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Auto Industry's Efforts to Lighten Up Create Lubrication Challenges

By: Dr. Nancy McGuire

Nancy McGuire is a freelance writer based in Silver Spring, MD. Contact her at nmcguire@wordchemist.com. This article was reprinted with permission from the November 2017 issue of TLT, the monthly magazine of the Society of Tribologists and Lubrication Engineers, an international not-for-profit professional society headquartered in Park Ridge, IL, www.stle.org.

Saturday, September 1, 2018
 

lightweight vehlcle parts drives changes in metalworking fluidsEfforts by automakers to develop lighter, more fuel-efficient vehicles continue, as do related ongoing challenges. Lighter materials such as advanced alloys can be more expensive. In addition, manufacturing processes must be adapted to work at higher temperatures, or to work with metals and alloys that oxidize more easily than steel.

As a result, companies adopt various strategies that often combine new and old technologies, processes and materials. Some companies focus on cost. If they are making vehicles for a mass market, they might be slow to adopt materials that require a change in processes or the purchase of new equipment. Others go for the lightest weight with space-age approaches for their pricier vehicles.

Global Competition

Key Concepts

  • Lighter metals and alloys pose new challenges in corrosion prevention and high-temperature processing.
  • Metalworking fluids are being reformulated for high-speed processes, downstream compatibility and regulatory compliance.
  • Global competition, industry consolidation and customer demand ensure that these trends are likely to continue.

Over the past several years, Ted McClure, technical resources manager for Sea-Land Chemical Co., Westlake, OH, has seen pronounced trends toward global competition, which puts pressure on domestic operations to increase their productivity. McClure deals mainly with clients having metalforming operations in the automotive industry. Companies must produce parts faster and more reliably with less downtime, he says. At the same time, they are working with new materials, new tools and tool coatings, new environmental regulations, and the latest performance requirements such as corporate average fuel economy (CAFE) standards.

Domestic OEMs use different strategies for lightweighting to meet CAFE standards, says David Budai, automotive industry manager, Americas, for Houghton International, Strongsville, OH. Some are making lighter engines, and some are making lighter structural parts such as body panels to get the CAFE numbers up. For example, Ford’s best-selling vehicle, the F-150 pickup truck, uses aluminum body panels, and it will be transitioning this over to other vehicles over the next 4 to 5 years, Budai says. The F-150’s cab and box are made from “high-strength military-grade aluminum alloys,” according to Ford’s website. That, and the high-strength steel in the frame, reduce the pickup’s weight by 700 lb. as compared to the previous generation of trucks. Other manufacturers are heading in that direction as well, but maybe not as quickly as Ford, Budai says.

OEMs are looking for strong materials with the desired degree of ductility and wear resistance. “We’re seeing more aluminum alloys containing titanium in specific engine components—power train valves, camshafts, pins, crankshafts, and parts such as exhaust and intake valves,” Budai says. His colleague, Yixing (Philip) Zhao, senior research scientist and innovation team leader, research and technology, at Houghton, adds that newer aluminum alloys have higher strength, different compositions and harder surfaces. The aerospace industry is the biggest user of titanium right now, Zhao says, but the knowledge that fluid developers gain in developing lubricants for this market eventually transfers to other industries.

Steel, Still Around

banana diagram presenting tradeoff between a metal or alloy's ductility and strength
Fig. 1—The banana diagram is a common way to present the tradeoff between a metal or alloy’s ductility (as percent elongation) and strength. This diagram compares magnesium alloys and 5xxx-, 6xxx- and 7xxx-series aluminum alloys with various types of steel. (Figure courtesy of R. Schneider, B. Heine and R.J. Grant (2014). Mechanical Behaviour of Commercial Aluminium Wrought Alloys at Low Temperatures, Light Metal Alloys Applications, Dr. Waldemar A. Monteiro (Ed.), InTech, DOI: 10.5772/58362.)
The auto industry is changing quickly, with newer vehicles made using thinner, stronger steel panels. Moving to high-strength steels is “a huge change in the way cars are made,” McClure says. In 2005, mild steel comprised about 85 percent of the mass of a typical auto body, with the rest being high-strength steel. By 2015, typical auto bodies were about 40-percent mild steel and more than half high-strength steel (HSS). Lighter metals such as aluminum and magnesium comprise the remainder, as much as 10 percent, but these metals are gaining, he says. Since then, the percentage of HSS has only increased, according to the Steel Market Development Institute (SMDI). For example, SMDI data shows the body of a 2017 Chrysler Pacifica as being composed of 28-percent mild, 23-percent HSS and 49-percent advanced-high-strength (AHSS) and ultra-high-strength steels (UHSS), while the 2019 Chevrolet Silverado’s body is 25-percent mild, 46-percent HSS and 29-percent AHSS and UHSS.

Mild steel has a low carbon content—a few tenths of a percent to a few hundredths of a percent. Mild steel is ductile, machinable and easily welded, but it has a low tensile strength, typically less than 200 MPa. Thinner sheets of steel can make a lighter vehicle, McClure says, but it takes a stronger steel to achieve the same or better crash worthiness. These HSS have tensile strengths in the range of 210-550 MPa. In this range, the steel is ductile enough to be shaped, but strong enough to meet performance standards. Some advanced third-generation steels have tensile strengths exceeding 1000 MPa, but they still have “decent formability,” he says ( Fig. 1).

Steel generally loses ductility as it gains strength, making it harder to form, McClure says. In addition, less-ductile steel tends to spring back after it is formed, making dimensional accuracy difficult. Solving this problem is an active area of research, he adds. HSS are more expensive and harder to work with, which drives up production costs. Higher tool temperatures from working with these steels and the higher stresses involved in forming parts requires tools, tool coatings and lubricants that can stand up to these conditions.

Aluminum Ramping Up

High-strength aluminum presents some of the same ductility and springback problems as HSS, along with higher costs, McClure says. These factors had been holding back the use of aluminum, but now that manufacturers are moving toward higher-strength materials, there is less of a barrier to using aluminum.

material trends—HSS and LW
Fig. 2—Many vehicle manufacturers are incorporating more HSS and aluminum into their vehicle frames.
(Figure courtesy of Houghton International)

“Aluminum is expanding into more applications,” at least for some vehicles, says Dianne Carmody, Americas marketing director, global product management adjacent businesses for Houghton International (Fig. 2).

Randy Sebastian, Houghton’s technical program manager, research and technology, concurs. He says that some semi-truck manufacturers have reduced the weight of their diesel engines by 50 percent by going from cast iron to compacted graphite iron for their engine blocks, but that this material is very difficult to machine. As a result, Sebastian says engine makers are moving toward aluminum for their gasoline engines. Among them--Mitsubishi and Cummins, who already have begun introducing aluminum diesel engines. Other aluminum engines are in the prototype stage.

Fluids Evolve

As materials and alloys change, the processes for working with them must change as well. Fluid formulators are looking for fluids with long service life to reduce downtime and increase productivity. Their customers are going toward faster speeds, using smaller sumps. Everyone wants low-foaming fluids that do not stain, emulsions that remain stable in hard water and additives that do not promote microbial growth (while still complying with regulations on allowable biocides). Not only that, but manufacturers are looking for compatibility with everything the fluid comes into contact with. “You have to look far downstream,” McClure says.

“All of the products have a place in the industry,” says Zhao. “North American and Asian companies use more water-based fluids, but European companies also are starting to move away from straight oils and toward water-based fluids.

Adhesives

HSS and aluminum parts are more difficult to weld than mild steel. Epoxy structural adhesives are sometimes used in addition to or instead of welding in situations where welding is difficult.

Epoxies are good sound deadeners and sealers, and they increase structural rigidity.

2015 automobiles typically had about 55 linear feet of structural adhesives. 2017 models have about 68 linear feet, and this is predicted to increase to 87 linear feet by 2020.

Some high-end, aluminum-intensive vehicles contain as much as 600 linear feet of structural adhesives.

Source: Ted McClure, Sea-Land Chemical Co.

“Water-based fluids are complex packages, though, and require users to balance all the components, Zhao continues. “Operations using high-strength materials generate more heat, and water-based fluids are better coolants than straight oils. Newer fluid technologies build in better detergency (prevention of particle agglomeration and coating formation on surfaces), dispersion and wetting, all needed for the lighter metals.”

Some operations still require straight oil metalworking fluids (MWFs), says Mark Soder, Houghton’s director of technical service, research and technology. Smaller parts and those requiring tight tolerances or a smooth surface finish are examples of this. Magnesium work often uses neat oil because it requires a lubricant that will not stain or release hydrogen gas. However, even here the trend is toward water-soluble fluids, for insurance reasons. Emulsion-based fluids are less flammable than neat oils and you do not have to evacuate the mist like you do with oils, he says.

High Temperatures, High Pressures

High-speed machining fluids for working with softer metals must have good cooling and lubricating properties to prevent excessive heat formation due to friction. Softer metals can expand and lose strength if the temperature rises too high. However, many of the new high-strength alloys must be heated to make them ductile enough to work with.

Aluminum alloys that were previously only used in the aerospace industry are beginning to be used in the automotive industry, McClure says (Fig. 3). The 6000- and 7000-series alloys, formed at elevated temperatures—500 to 750 F)—don’t always require a lubricant. When they do, the lubricant may include more solids, inorganics and phosphates. (More on no-lubricant scenarios later.)

Lubricants for the automotive industry have to perform at higher temperatures and pressures, McClure says, while maintaining compatibility with adhesives, cleaners, primers and welding. Straight oils don’t encounter the biological or hard-water problems that water-based fluids do, he adds, but they often become volatile at higher temperatures, producing fumes and smoke, and they may be prone to oxidation.

Oxidized lubricants are harder to clean off of parts, McClure adds. Lubricants and other fluids must not interfere with adhesives, including structural adhesives, which are seeing increasing use as a replacement for welding. This is an important research area, McClure says (see Adhesives).

Dave Slinkman, senior vice president, global research and technology for Houghton, notes that rolling HSS or aluminum sheet requires greater force, but at the same time body panels need a smoother surface finish than other parts might require. “We’ve had to make significant changes to the lubricity of our formulas to be able to work in those environments,” he says.

aluminum alloys in aerospace applications
Fig. 3—Aluminum alloys that formerly were used only in aerospace applications are finding their way into automobile manufacturing. ©Can Stock Photo/Leaf

Zhao says that working with lighter, stronger metals increases the need for boundary/EP lubrication additives. The hard surfaces of HSS, aluminum alloys and the titanium alloys used in the aerospace industry require a different lubrication mechanism. “We test additives by themselves and as parts of formulation packages for specific applications, and we use design of experiments to optimize our formulations,” he says.

Taming Rogue Ions

Hard-water cations (positively charged ions) can cause emulsions to separate, and they can leave mineral deposits on tools and workpieces. Calcium and magnesium are the usual suspects, but aluminum ions can cause hard water as well. Cast aluminum alloys contain magnesium, notes McClure, and magnesium is particularly hard on water-soluble cutting fluids. Formulations must contain additives that can stabilize emulsions in this environment.

Lightweight metals such as aluminum and magnesium corrode more easily than steel, and their protective oxide coatings are stable over a much narrower pH range than iron oxides. Semi-stable emulsions that deposit an oil coating on the workpiece are one option for preventing corrosion, though cleaning off the oil film is necessary before applying coatings or paints.

Even a small amount of corrosion can present a problem in the form of staining. Aluminum, magnesium and their alloys are prone to staining. Aluminum is a more reactive metal than steel, so processing fluids must contain corrosion inhibitors to prevent staining, says Hoon Kim, senior principal R&D scientist with Chemetall, Jackson, MI (see Corrosion Inhibitors: A Primer).

Magnesium 101

  • Magnesium

–Third most commonly used structural metal after steel and aluminum

– Mainly used for making aluminum alloys 

– Lightest metal used in the production of structural components

– Extremely high strength-to-weight ratio 

– Used for making wheels, radiator supports and engine blocks

– Alloys among the easiest metals to machine

– High thermal conductivity and dissipates heat rapidly

– Sometimes machined dry without using any sort of metalworking fluid • Water-based fluids can stain magnesium more easily than they can aluminum or zinc.

  • Direct contact between magnesium and water can release flammable hydrogen gas. Aerating the fluid can minimize this.
  • Newer water-based fluids use slightly unstable emulsions that deposit a thin oil coating onto the metal surface, which limits contact with water.
  • Magnesium corrosion generates heat, so drums full of scrap or chips can catch fire.
  • Magnesium fires are very hot and difficult to extinguish—using water or a carbon-dioxide fire extinguisher actually feeds the flame, sometimes explosively so.
Source: Randy Sebastian, Houghton International

Ferrous alloys corrode in neutral to acidic environments, but not in highly alkaline environments, because their surface oxide layers are stable at high pH. Aluminum readily forms a protective oxide layer, but the oxides are only stable in a fairly narrow pH region on either side of neutral. This can present a problem when MWFs are kept at a pH above 9 to protect expensive steel tools, Kim says. At this high pH, aluminum workpieces stain, and high-alkaline fluids can dissolve the protective aluminum-oxide layer as fast as it can form, so MWFs require corrosion inhibitors.

Processes that generate fresh metal surfaces by machining or grinding away the oxide layer require the use of fluids that protect these surfaces from direct contact with tool surfaces or chips to prevent welding or adhesion, often referred to as “sticking.” Because stainless steels, aluminum and titanium form a pure oxide skin, the sticking effect is more pronounced for these metals than for steel or copper.

Mild corrosion leaves a yellow or gold stain on aluminum. This can occur when using the right type of MWF but for too long, resulting in depletion of the corrosion inhibitor. Using MWFs meant for ferrous metals can cause more severe corrosion that leaves a gray or black stain. Even the right MWF can stain an aluminum workpiece if certain additives (triazine biocides, for example) raise the pH of the fluid too high.

Semi-Solids? Solids? No Lubricant?

Not every metalforming operation uses a lubricant, McClure says. HSS containing boron (in the gigapascal-tensile-strength range), formed at 1202-1562 F, are quenched in the die to form very strong parts. Conventional lubricants cannot withstand these temperatures.

Magnesium often is machined dry, without a cutting fluid, because it does not need the cooling or the lubricating effects. MWFs are sometimes used in the more difficult machining operations (e.g., deep-hole drilling) or work at high spindle speeds, but here the MWF is mainly a coolant, especially to keep the chips from igniting (see Magnesium 101).

Some aluminum-forming operations use semi-solid, half-hard lubricants, McClure says. Semi-solid or solid lubricants overcome some problems because they stay put rather than migrate within a coil, preserving an even coating, and they can provide excellent lubricity. However, these coatings can be costlier to apply and, like traditional fluids, must be compatible with assembly, cleaning and painting.

Adaptability Issues

Some fluid formulations advertised as multipurpose and suitable for a wide range of metals and applications may not live up to the one-size-fits-all claim. “It’s always a balancing act,” McClure says. You can optimize your formulation for one type of metal or one operation, but “users are reluctant to inventory too many different fluids in the plant.” Thus, it’s a trade-off between simplifying inventory and optimizing performance. “If it’s a critical operation,” he says, “you might use a fluid that’s specific to one type of material in a particular type of operation.”

A new lubricant must be approved for use before an OEM introduces it onto the factory floor, where making changes involves a long process, McClure says. Changes in the factory tend to be incremental to prevent issues in the manufacturing process. For example, fluids must be compatible not only with workpieces (which can have various surface treatments, including the galvanized coatings on HSS), but also perform well with the various tool materials and die coatings with which they come into contact (Fig. 4).

Environmental Considerations

Scott Lay inspects advanced high strength steel cup
Fig. 4—Houghton chemist Scott Lay inspects an AHSS cup made with the state-of-the-art Valley Forge Labs 40-ton stamping press that the company uses to evaluate new technologies. (Figure courtesy of Houghton International)
Environmental considerations affect fluid formulations as well. Carmody sees a trend toward “minimum quantity lubrication” and away from “flood lubrication.” This approach typically requires a change in manufacturing equipment as well as different fluid formulations, she says, so operations generally do not adopt this approach until they are ready to replace their equipment.

Dry machining has proven successful in numerous operations, but MWFs still are required for applications that require cooling and lubrication, including titanium milling for aerospace applications or working with compacted graphite iron for engine parts. Using an MWF reduces tool wear (thus reducing tool replacement and disposal costs), and produces better parts by reducing residual stresses and dimensional errors, and improving surface finish. Fluids also allow processes to run at faster speeds without building up excessive heat. These factors can more than balance out the environmental impact of using fluids in the process.

Environmental considerations also affect what additives are used. For example, McClure says, conventional EP additives, including sulfurized, chlorinated and phosphorus-bearing additives, can react with steel but not necessarily with nonferrous metals, zinc-coated steels and tool coatings. OEM plants have gotten away from chlorine, he adds, but it is difficult to replace chlorinated fluids in some severe operations, including fineblanking. Also difficult, he says, is replacing chlorine for severe operations on some stainless steels as well.

Regulatory requirements differ from one region to another, says Slinkman. Older fluids, including those that contain chlorinated paraffin, may be acceptable in one region, but fluid suppliers in other regions may have to make significant changes to their products. Products for sale in Europe must comply with the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulations.

“We have to conduct a complete review of our product lines to make sure that we are compliant,” Slinkman says. “We expect this to have a huge impact on the industry in terms of what products are available for these new uses.”

Often an old fluid that works well in one region of the world is heavily reformulated for sale in another region, Slinkman continues. “That’s a big change from what we’ve seen.” To keep up with the regulatory changes while reducing the amount of reformulation work, fluid development begins by finding “a basket of raw materials that we can use in five or six different regions,” he says.

Corrosion Inhibitors: A Primer

Various classes of chemical compounds can prevent corrosion and staining on aluminum metal and alloys. Each has advantages and disadvantages.

Inorganic compounds

  • React with ions in solution before they can reach the aluminum surface
  • Many of these compounds (including arsenates and hydrazine) are toxic

Heterocyclic organic compounds

  • Good for copper-containing aluminum alloys
  • Prevent galvanic corrosion caused by close contact between aluminum and copper

Sulfonates and phosphates

  • Work well as corrosion inhibitors
  • Encourage microbial growth: biocides required
  • Solutions with less than 2-percent phosphates effectively prevent stains
  • Sulfonate-containing fluids can have an unpleasant odor, and they can corrode copper

Amine carboxylates

  • Very effective at keeping aluminum from staining
  • Don’t require biocides (if the proper amine chemistry is selected)
  • pH buffering action, which improves the overall stability of the MWF
  • Relatively large amounts required, typically as much as 10 - 15 percent of the formulation
  • Changing to a different amine has a big impact on the buffering and stain-prevention properties of the fluid

Complex esters

  • Small oxygen content requires a relatively large amount (6 - 7 percent) in the fluid to be effective at preventing stains
  • High cost means that they are used mostly as lubricants rather than corrosion inhibitors

Silicates

  • Similar chemically to phosphates and sulfates
  • Microbes don’t use them as food
  • Inexpensive
  • High oxygen content means you can use less to get the same level of corrosion protection
  • Tend to form gels in water-based fluids
  • 0.8-percent tetraethyl orthosilicate (TEOS) can prevent staining on aluminum alloys without forming a gel

Silanes

  • Lower oxygen content than silicates
  • Less prone to forming gels but must use more to prevent corrosion
  • Bonding at least one long-chain organic group to the silicon atom further disrupts gel formation (but can drive up the cost of the additive)

Compounds with branched organic substituents

  • Degree of branching has little effect on pH
  • Highly branched organic substituents are significantly better at preventing aluminum staining than their straight-chain counterparts

Amine-functionalized organosilicates

  • Still under development as fluid additives
  • Combine the beneficial properties of amines with those of silanes
  • Do not require biocide
  • Good stain protection
Source: Hoon Kim, Chemetall
Carmody notes that many of Houghton’s customers want products that they can use in most or all of their locations around the world. Sometimes, however, specialized regional formulations are required to accommodate differences in things such as water quality, she adds.

MWF Market

Competition and regulation come together to drive changes in MWFs, McClure says. Changes in fluid formulations, driven not only by new demands from parts manufacturers (who, in turn, are driven by marketplace and regulatory demands), also are pushed by new product offerings from the chemical companies’ R&D labs. “I hear from both sides,” says McClure.

It’s not enough just to watch overall industry trends, says Chuck Faulkner, Houghton’s product marketing manager for metalforming, forging and heattreatment. We watch it all,” he says. Individual OEMs do things differently. We work with colleagues all around the world, and they all work with different materials and processes.”

In a 2016 press release, Gaia Franzolin, global marketing manager for Swedish specialty-oil manufacturer Nynas, cited a prediction for a 5.5-percent global increase in demand for private cars and light commercial vehicles over the next 5 years. Franzolin predicted a concurrent growth in demand for MWFs, driven in part by new materials such as the lightweight aluminum and titanium alloys for weight reduction and fuel efficiency. “Ultimately, it’s important to remember that technological advancements also bring the need for improved MWF performance (longevity and stability). And, their lower consumption will be compensated by higher costs and improved tool life,” she says.

Lubrication: Looking Ahead

Budai notes that the trend to reduce weight in automotive powertrains continues, with the downsizing of engines from V8 engines to V6 and I4 engines and the addition of turbocharging. The smaller turbocharged gasoline engines generate more heat, which raises the under-hood temperature. A few years ago, he says, OEM manufacturers were replacing steel engine parts with thermoplastics, but now some of the commonly used thermoplastics can’t always deal with the higher heat. So, aluminum and magnesium are, in some cases, seen as more cost-efficient alternatives for replacing the higher-priced, high-performance thermoplastics.

Kim sees a role for polymer composites; he notes that the aerospace industry uses carbon-fiber reinforced plastic (CFRP) composites, in addition to aluminum, titanium and HSS. Automaker BMW uses some CFRP composites in its passenger compartments but not for structural components, Kim says. Because CFRPs are so easy to form, the aerospace industry has moved from steel to aluminum and then toward CFRP. It is possible that the automotive industry could follow this trajectory as well.

As challenging as today’s high-speed, high-temperature manufacturing processes are, even bigger changes are on the horizon. Carmody notes that the aerospace industry is beginning to use 3D printing for prototyping and part production, and the medical industry already uses 3D printing with metals and composites to make joint replacements. Printed automotive components would require no processing fluids, and complex parts could be formed as one solid piece, with little to no need for machining, forming, stamping or other processes that generate heat and waste material.

The challenges involved in integrating these new materials and processes into efficient, reliable vehicles are becoming increasingly complex. Fluid formulators who work with the automotive industry clearly have their work cut out for them. MF

 

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