Aluminum Metalworking Lubricants

19Aluminum

MetalworkingLubricants

James R. AnglinAluminum Company of America

19.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19-1

19.2 General Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19-2

19.3 Detailed Discussion of Lubricant Requirements . . . . 19-4Formulation Chemistry • Regulatory and Customer

Requirements • Process Equipment Lubricants •

Filtration • Waste Treatment

19.4 Lubrication in Selected Aluminum MetalworkingProcesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19-12Rolling • Lubricants in High-Temperature Processes •

Lubricants for Forming of Finished Food and Beverage

Containers

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19-19

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19-19

19.1 Introduction

This chapter addresses formulation concerns related to the metalworking of aluminum and its alloys, with

an emphasis on processes commonly performed by or focused on by aluminum producers. Included is

a general discussion as well as more detailed discussions for rolling, hot processing other than rolling,

and forming of finished food and beverage containers. This information should provide a basis for

understanding the requirements for process fluids used in other applications such as wire drawing, general

sheet forming, and machining, which are not discussed here but are described elsewhere, such as in Schey’s

monograph [1]. The following discussion will apply generally to aluminum and its alloys, except where

specific alloys are indicated.

While the primary function of an aluminum metalworking lubricant is to facilitate the fabrication of

aluminum products, there is a lengthy list of requirements built into lubricant performance whose relative

importance depends on the specific application. The lubricant must contribute to process robustness and

metal surface quality as well as provide environmental, health, and safety (EHS) suitability, regulatory

compliance, and acceptable cost. It must further be possible to remove lubricant residues as required for

subsequent processing to be performed on the material. Finally, it must be possible to reclaim or dispose

of the used lubricant economically and in compliance with applicable regulations.

19-1

© 2006 by Taylor & Francis Group, LLC

19-2 Handbook of Lubrication and Tribology

-- Oxide layer

-- Near-surface metallurgical properties

-- Bulk metallurgical properties

-- Adsorbed water, lubricants, organic materials

FIGURE 19.1 Schematic representation of aluminum surface.

19.2 General Overview

A properly formulated lubricant can be expected to act in concert with well designed and maintained

tooling to enable the metalworking operation to proceed successfully. It will control friction in the tool-

workpiece contact and resist the onset of adhesive wear, where aluminum adheres to the tooling and

its surface is torn as it moves relative to the tool surface. This can occur when the oxide layer on the

aluminum has been breached and the softer, more reactive underlying metal has been exposed. Certain

alloys, such as 6262, incorporate moderate levels of lead and bismuth to facilitate metalworking. However,

in general, only very mild metalworking of aluminum alloys is expected to succeed without lubricant. In

certain hot forming processes, the lubrication can be provided by lamellar solids such as graphite, boron

nitride, and molybdenum disulfide. These alone, or in combination with other components, provide

reduced friction and reduced aluminum transfer to the tooling as the layers of the easily sheared solid

slide over one another. On the other hand, certain forming operations with polymer-coated metal rely on

thin layers of lubricant to reduce the friction between the polymer and the tooling, with no actual contact

of the aluminum and the lubricant. In most instances, however, a liquid or dry film of lubricant directly

lubricates the aluminum surface.

The surface of the aluminum workpiece presents a complex system to the lubricant and tooling, as

shown in Figure 19.1. The composition and properties of the metal will increasingly depart from the bulk

material and reflect its processing history as its surface is approached. Similarly, the oxide layer can vary

in composition and thickness based on processing history. As an example, alloys containing magnesium

can have a higher concentration of that element at the surface. Friction coefficients have been linked to

both the underlying strength of the alloy and to the levels of MgO found on the surface [2]. The surface

may contain other elements, such as fluorine from furnace treatments to minimize hydrogen uptake by

the metal in moist atmospheres [3]. In addition, adsorbed water, organic materials, or residual lubricants

can be present on top of the oxide. Further, the topography of the surface can have a strong influence on

the distribution of the lubricant, with depressions providing reservoirs and asperities controlling the flow

of the lubricant in the contact. Asperities aligned across the direction of travel will tend to lead to thicker

lubricant films [4].

The surface of the tooling has an important influence on the metalworking process. While generally

outside the scope of this review, it should be noted that the surface composition and topography of the

tooling are very important. The abrasiveness of the surface oxides on aluminum leads to the use of a

variety of strategies for reducing tooling wear, including the use of hard coatings such as chromium or the

use of wear-resistant materials such as chromium carbide. The tendency for adhesion of the aluminum to

the tooling is dependent on the composition of the tooling surface as well as its topography, since rougher

features can provide an opportunity for transferred aluminum to be held more securely. The directionality

of tooling surface features will also influence the flow of the lubricant during the process and influence

its film thickness [4]. In addition, the angle at which asperities rise from the tooling will determine the

relative tendency to cut the workpiece and generate abrasive wear debris.

Although machinery lubrication is designed where possible to occur under hydrodynamic lubrication

conditions, this can be troublesome for aluminum metalworking. In hydrodynamic lubrication, the

lubricant film thickness exceeds about three times the composite roughness of the lubricated surfaces.


Aluminum Metalworking Lubricants 19-3

6060

FIGURE 19.2 Transverse fissures.

FIGURE 19.3 Orange peel.

In rolling, for example, this will lead to slip in the mill and general loss of control of the metal being rolled.

Low friction may also lead to bite refusals, especially with thicker slabs, where the rolls spin against the end

of the workpiece without being able to draw it into the roll bite. The application of kerosene can be used to

overcome bite refusals, but the possibility of ignition of the solvent must be anticipated. With the greater

lubricant film thickness, transverse fissures (Figure 19.2) can be generated on the aluminum surface

because of uneven expansion of the metal surface not in direct contact with the tooling. This imparts

a grey appearance to the finished metal. Conversely, too thin a film can lead to primarily boundary

lubrication, where higher friction leads to greater energy usage and heat generation, along with excessive

transfer to the die or dislocation of surface material. This can give rise to scuffing or galling of the

workpiece surface or a rough surface condition often referred to as “orange peel.” (Figure 19.3) For many

applications, mixed lubrication, in which the lubricant film thickness is typically 1 to 3 times the aggregate

surface roughness, appears to provide the best compromise.



Liquid lubricants in mixed or boundary conditions rely for their effectiveness on fluid rheology and

boundary additive performance, the combination of which is referred to here as film strength. Boundary

additives have a strong affinity for existing and freshly formed metal surfaces under the conditions of the

metalworking process. In work with steel, adsorbed fatty alcohols and fatty acids gave reduced friction

coefficient with increasing chain length over the range C12 to C18 [5]. Work has also been performed

with aluminum surfaces [6]. Guangteng and Spikes [7] have reported a tendency for esters in formulated

products to concentrate preferentially at surfaces and, for thin films, to provide film thicknesses approach-

ing that of the pure ester. In formulated process lubricants, just as in machinery lubricants, understanding

the contributions and synergies among the various components remains a challenging area.

Oleochemical products such as fatty acids, esters, and alcohols are the most commonly used boundary

additives in aluminum metalworking along with phosphate esters and their derivatives in some instances.

Molecules with long, linear chains afford the opportunity for a more densely packed layer of additive

on the surface with the polar end groups of the acids and alcohols providing attachment and lateral

forces among chains providing additional film strengthening. The esters appear to act synergistically in

combination with the fatty acid or alcohol additives. With increasing temperature, the opportunity for

chemical reactions involving the polar groups increases, such as soap formation with adsorbed fatty acids.

Other mechanisms of action of these species have been proposed and are summarized by Schey [1].

One explanation presented by Rebinder [8] relates facile metalworking to a reduction of the surface energy

of the workpiece with the adsorbed additives. It is likely that this effect is sensitive to the nature of surface

oxides that are present.

Many other types of compounds are also used in aluminum metalworking. Paraffin and other waxes

have a crystalline structure that contributes to low friction, but hydrocarbon waxes are poor boundary

additives and provide limited protection against adhesive wear. Other chemistries can be expected to

provide breakdown products or reaction products at the aluminum surface that participate in the lubric-

ation, including metal carboxylates, such as those of lead (now little used), tin, and bismuth. Boric acid,

a lamellar solid, has also been discussed recently [9]. Evidence was presented that its performance varies

with alloy, an observation that may reflect different interactions with the varying oxide compositions

present on different alloys. However, the low temperature (169◦C) for boric acid dehydration and loss of

lamellar structure limits its effectiveness to relatively low temperature processes.

19.3 Detailed Discussion of Lubricant Requirements

Key functions of lubricants in aluminum metalworking are the control of friction and the minimizing

of wear, especially in sliding contact situations. The selection of lubricant components is determined in

large part by temperature considerations. At higher temperatures where organic formulations may not be

successful or ignition of the lubricant cannot be tolerated, water-based products or formulations based in

part or wholly on inorganic solids can be used. In principle, inorganic liquids can also be used in such an

application; however, products with melting behavior suitable for aluminum metalworking appear to be

few. At lower temperatures, the formulation options are much broader but, as temperatures decrease, the

tendency of some oleochemicals to solidify can be a limiting factor.

The requirements for lubricant formulations are quite diverse and depend on the nature of the process

in question. While lubricants used in many operations are consumed in the process, with residues helping

to lubricate the tooling for subsequent parts, some processes, such as rolling and can bodymaking, use

systems with large volumes of fluid that can continue to be used for many months without wholesale

replacement. In these instances, additional concerns take on importance, such as oxidative and thermal

stability, along with hydrolytic stability and biostability for water-based lubricants. The ability to filter

well enough to control the level of debris becomes more important in these systems since continued

increases in debris level will at some point adversely affect fluid handling or the metalworking process.

It also becomes important to be able to monitor and control the composition of the lubricant so that

performance over time remains as consistent as possible.



19.3.1 Formulation Chemistry

While many applications have typically used formulations based on petroleum-derived base oils, interest

is increasing in the use of alternate materials such as oleochemicals that can reduce dependence on

petroleum and provide improved biodegradability. Any changes need to be made in keeping with modern

production strategy, which places a premium on consistent performance, enabling the process to be

switched rapidly among products according to customer demand regardless of their sensitivity to lubricant

system condition. The drive for lower costs may favor the use of oleochemicals containing significant levels

of linoleic and linolenic acid and their derivatives, but it must be recognized that such products can detract

from oxidative and thermal stability, a significant concern in systems where consistent performance over

time is needed. Antioxidants can be a valuable tool in this situation as can the use of oleic acid based

products with reduced levels of polyunsaturated components that are now becoming more routinely

available. Care must be taken to choose antioxidants carefully, given that some can be water soluble [10]

and amine-based products have the potential to react with fatty acids in the system to form amine salts

that can affect metalworking performance and emulsification properties.

The desire for biodegradable lubricants, whether process fluids or machinery lubricants that can leak

into them, also presents a tradeoff, since this desirable property can lead to significant problems with

microbial growth in water-based lubricants. The problems can include odor, chemical degradation, system

corrosion, difficulty in filtering, and the costs as well as the hygiene considerations with the handling of

biocides. The biocides themselves can also have a chemical influence on the lubricant [11]. These concerns

must be balanced against the advantages afforded in waste fluid treatment and the management of

spills.

Fatty acids are well known to be excellent boundary additives, with commonly used products being

those from oleochemical sources with chain lengths of 10 to 22 carbon atoms. Also well known are

products with multiple acid groups, such as dimer acid and other oligomeric products obtained from

unsaturated C18 fatty acids by thermal processes. Products of the latter type can also be partially esterified,

generating a distribution of components ranging from the free fatty acids to fully esterified material. The

linear saturated fatty acids have higher melting points and decreasing solubility as chain length increases,

requiring that the formulator control the content of such materials to keep the lubricant fluid under the

process conditions. Alternatives with improved solubility properties are oleic acid and erucic acid, along

with isostearic acid, which is a complex mixture obtained by hydrogenating a highly unsaturated C18 fatty

acid product known as monomer acid. Isostearic acid comprises primarily branched chain structures,

some of which include a cyclic component.

The use of fatty acids in lubricants is a two-edged sword from the perspective that their excellent

metalworking performance is accompanied by a tendency to generate hard-to-filter fine debris and a

tendency to undergo chemical reactions. They can form soaps or metal salts, including oil-soluble soaps

based on aluminum or iron as well as poorly soluble soaps based on such water-hardness cations as

calcium. Aluminum soaps are known to have a variety of possible structures that can be distinguished

by differences in the infrared absorption energies of their carbonyl functions. Their molecular weights

can range up to more than 1,000,000 [12]. As a result, they can significantly raise fluid viscosity. These

materials, especially when derived from longer chain linear fatty acids, can have poor solubility near or

below room temperature, a troublesome feature that does, however, afford an opportunity for controlling

soap levels.

Fatty acids can also be combined with amines to provide emulsifying behavior. Commonly used amines

include triethanolamine, 2-amino-2-methyl-1-propanol, and isopropanolamines. Where triethanolamine

is used, product that is low in diethanolamine and monoethanolamine is preferred for hygiene reasons,

and where amines in general are used the use of nitrites needs to be avoided to eliminate nitrosamine

formation. The levels of fatty acid amine salts can typically be monitored by infrared techniques. The

amine salts of fatty acids, like the free fatty acids, are subject to depletion by reacting with water hardness

ions and depositing on mill equipment or being filtered out of the system. This can lead to product quality

concerns if deposits on rolling mill equipment subsequently fall or transfer onto the rolled product.



Fatty acids and their salts are known to undergo additional degradation reactions at higher temperatures,

including decarboxylation and formation of ketones [13]. Similarly, amine salts of carboxylic acids, other

than those of tertiary amines, have the potential to be converted by heat to amides. They also can undergo

reaction with other boundary additives as discussed below. The reactivity of these species increases the

challenge of maintaining them at consistent levels for optimum formulation performance.

The use of fatty alcohols is also well known, with the materials including both synthetic products

and products formed by the reduction of species such as the methyl esters of the naturally occurring

fatty acids. The natural products are highly linear but may contain unsaturation from the precursor

compounds. On the other hand, the synthetic products can contain primary alcohols with differing

amounts of branched chains as well as minor amounts of diols depending on the specific manufacturing

method. While most synthetic procedures generate product with even numbers of carbon atoms similar to

natural alcohols, some also produce the odd-numbered chain lengths. Like the corresponding fatty acids,

saturated linear alcohols have the drawback of reduced solubility in typical organic media with increasing

chain length, requiring that formulations with longer chain length alcohols be maintained warm. Some

advantage in handling comes from blends of alcohols, such as mixed C12–C14 alcohols, which have a lower

freezing point than the pure individual compounds and can be fluid at room temperature. Higher levels

of branching would also decrease the freezing point, but compositional constraints associated with FDA

compliance limit the extent of branching permissible for FDA-sensitive applications. The alcohols have a

tendency to oxidize to aldehydes and other species, and are commonly used with an antioxidant present.

Finally, they can react with fatty acids to form esters, thereby depleting the levels of both components in

favor of the product.

The third common type of additive, whether of oleochemical or synthetic origin, is esters. These are

commonly used in combination with fatty acids or alcohols, where they may provide a synergistic boost

in film strength performance, but are at times used alone. Esters are available in a wide range of struc-

tures and viscosities and range from the methyl esters of common fatty acids to higher viscosity polyol

esters. The latter can be derived from synthetic polyols such as neopentyl glycol, trimethylolpropane

(TMP), and pentaerythritol or from glycerine. Esters based on the above synthetic polyols have excellent

thermal stability because the structures afford no opportunity for degradation by a β-elimination mech-

anism that releases an olefin. Esters can also be formed from multifunctional carboxylic acids, such as

dimer and trimer acid, and very high viscosity products can be made from the combination of polyols

with multifunctional carboxylic acids. It should be recognized that the contribution of esters to film

strength can easily be overestimated when low to moderate levels of unreacted fatty acid are present as an

impurity.

Esters are capable of a variety of chemical reactions related to their use in lubricants. Among these are

transesterification, in which either the acid or the alcohol moiety in the ester exchanges with other acids

or alcohols present in the system to form different esters (Equations [19.1] and [19.2]).

RC(O)OR′+ R′′C(O)OH → R′′C(O)OR′

+ RC(O)OH (19.1)

RC(O)OR′+ R′′OH → RC(O)OR′′

+ R′OH (19.2)

In the presence of water and particularly in water-based formulations, esters can be prone to undergo

hydrolysis, with the formation of the component fatty acids and alcohols. This can change the film strength

and other properties of the formulation depending on the specific ester. The generation of a short-chain

alcohol with a low boiling point can contribute to air emissions, whereas partial hydrolysis of a polyol

ester can generate species that act as surfactants. In addition, as has been reported for turbine lubricants,

the heating of TMP esters together with phosphate esters has been found to form the potent neurotoxin

TMP phosphate [14].

The base oils used in aluminum metalworking are quite diverse. In cold rolling and foil rolling, linear

paraffins are the leading products; however, hydrotreated kerosene streams, often with aromatics levels

of less than 1% and very low heteroatom content, also have a place. The linear paraffins commonly



used in aluminum metalworking are isolated from kerosene streams and fractionated to provide relatively

narrow boiling range products. These cover a range of flash points and viscosities and have linear content

typically above 97%. They are low in odor and staining tendency and have relatively high flash points

together with relatively low viscosities compared with typical hydrocarbon streams of similar boiling point

range. This combination can enable higher cold rolling speed where surface appearance issues related to

thick lubricant films come into play. Some products have very narrow distillation ranges that, together with

careful fractionation at the low end, further improve flash point [15]. Additive response can be expected

to be very good in these highly nonpolar products. One disadvantage associated with the highly linear

structures when compared to hydrotreated kerosene fractions is the relatively high freeze point. Fractions

with substantial content of C15 or higher homologues need to be insulated or warmed under cold winter

temperatures. In the future, suitably fractionated products may become available from natural gas or

coal sources using the Fischer-Tropsch process to generate aliphatic hydrocarbons. These gas-to-liquids

products are highly linear although not typically to the level of the linear paraffin products isolated from

kerosene. While such base oils are generally resistant to oxidation, it has been shown that aluminum can

catalyze the oxidation of base oils as well as additives [16]. As an approach to formulating lubricants with

lower volatility, ionic fluids have been evaluated as base oils [17].

Staining of aluminum can occur when lubricant residues remaining on the metal during a high-

temperature process such as annealing give rise to a discoloration of the aluminum surface. The color

can range from a light stain, commonly yellow, to darker colored stains with a variety of colors. Stain

is typically measured either visually or gravimetrically using samples with bright, clean surfaces. Among

base oils, the lower boiling and fully saturated products such as cold rolling oils are most likely to have

minimal stain. Among higher viscosity products, polybutenes yield minimal stain or residue if the high-

temperature process is sufficiently high in time and temperature to convert the residues to low molecular

weight, volatile products. Leakage of standard machinery lubricants into cold rolling oils can normally be

tolerated only at low levels before stain becomes excessive, even where highly refined or other synthetic

hydrocarbon oils are used.

Naphthenic base oils are most commonly used in water-based emulsion products because of their rela-

tive ease of being emulsified. However, current refining practices are producing more severely hydrotreated

products with improved hygiene characteristics and with improved stability. These changes can require

adjustments to the emulsifier package. Naphthenic oils are available in a relatively wide range of viscosities,

providing considerable latitude to the formulator to tailor the viscosity and film thickness of the lubricant

film in the contact.

A key additive type required for many water-based formulations is the emulsifier. The emulsifiers

typically used in these applications enable insoluble or poorly soluble materials to be chemically stabilized

in a water medium. Applicable fluid types, such as emulsions (often termed soluble oils), microemulsions,

and micellar solutions have been reviewed by Laemmle [18]. These fluids differ in composition, droplet

size, and thermodynamic stability, and ultimately in lubrication performance and waste treatability.

Lubrication performance will generally be stronger with emulsions of relatively low stability, where the oil

phase separates more readily from the emulsion. Lubrication performance may be weaker on moving to

more stable emulsions, microemulsions, micellar solutions, and true solutions.

Common types of emulsifiers include:

• Nonionic, including such materials as ethoxylated alcohols and ethoxylated fatty acids

• Anionic, commonly salts of longer-chain fatty acids or polyalkoxylated phosphate compounds

with amines such as triethanolamine, 2-amino-2-methyl-1-propanol, and isopropylamines, but

also including sulfonates

• Cationic, such as quaternary ammonium compounds

• Combinations of nonionic with ionic emulsifiers

In addition, emulsifier properties can be built into other components of the system, such as boundary

additives, to provide multifunctional performance. To assist in initial emulsification, lower molecular

weight diols and triols can be included as coupling agents. The anionic emulsifiers generally have the



drawback noted previously for amine carboxylates that they can combine with water hardness cations,

such as calcium ions, to form insoluble calcium salts. This will deplete the emulsifier and, for amine

carboxylates, alter the balance of fatty acid and emulsifier and further affect lubricant performance. For

this reason, deionized water is preferred for blending with the oil and for water make-up since certain

processes such as hot rolling require large water additions to replace evaporation losses.

For water-based formulations, consistent performance is highly dependent upon consistency in the

formulation chemistry. Changes in performance resulting from depletion of the emulsifiers or leakage of

machinery lubricants into such systems can be difficult to correct through adjustments to the formulation

chemistry. Either of these events can initiate a performance pattern where droplet sizes increase during

usage, often giving improved metalworking performance until the product starts to become too unstable

and performance degrades. Leakage of high viscosity products, such as gear oils, into a formulation can

greatly affect lubrication performance. Skimming of unemulsified or separated material, often termed

tramp oil, from the surface of the mixture is routinely performed. To the extent that the formulation

components are removed with the tramp oil, they need to be replaced with fresh or possibly reclaimed

product. Consistent emulsifier performance can be expected to require careful control of both the emulsi-

fier content and composition. This is challenging because of the chemical complexity of many commercial

emulsifiers and the difficulty in obtaining detailed compositional information on the emulsifiers present

in emulsions using routinely available, inexpensive methods. Fortunately, levels of fatty acids and esters

can be readily monitored by infrared techniques and adjustments made to the composition as necessary

to keep these at target levels. This becomes more difficult if the formulation contains multiple products

that cannot be distinguished by infrared or are too low in volatility for GC analysis. High levels of fatty

acid can build up with rapid ester hydrolysis and high levels of ester can occur from contamination by

ester-based mill equipment lubricants.

The importance of the water phase chemistry in water-based formulations should not be overlooked.

The pH affects equilibria involving, for example, fatty acid and amine species and ultimately emulsion

stability and oil droplet size. It can also influence corrosion and the rates of hydrolysis of esters. Conduct-

ivity affects emulsion stability as well as corrosion, especially if certain anions such as halides are present at

significant levels. Conductivity can be expected to rise gradually over time from pH adjustments and dis-

solution of alloy components, but will increase more rapidly if soft or potable water is used for additions.

Water hardness cations, as previously noted, can react with and deplete certain components.

In water-based formulations, certain problem areas, such as rust and corrosion, microbial growth, and

foam, can require special attention. A variety of additives are available for addressing rust and corrosion

issues and these can be tested for efficacy by using methods such as ASTM D 665 and D 130 or variants of

them that simulate process conditions. Performance can be expected to depend on both oil and water phase

chemistry. Special care should be taken to address any conditions that might give rise to pitting corrosion,

which can rapidly lead to perforation of system hardware, especially if made of mild steel. While the

preference is to formulate products that resist microbial attack, an array of biocides is available if needed.

As noted above, amine-based products, including biocides, can react with fatty acids in the formulation

[11]. Normal biocide strategy is to introduce a second biocide at intervals to reduce populations of

microbes developing tolerance for the primary product. If foam issues occur, mechanical causes such as

turbulence should be minimized first. If not successful, reformulation to minimize foam may be preferable

to implementing formulations with significant foaming problems since excessive antifoam additive use

can lead to air entrainment problems. Furthermore, the use of silicone-based products, even in seemingly

inconsequential amounts, can lead to severe adhesion problems for products that will subsequently be

coated or painted. Nevertheless, a range of defoamers, including ones free of silicones, is available.

An additional concern with water-based formulations is the potential for formation of solution stain

or water stain, which is commonly a whitish appearing surface blemish on the formed product. This is

unacceptable for products requiring good surface appearance, and differences in friction performance

over a partially stained surface can lead to problems in forming. The formation of this defect is related to

the time of contact of aqueous materials with the aluminum surface at temperatures near 100◦C. It can

be addressed by rapid and efficient removal of aqueous metalworking products from the surface to limit



the opportunity for the stain to form. Alternatively, reformulating or adjusting process conditions can be

considered to reduce or eliminate the stain.

19.3.2 Regulatory and Customer Requirements

It is important to be aware of a variety of considerations unrelated to lubricant performance that

nevertheless are important when lubricant formulations are developed.

For applications where aluminum products will be used in food or drug packaging, base oils and

additives must comply with appropriate regulations. While this is a complex area, for United States and

many international requirements, the information in the most recent issue of Title 21 — Food and Drugs of

the Code of Federal Regulations (CFR) will apply. Key information can be found in 21 CFR 178.3910, which

discusses surface lubricants used in the manufacture of metallic articles. Lists of substances and their usage

limitations are presented, along with limits for the levels of residue on the formed product as it contacts

the packaged product. The allowable levels depend on whether the lubricant function is to roll sheet or

foil (0.015 mg/in.2 or 23 mg/m2) or to draw, stamp, or form articles from it (0.2 mg/in.2 or 310 mg/m2).

Additional listings of potentially acceptable components can be found in Sections 172 (food additives

permitted for direct addition to food for human consumption), 182 (substances generally recognized as

safe), and 184 (direct food substances affirmed as generally recognized as safe). Other sections within

other parts of Title 21 may provide additional materials for consideration, including 178.3570, which

discusses lubricants with incidental food contact. These are components of lubricants used to operate

machinery used in food applications, which differs from 178.3910 in that intentional contact with the

packaged product is not expected. Careful interpretation of the sections and the precedents associated

with their use is needed in making component selections.

An actively managed program to ensure that good manufacturing practices are followed needs to be

in place for the manufacture of food packaging products. It is very important to avoid the presence of

components or contaminants that can cause the lubricant residues on the packaging material to be deemed

unsafe and products contacting them to be deemed to be adulterated.

Additional regulatory and customer requirements can also apply for food and drug packaging applic-

ations. For example, a need to meet kosher or halal requirements will require careful selection of

oleochemical and other components as well as attention to all stages of processing and transportation of

such materials to ensure suitability for use. Legislation enacted by the Coalition of Northeast Governors

(CONEG) restricts the content of lead, cadmium, mercury, and chromium(VI) in packaging materials in

many U.S. states. The presence of compounds listed under California Proposition 65 can require labeling.

In addition, the use of products that are associated with allergies can be of concern, whether as lubricants

or in other manufacturing applications where contact with the metal occurs. Among the most common

allergens are latex, as well as food allergens, such as milk, eggs, soy, wheat, fish, shellfish, peanuts, and

treenuts. Furthermore, it should be noted that certain families of esters and surfactants, such as phthalate

esters and nonyl phenol derivatives contain members that are suspected to act as endocrine disruptors.

19.3.3 Process Equipment Lubricants

Although equipment suppliers will normally have lubricant recommendations for use in their machinery,

it may be desirable to lubricate certain process equipment with the metalworking formulation or a close

approximation to it. This can, for example, apply to mill situations where connections in the spindles

driving the rolls may be lubricated by the mill lubricant. Similarly, separate systems performing hydraulic

or gear oil functions with a tendency to leak into the metalworking fluid or onto the aluminum part

may be lubricated with the process fluid composition or simply the organic components of a water-

based product. This can provide advantages in the control of the process fluid composition and, through

careful formulation, provide a means to address U.S. Food and Drug Administration (FDA) concerns or

metal staining issues resulting from contamination. The formulations of these fluids may preclude them

from fully meeting normal industry performance standards and their limited commercial use may not be



sufficient to justify extensive performance testing. Nevertheless, they may provide adequate performance

and be a good choice when the advantages provided on leaking into the process fluid are considered.

An alternate approach to this challenge is the use of commercial machinery lubricants formulated to

provide some of the following properties: low stain, FDA compliance, efficient misting, and good EP

properties. As with the above products, their performance is likely to fall short of that of fully qualified

machinery lubricants. When leakage into the process fluid or contact with the aluminum surface occurs,

the impact of these products, based on such base oils as linear paraffins, hydrotreated light petroleum frac-

tions, and polyalkylene glycols, should be less troublesome than conventional mineral-oil based products.

Polybutenes can also be used as a base oil or, where compatible, as a thickener for other base oils. The

potential effect of the polyalkylene glycol products on the emulsion stability properties of water-based

formulations would need to be checked before use.

One approach to the control of contamination, particularly for cold rolling formulations, is distillation

and reuse of the recovered distillate in the process. For products whose components distill over a narrow

range, this is an efficient means to separate the product from heavier machinery lubricants and higher

molecular weight materials such as soaps that can form in the process. Distillation is typically performed

under vacuum to minimize degradation reactions in the still that can reduce yield and also generate a

burned odor that can be very persistent. Distillation may also be used to recover rolling components

from control systems that use oil to capture organic materials in the mill exhaust. In either instance, it is

important to analyze the distilled material to establish its composition since some components may not

be fully recovered. Verification that the products still meet applicable FDA requirements, such as the UV

absorbance limits for base oils, should be performed.

A significant challenge occurs where the use of fire-resistant fluids in process equipment is deemed

necessary and the potential exists for them to leak into the process fluid. The water-based products such

as water-glycols, soluble oils, invert emulsions, and high water-based fluids, can be expected to alter the

stability and performance of water-based process fluids and, therefore, not be preferred choices. Among

anhydrous fluids, phosphate esters are subject to hydrolysis to generate phenols, a potential wastewater

concern, along with acid phosphates, which are strongly acidic and can subsequently form salts similar

to those of fatty acids. Other anhydrous fluid types are available that afford improved fire resistance

over mineral oils in many situations. If leaked into a process lubricant, polyalkylene glycol derivatives

(polyether polyols) and polyol esters have the potential to contribute to aluminum metalworking per-

formance, although the former may also affect water-based fluid stability. The latter are subject to the side

reactions noted above for esters and, if based on unsaturated fatty acids, are subject to oxidation and poly-

merization. Amine-based additives in these products, such as antioxidants in the polyol esters, have the

potential to form salts with fatty acids in the formulation and alter its performance. An option to be con-

sidered is the implementation of improved equipment and personnel safeguards at the process to enable

the use of mineral oil based products or other more compatible machinery lubricants with reduced fire

resistance.

The use of misting systems for process equipment lubrication can have the drawback of fugitive mist

issues including inhalation concerns as well as potential deposition on the sheet. This may raise customer

acceptance concerns in such products as foil for packaging applications. The use of air/oil systems to

deliver metered volumes of oil in such applications may be a good alternative.

19.3.4 Filtration

Metalworking processes typically generate debris that partitions among the metalworking fluid, the surface

of the tooling, and the surface of the workpiece, where it may be termed smudge or smut. The debris

consists primarily of particles of a mixture of aluminum or aluminum alloy and oxides as typically found

at the metal surface. The composition of this surface material and the ease with which it is removed to

generate debris will vary with alloy and processing history. The debris may also include particles from

wear and corrosion of the tooling and system components as well as filter media. Most commonly, debris

levels are controlled through filtration.



The nature and size of the debris generated are closely linked to the contact conditions in the process

and lubricant chemistry. In some processes, large amounts of relatively fine debris are formed, including

submicron debris that settles very slowly and filters poorly. Fatty acids are known to contribute to the

production of fine debris. The chemistry will also influence the degree to which those particles are

dispersed individually or agglomerated into larger, more easily filtered particles.

Increasing levels of debris in the fluid can lead to a number of concerns for the process. It can be expected

at some concentration to lead to the generation of product with poor surface quality. In addition, high

debris levels can lead to changes in the stability of emulsion products and the increased viscosity of debris-

laden fluid will further reduce filtration speed. On the other hand, if much of the debris is relatively large,

a settling step may be at least a partial answer to the need for cleaning the fluid. However, deposition of

larger or agglomerated particles on mill equipment can open the door for accumulations to retransfer to

the product and lead to surface quality problems. Larger and more numerous debris particles also may be

more likely to lead to wear in any process equipment lubricated by debris-laden fluid [19].

Common types of filters include flatbed filters, with vacuum applied to the bottom side, and plate-and-

frame filters that operate under moderate applied pressure, often with the use of added filter media. The

former can index the filter paper automatically based on the level of fluid on the bed, whereas the latter

require “blowdowns” at intervals. When the pressure differential across the filter reaches the design limit,

the filter is taken off line for renewal of the paper, followed by the deposition of a uniform layer of the filter

media. Plate-and-frame filters are mostly used for neat oil streams and can be very efficient at removing

even very fine debris. Commonly used filter media include diatomaceous earth, a product that must be

handled carefully because of the inhalation hazard associated with its content of crystalline silica, and

cellulose products. Diatomaceous earth is supplied in a variety of grades varying in their ability to filter

out fine particles. Cellulose products have the advantage of being biodegradable and can be impregnated

with citric acid for conversion of fatty acid salts in the lubricant back to the fatty acid. Two drawbacks

with the use of filter media are increasing disposal costs and the value of the significant level of entrained

lubricant in used product, which can be on the order of 30 to 40% by weight for used diatomaceous earth.

Recovery of the lubricant or its energy value may be desirable.

The conductivity of neat oil lubricants is of importance, since low conductivity can lead to static charge

buildup during processes such as filtration. This leads to a risk of fire from static discharge, especially with

products of relatively low flash point. Increasing the conductivity, either through fatty acid use or the use

of conductivity-enhancing additives, along with grounding at key locations in the system, can help.

The choice of filter paper needs to be made carefully so that its capacity and pore sizes are well matched

to the needs of the system. The chemistry of the paper also needs to be tailored to the lubricants to be

filtered to ensure that its wetting properties are well matched to the fluid and that it does not affect the

stability of water-based products.

In the event that solids loading and fluid viscosity are both high, the solids content may simply increase

over time because conventional filtration is ineffective or impracticable. Continued development of

alternate filtration techniques, such as electrostatic methods or membrane methods, may provide an

answer to these shortcomings. In some instances with higher cost formulations, it may be economical to

chemically remove the fines by treatment with acid or base, recognizing that this may lead to chemical

changes in the lubricant, including changes in the relative levels of fatty acids and their soaps as well as ester

hydrolysis.

19.3.5 Waste Treatment

Minimizing the waste from metalworking operations is highly desirable from both an environmental

and economic perspective. It may be feasible simply to burn waste oil or oil-solids mixtures in suitable

burners to recover their heat value. For water-based formulations, it can be a significant challenge to

achieve efficient separation of the oil and other organic components from the water to obtain suitably

pure water and oil phases for subsequent treatment. A common approach is the use of heat together

with acids or surfactants, or both, perhaps with the aid of flotation technology. Ultrafiltration is another



option for concentrating water-oil mixtures, as is evaporation of the water phase, a process that may be

economical using waste or renewable energy heat sources.

The quality of the water obtained will depend on its level of contamination and the capability of the

available treatment processes to purify it. In general, the water can be expected to contain a fraction

of those components that are not exclusively oil soluble. This can include a portion of the surfactants,

particularly those with high hydrophile-lipophile balance (HLB) values, as well as amine salts to the

extent they are not associated with the organic phase. It can also include products such as low molecular

weight alcohols generated by hydrolysis of esters based on them or materials present as coupling agents, or

products of oxidation or microbial activity. Low levels of rust and corrosion inhibitors, antioxidants, and

biocides from the process oil as well as additives from contaminant (tramp) oils might also be present,

and can all contribute to the biological oxygen demand (BOD) of the water.

After initial separation from any oil, further treatment of the water may be required to minimize

disposal costs, meet local discharge regulations, or render it suitable for continued use in order to meet

water conservation goals. This may be simply an adjustment of the pH to the desired range, a process that

may precipitate excess levels of certain metal ions whose concentrations in the wastewater are regulated.

Additional methods of wastewater treating to reduce BOD or oil and grease content can include microbial

treatment or oxidation or adsorption processes using, for example, lime or activated carbon. In some

instances, reverse osmosis may be a means to achieve the final purity needed. The choice of a treatment

option could be based on both the water quality obtained and the options for disposal of the concentrated

waste.

It is important when formulating or evaluating a water-based lubricant to assess its ability to be treated

by available wastewater systems. The compositions and amounts of the surfactants in the formulation have

a strong influence on how successfully the system will separate the phases. It should also be recognized

that surfactants vary considerably in their toxicity to aquatic organisms. The formulation with the most

robust metalworking performance may not be the best for waste treating and a compromise is likely to be

needed that balances those requirements.

19.4 Lubrication in Selected Aluminum Metalworking Processes

19.4.1 Rolling

Modern rolling technology integrates a complex blend of lubricant technology with surface technology

and metallurgy together with very precise process control. Cold mill sheet exit speeds of 1500 m/min are

possible while sheet thickness is controlled to within a few microns. In modern continuous mills, feed

coils can be joined without stopping the mill through the use of accumulators that maintain a continuing

supply of metal to the mill during the coil joining process. Multi-stand or tandem mills provide very

efficient production of many alloys at finished hot or cold mill gauge.

In the rolling process, the lubricant is called upon to deliver consistent lubrication, to assist in controlling

roll geometry by controlling roll temperature, and to aid in the management of debris generated in the

contact. In hot rolling, water-based formulations are necessary in order to minimize fire risk. However, in

cold rolling, the use of water makes possible increased reduction at high rolling speed through increased

heat removal compared to neat oil formulations. The use of water-based formulations also provides an

opportunity for reduced rolling emissions compared with neat oil formulations. As discussed earlier,

water-based lubricants can lead to water stain on the finished product, especially if care is not taken to

remove all water quickly and efficiently from warm aluminum surfaces.

The surface quality of the rolled product at high levels of gauge reduction is very sensitive to the

composition and the thickness of the oil film present in the lubricated contact. While marginal lubrication

may be tolerated and can be beneficial in avoiding refusals in early hot rolling passes, as one progresses

towards the final hot rolled gauge, the quality of the lubrication and its influence on the metal surface

become increasingly important. Significant surface flaws from hot rolling may persist during cold rolling

such that the metal will not meet finished product requirements. One approach to this need that is easier



if more than one hot mill is available is the use of relatively stable emulsions for the early passes on a

reversing mill, but less stable emulsions with better oil availability for later passes or coiling passes.

After hot rolling, surface quality can be evaluated by anodizing, a process that highlights discontinuities

in the surface that can be present even if the as-rolled surface appears to be uniform, and by examining for

surface smoothness. A common surface feature that reflects weak lubricant performance is a topography

resembling orange peel (Figure 19.3). This has been studied as a function of lubricant composition and

properties [20]. An inadequate lubricant film is unable to prevent adhesion of the hot sheet to the work

roll, leading to the dislodging of surface metal and oxide that subsequently can be repositioned on the

surface as sliding occurs or transferred to the roll where it may be retransferred to the sheet.

The casting of strip that is relatively thin through the use of belt or roll or block casters can dramatically

reduce the need for extensive hot rolling compared with ingots that can range up to about 600 mm thick.

In this streamlined process, there is much less opportunity for the elimination of surface flaws with the

reduced number of hot rolling passes.

In cold rolling, as in hot rolling, lubricant chemistry strongly influences how much gauge reduction

is possible before lubricant failure occurs. Rolling in a mixed lubrication regime enables new surface to

be generated under controlled contact conditions and generally provides acceptable surface quality. This

minimizes problems related to the dull surface caused by transverse fissures generated with the thicker

lubricant films present in hydrodynamic lubrication. It also limits the adhesion and abrasion that can lead

to higher friction and greater heat generation occurring with minimal lubricant films. A pattern of marks

in the sheet called herringbone [21] can signal lubricant failure and a likelihood of strip breakage. The

greater power needed to drive the rolls under these boundary lubrication conditions may limit the output

of mills operating at or near peak power.

In the rolling process, the surface of the rolls also plays an important role. Rolls with freshly prepared

surfaces will have a higher roughness and generally more angular surface asperities that will tend to cut

into the aluminum surface more than rolls whose surface topography has become smoother in the course

of an extended rolling campaign. Thus, as the nature of the roll surface changes, so will the characteristics

of the debris formed. If a ground rolling surface is modified to provide improved wear life, through such

means as chrome plating, changes in friction can result from the material change as well as changes in

the topography resulting from the plating process. Both can affect the nature of the contact and debris

formed. As roll wear proceeds, the lubrication regime will shift towards hydrodynamic and when the limit

of the acceptable process window is reached, resurfacing of the roll will be required.

Most rolling is performed using rolls whose surfaces are prepared using grinding wheels that generate

a pattern resembling an array of canoes of different lengths aligned in the rolling direction. Alternate roll

surfacing techniques are also available, such as shot peening, electron beam, electro discharge, and laser

texturing. These methods provide an opportunity for alternate surface appearances in the finished part,

including more isotropic surface topography, and have begun to find commercial application. The targeted

tooling surface roughness will be based on the specific product and its surface finish requirements. Typical

ground surface roughnesses range from an Ra of about 1 to 3 µm for hot rolling down to about 0.1 µm

for foil. Very detailed decorative images can be imparted to sheet surfaces using embossing techniques.

The amount and characteristics of debris formed in the rolling process can be expected to depend not

only on lubricant chemistry, roll material and topography, alloy, and processing history but also on rolling

conditions. In the rolling process, sliding of the workpiece against the roll will occur at all points in the

arc of contact except for the neutral point, where the sheet and roll surface are moving at the same speed

(Figure 19.4). At this point, the sheet is changing from moving more slowly than the roll to faster. For

a given pass, higher levels of slip on entering and exiting the contact provide an opportunity for more

abrasive wear, as does the use of larger diameter rolls providing a longer arc of contact.

If rolling conditions are such that the roll surface develops and retains a significant level of aluminum

alloy and oxides, this roll coating can leave an imprint in the metal contributing to the orange peel

appearance. In hot rolling, where the roll coating is often substantial, scratch brushes are commonly used

to perform a continuous scrubbing action on the roll surface. This provides control of the amount of

roll coating and thus provides a roll surface to the rolling contact with more consistent topography and



Direction of travel

Neutral point

Direction of

rotation

Arrows adjacent to neutral point

show relative amount and

direction of metal speed

compared with roll speed

FIGURE 19.4 Sliding of strip on the roll bite.

friction characteristics. The brushes can be made of steel wire, which can lead to product quality problems

should any of the wires become detached and enter the roll bite, or a polymer such as nylon impregnated

with abrasive particles.

On the aluminum surface, some debris is relatively loosely bonded and can be removed and measured

as smudge by various wiping or cleaning procedures [22]. The ease of removal of this material tends to

diminish over a period of days. Some debris is pressed into the surface, but may be released by further

rolling contacts or stretching of the surface in which it is embedded. Where subsequent processing of the

rolled metal requires a cleaned surface, lubricant residues, including associated smudge, can be removed

using mildly to strongly etching basic or acidic cleaning media. A variety of solvents from petroleum

or renewable resources can be used to remove lubricant residues; however, such processes need to be

designed carefully to minimize emissions and fire risk. The use of chlorinated solvents for this application

is diminishing because of hygiene and environmental concerns. The cleanliness of the surface will reflect

the level of residue present in the cleaning medium and the efficiency of its removal, and can be assessed

by techniques measuring the water wettability of the surface.

It is a significant challenge to provide an optimized lubricant film throughout the rolling process that

enables finished product requirements to be met. Whether in hot or cold rolling, the lubricant or lubricants

must accommodate the needs of multiple passes through the rolls, whether in single stand or tandem mills,

as process conditions vary over a wide range. Wilson and Walowit [23] developed relations linking film

thickness for the rolling contact to a series of parameters, with the relation for inlet film thickness being

given in Equation (19.3).

h1 =3µ0γ a(U1 + V )

x1(1 − e−γ (σ−s))(19.3)

where h1 is film thickness at the inlet edge of the work zone, µ0 is fluid viscosity at atmospheric pressure,

γ is pressure coefficient of viscosity, a is roll radius, U1 is strip velocity at inlet, V is roll velocity, x1 is

distance to inlet edge of work zone (from line connecting axes of top and bottom rolls), σ is material flow

stress, and s is back tension stress.

An inspection of the equation provides insight into the influence of not only viscosity and speed

considerations, but other parameters, such as material flow stress, roll diameter, reduction, and the

pressure–viscosity properties of the lubricant. Fluid viscosity at temperatures found in cold rolling can

increase by roughly an order of magnitude under typical aluminum forming pressures.

Until relatively recently, experimental verification of the thickness of oil films in the roll bite had not

been achieved. However, x-ray work by Dow [24] and optical interferometry developments by Cameron

and coworkers [25] have enabled new insights into this area. In the interferometry method, the analysis

of interference patterns of light reflected from the contact between a roller and a partially reflecting

transparent disk enables a calculation of the lubricant film thickness. The method has been refined by

Spikes and coworkers [26] to enable the measurement of film thicknesses of under 1 nm under carefully



controlled conditions. This equipment, together with equipment that can monitor traction between metal

specimens under similar contact conditions, enables the determination of the influence of formulation

and surface topography over a moderate temperature range.

In the past decade, interferometry work has led to significant new understandings in the performance

of emulsions in aluminum metalworking. At lower speeds with oil-in-water emulsions, Zhu, et al. [27]

observed the formation of an oil pool in the inlet zone. The measured film thickness initially increased with

speed and was usually quite similar to the film thickness for the neat oil until the speed reached the “first

critical speed.” At this point, the film thickness was observed to decrease with increasing speed, although

in some instances it continued to increase at a reduced rate before beginning to decrease. As rolling speed

increased further, it was observed that a “second critical speed” was reached, at which point the film

thickness again increased with increasing speed. The observed film thicknesses remained less than those

for neat oil but were greater than those for pure water. However, the approach to the values for oil was

closer for destabilized emulsions or emulsions of higher concentration.

The mechanism of formation of the oil pool observed at lower speeds at the inlet is not well understood

in terms of the relative contributions of contacts of oil droplets with the surfaces outside the contact

region and the alternate mechanism of entraining droplets in the inlet region to the contact. In emulsion

lubrication, it has generally been believed that plate-out, which is the deposition of an oil film or the

wetting of the metal surfaces by the oil, is a key to providing an adequate oil supply to the contact. The

process of generating an oil-rich phase that enters the contact represents an inversion of the oil-in-water

emulsion. The efficacy of the plate-out process, which will influence the first critical speed, has been

related to the wetting of the metal surface by the oil droplets according to the nature and concentration of

the emulsifier system, as well as pH and water hardness [28]. These properties also influence the droplet

size and relative looseness or tightness of the emulsion.

A preference for oil rather than water to be entrained into the contact has been shown in studies with

single emulsion droplets [29,30]. However, above the first critical speed, which is variable and depends on

the specific conditions, there is insufficient oil available to the contact for a full film to be present. At this

point, the greater oil volume needed for a full film as both speed and film thickness increase could now

be considered to exceed the volume that is plating out. Alternatively, for a point contact situation such as

a bearing, a deficiency of lubricant or starvation might take place if repeated contacts by rolling elements

occur before surface tension can draw back the lubricant displaced in the previous contact [31].

Speeds near or above the second critical speed values of 0.15 to 5.0 m/sec of Zhu, et al. are representative

of commercial rolling speeds. For an adequate film to form at higher speed conditions, a mechanism of

wetting and film formation is needed that applies under relatively high shear conditions. A dynamic

concentration theory has been proposed that particularly applies to line, rather than point, contacts [29].

Since the film thickness in rolling contacts is commonly less than the emulsion droplet size, droplets

approaching the inlet will bridge the gap between the roll and the sheet. This is shown schematically in

Figure 19.5. With the narrowing of the gap as the contact zone is approached, the oil droplets will be

squeezed to a flatter shape and continue to be drawn in with their viscosity increasing exponentially with

pressure. As droplet concentration and size increase, the film thickness also will increase. The water is

preferentially left behind, inverting the emulsion. Continued work is needed to provide a clearer picture

of the mechanism(s) of lubrication at higher speeds.

OilWater

Phase inversion

FIGURE 19.5 Schematic representation of two-phase lubricant in the rolling contact.



19.4.2 Lubricants in High-Temperature Processes

Lubricants are used as forming aids and release aids in a number of higher temperature aluminum

metalworking processes that involve metal temperatures ranging from about 300 to over 700◦C. Such

processes include casting, extrusion, forging, and superplastic forming.

Aluminum is cast commercially in a variety of ways that place a range of demands on the lubricant.

Often, it is important that lubricants for casting act as release agents. However, where sliding occurs,

they must also provide lubrication and, in combination with the choice of tooling material, serve to

minimize adhesion of the very soft hot metal to the tooling and erosion of the tooling by the metal. In

roll casting, lubricant is continuously applied to the rolls that contact and squeeze the solidifying metal

to the desired thickness. Materials used in such applications often utilize carbon, whether as graphite,

which may be applied as an aqueous suspension, or soot deposited from a smoky flame translated across

the roll surface. Similarly, lubricants containing solids are commonly used with block or belt casters,

with a significant concern being the avoidance of excessive levels of residue that may impart a dirty

appearance to the aluminum surface after rolling. In die casting, a variety of water-based products are

used to lubricate moving parts used for metal transfer and to facilitate release of the solidified part with

good surface appearance. Lubricants used in direct chill ingot casting molds facilitate the formation of a

uniform surface layer on the solidifying ingot and the smooth travel of that surface against the mold while

controlling the formation of flaws that can lead to cracks in subsequent processing [32]. Although greases

have been used, commonly used lubricants include vegetable oils and derivatives based on oleates and

ricinoleates that have high flash points and are supplied to the metal-mold interface. Preferred materials

have high decomposition temperatures and function without leaving undesirable residues on the mold

surface or tenacious stains on the ingot surface.

In the hot extrusion process, lubricants containing solids such as graphite or boron nitride can be used

to lubricate the die surfaces, but frequently no lubricant is used [1]. With no facile way to apply lubricant

continuously to the die surface during the extrusion process, a functionally useful amount of lubricant can

be expected to persist only for a limited length of extrusion. The use of excess lubricant may give surface

quality defects on the extruded part. Cleaning the die with caustic material can provide the benefit of

removing aluminum pickup; such compositions can be combined with graphite. Tooling considerations,

such as the die surface composition and roughness, are important factors for minimizing adhesion of the

hot aluminum to the die.

The hot forging process also provides an array of significant lubrication challenges, which has led

to the development of products tailored for specific forging jobs. In certain applications, the lubricant

must provide excellent forming performance in parts where large amounts of fresh aluminum surface

are generated at die temperatures that can exceed 425◦C. In precision hydraulic forging applications, the

lubricant needs to provide good performance for net or near-net finish on key surfaces and may be called

upon to function during a contact time that can extend to tens of seconds. For such products, the ability

to capture fine detail is more important than excellent forming performance, but in either instance it

is important to avoid adhesion of the workpiece to the die and subsequent scuffing of the surface as it

slides over the die surface. The die surface material and topography also play a role here. The surface

requirements of many forgings require facile lubricant residue removal that does not rely on aggressive

etching.

Oil-based forging lubricants containing lubricious suspended solids such as graphite have typically been

applied to the die and often to the billet to facilitate forming. Hygiene and environmental improvements

have been made by switching from lead-based additives to products containing other carboxylates such

as those of tin and bismuth and by adjusting formulations so that they generate lower levels of particulate

and organic emissions. Increasingly tight environmental limits have driven an aggressive pursuit of water-

based products in recent years. A partial answer comes from the use of water-based precoats containing

solid lubricants that can be applied to billets that have been warmed to assist in forming an even coating.

Precoats can assist in making preforms or, where the forging requirements are demanding, can act as a

supplement to the usual lubricant.



With water-based lubricants, the rapid boiling and draining of the water upon contact with hot surfaces

inhibits efficient and uniform deposition of the lubricating components. Therefore, a key to success in

this process is the efficient deposition of lubricating components on hot die and billet surfaces without

resorting to high levels of organic components that contribute to emissions. Progress continues to be made

in the application and composition of water-based products such that they are approaching equivalence

to the older generation oil-based products.

Hot forming processes, such as superplastic forming of aluminum alloys for aerospace or automotive

applications, are typically lubricated with formulations based on boron nitride. Graphite has also been

used in recent works by Davies, et al. [33] and by Osada and Shirakawa [34]. In this process, the periphery

of the sheet is sealed in the die and the forming can be done using a gas pressure of no more than a

few megaPascals. The lubricant must facilitate the smooth movement of the metal over the die features

during a time period that may last up to tens of minutes. It must complement the die design to enable

forming without excessive part thinning, and should enable facile release of the hot and very soft formed

part from the die without distorting its shape. In addition, for faithful replication of the detailed shape

and good surface appearance, the lubricant must be applied in a very thin evenly distributed layer that

needs to adhere well to the aluminum surface. It must function without allowing a transfer or buildup of

aluminum on the die that can scuff the soft part as it forms and without the generation of accumulated

lubricant on the dies that can lead to imprinting of those residues into the formed part.

19.4.3 Lubricants for Forming of Finished Food and Beverage Containers

As noted previously, the lubricants used for the manufacture of sheet and foil products for food and

beverage packaging must comply with regulatory requirements. In the United States, the components

and limitations in 21 CFR 178.3910(b) provide a starting point for formulating lubricants for draw-

ing, stamping, or forming these articles. For components listed therein, the lubricant residue limit

on surfaces contacting the food or beverage is 0.2 mg/in.2 (310 mg/m2). In addition to good

metalworking performance, customers will frequently have further requirements based on dietary

or product compatibility considerations, or on organoleptic requirements involving taste and odor

evaluations.

A major share of the sheet rolled for food and beverage applications is used in the manufacture of

drawn and ironed cans, with U.S. annual production being in excess of 100 billion cans in recent years.

To make the can bodies, a series of lubricants are used that in most instances are not FDA compliant;

however, the finished can bodies are cleaned using etching solutions and then coated with an FDA-

compliant polymeric film that serves as a protective barrier to minimize any potential interaction with the

packaged product. Commonly, a reoil is applied to can body stock after rolling that serves to protect the

metal from scratching and moisture during handling, transporting, and storing. Together with the cupper

lubricant, it aids in the forming of a cup or preform that is shorter and wider than the final shape. The

cups are then converted into cans having the final diameter and full wall height by redrawing and ironing,

a high-speed operation that uses water-based lubricants [35]. These bodymaking lubricants can range

from soluble oils to synthetics (oil-free water-based formulations). Like some hot rolling formulations,

these compositions are often based on an emulsifying combination of fatty acids and amines, and can

also contain additional surfactants, such as nonionics, along with esters and oxidation and corrosion

inhibitors. Satisfactory performance in the bodymaker process requires compatibility with the reoil and

cupper lubricants, which are brought into the lubricant with the incoming cups, as well as an ability to

function well in the presence of gear oil that can leak in from the tooling. The avoidance of any additional

contaminants and the skimming of insoluble contaminants and byproducts from the lubricant surface

can aid in prolonging lubricant life. Microbial control is also a key for these products as is good control

of debris through filtration. The necking operation, in which the trimmed top of the cup wall is reduced

in diameter to enable the use of a smaller, lighter lid, can be aided with a light hydrocarbon solvent as

lubricant.



To the extent possible, good control of the composition is a key to consistent performance in the

bodymaker. As in the rolling process, the details of the lubricant composition and the droplet size distri-

bution of emulsion products can be expected to influence the lubricant film thickness and the tribological

conditions in this high-speed process. One flaw, referred to as bleed through, is a dark and irregular under-

cast on the decorated sidewall. This has been attributed by Knepp [36] to a high degree of hydrodynamic

lubrication during the deforming of the metal; however, Japanese workers have ascribed the problem to

lack of lubrication in the drawing and redrawing process [37,38].

High productivity in can plants requires a very low level of down time through the minimization of

problems such as unsuccessfully formed cans, known as tearoffs, or excessive wear of the tooling, which

is made of highly wear resistant material such as tungsten carbide. Consistent metal surface topography,

carefully tuned metallurgy, and careful filtering of the molten metal to limit inclusions are important for

both high productivity and the formation of cans with good surface characteristics. A second surface flaw

is looper lines, which are visible patterns in the can wall showing the roughness in the sheet imparted by

the ground rolls. If pronounced, they may not be satisfactorily covered by the can decoration. An alternate

strategy for the formation of drawn and ironed cans is the application of a polymer to the metal prior to

forming [39].

In many instances, the lubricant used for the forming operation is combined with or applied over a

polymeric coating on the aluminum. For example, aluminum can ends typically have a coating based

on epoxy, acrylic, or vinyl polymers that can contain an internal lubricant that blooms to the surface

as the polymer is cured. Such lubricants include waxes, lanolin, and polyethylene. External lubricants

or postlubricants can be applied onto the cured polymer, using electrostatic or roll-coating techniques,

with warming as necessary to facilitate application. Application from solvent solution is well suited to

high-speed lines but requires the management of significant environmental, health, and safety concerns

associated with the solvent. To a certain extent, the polymeric coating reduces the need for good film

strength in forming lubricants applied over it. Commonly used compositions include petrolatum and

paraffin, or blends of paraffin with materials such as lanolin. For waxy compositions, such as those

containing sizeable levels of paraffin, the potential for the build up of tenacious residues on the tooling

must be recognized. The lubricant, in combination with the coating and the metal, can be looked at as a

system that enables facile high-speed formation of high-strength ends with very low levels of flaws such

as splits in the more severely formed areas. The lubricant requirements include:

• Regulatory compliance

• Sufficient lubrication performance

• Ease of uniform application

• Absence of deleterious effects on coating performance

• Absence of undesirable interaction with the packaged product

• Absence of negative organoleptic effects

• Reliable supply of consistent product

• Ability to decorate ends over the lubricant if necessary

In the instance of packaged beverages, undesirable interactions can include such visual concerns as

an appearance of higher turbidity, undesirable changes to the consistency and longevity of foam on

beer, or the occurrence of “film float,” which is a visible film of lubricant on the surface of the product.

The presence of polyunsaturated fatty acids and their derivatives, typically linoleates, has been linked to

the formation of unsaturated aldehydes, which in trace amounts can have a very deleterious effect on

beer taste. Such products must be scrupulously avoided as lubricants or potential contaminants in this

application [40]. Minimizing or eliminating lubricants on the surface of finished can ends that contacts

the product can aid in minimizing undesirable product interactions.

Where no polymeric coating is present, the lubricant may be relied on to provide a higher level of

performance or film strength than is normally provided by such products as petrolatum and waxes. This

applies to the multi-step formation of the tabs used for easy-open aluminum can ends, since much of the

metal supplied currently is not coated with polymer. Because the potential exists for transfer of lubricant



from the tab on one lid to the product side of the end nested next to it in stacks of finished ends, the above

concerns about product compatibility can extend to tab forming lubricants. In certain other applications,

such as aluminum pie pans, the lubricant is called upon to minimize the release of any smudge remaining

on the sheet and, in effect, also serve as a coating. This provides a difficult challenge since boundary

additives tend to generate smudge and in many instances serve to solubilize existing smudge. On the other

hand, waxy materials, such as paraffin, can serve as a coating but provide little film strength by themselves

and are prone to cause die buildup.

AcknowledgmentsThe author thanks Alcoa Inc. for permission to publish and Simon Sheu for assistance with the figures.

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Aluminum Metalworking Lubricants