEMs are showing real interest, though he will not be drawn on names. In short, a manufacturing technique that he believes can legitimately be described as “seriously under-utilised” may be about to enjoy a genuine renaissance based on a demonstrable potential to generate real benefits.