Systems for Cold Forging
The choice of which lubricant to use for a cold forging process depends on the severity of the operation (i.e., process parameters such as inter- face pressure and surface expansion) and the pa- rameters associated with the billet itself (i.e., hardness). For example, upsetting to small di- ameter-to-height ratios does not require the same lubricant performance (lubricity) as backward extrusion processes. In addition, the lubrication systems used for steel may be very different from those used for aluminum or titanium [Schey, 1983]. Figure 7.4 shows an example of this lubricant selection process for cold forging of aluminum alloys [Bay, 1994].
Fig. 7.4 Lubricant selection based on deformation severity. [Bay, 1994]
7.6.1 Ferrous Materials
Carbon Steels. Because of the severe defor- mation conditions typical of many cold forging operations, the most widely used lubrication sys- tem in the cold forging of carbon steels is a zinc phosphate coating and soaping system (Fig. 7.5). However, simple forging processes such as light upsetting may be completed without lubri- cation or with a simple mineral oil. In the zinc phosphate coating lubrication system, the basic workpiece material is first cleaned to remove grease and scales and then dipped in a zinc phos- phate solution. Through chemical reaction, a zinc phosphate layer on the order of 5 to 20lm is formed. The coated billets are then dipped in an alkaline solution (usually sodium or calcium soap), resulting in firmly adhered and adsorbed layers of alkaline soap, zinc soap, and phosphate crystals with very low shear strength. The typi-
cal procedure for applying this lubrication sys- tem to a billet is described in Table 7.1 [Altan et al., 1983] [Bay, 1994] [Manji, 1994] [ICFG, 1996].
Process parameters such as bath age and tem- perature, process time for phosphating and lu- brication, and type of activators influence the properties of the zinc phosphate coating and soaping lubrication system. In general, thicker coating or soaping layers are obtained by allow-
Fig. 7.5 Zinc phosphate coating and soaping lubrication sys- tem. [Bay, 1994]
Table 7.1. Treatment sequence for zinc phosphate coating of steel billets for cold forging
Bath temperature
Operation ⬚F ⬚C Process time, min
Cleaning
Degreasing in alkaline solution 140–205 60–95 5–15
Rinsing in cold water N/A N/A N/A
Removing scale, usually by pickling but occasionally by shot blasting 104–160 40–70 1–5
Rinsing in cold water and neutralizing (if pickling used) N/A N/A N/A
Dipping in warm water with activators N/A N/A N/A
Phosphating
Dipping in zinc phosphate solution 130–205 55–95 5–10
Rinsing in cold water and neutralizng N/A N/A N/A
Lubrication
Lubricating with sodium soap 160–175 70–80 0.5–5
Drying N/A N/A N/A
Source: [Altan et al., 1983] [Bay, 1994] [Manji, 1994] [ICFG, 1996]
ing the billets to remain in the zinc phosphate solution or the sodium soap solution for longer periods of time. However, care should be taken because excessive amounts of lubricant could cause dimensional tolerance errors or unsatis- factory surface finish [Bay, 1994] [Lazzarotto et al., 1999].
Despite its success as a cold forging lubrica- tion system, the zinc phosphate coating has sev- eral disadvantages. Hence, today, lubricant man- ufacturers are attempting to design replacements for this lubrication system [Ngaile et al., 2002]. The disadvantages are summarized as follows [Schmoeckel et al., 1997]:
Profitability
● The initial purchase, as well as the mainte- nance, of the zinc phosphate coating and soaping line is expensive.
● Removal of the zinc phosphate layer is dif- ficult and thus expensive.
Productivity
● The zinc phosphate coating and soaping pro- cess is time consuming.
Energy Usage
● It is necessary to heat multiple baths to tem- peratures between 105 and 205 ⬚F (40 and 95⬚C).
Worker Environment
● Dust accumulates as a result of surface en- largement during forging. This dust is a health risk to the workers in the facility.
● The baths are a source of toxic chemicals and fumes, which lead to unhealthy working conditions.
Waste Removal
● The baths contain acids, ion of the basic metal, the alloying constituents, and phos- phates. The wastewater contains organic compounds and emulsifying agents. After phosphating, the baths become polluted with heavy metals like lead and cadmium. The wastewater treatment and the baths result in solids, which contain metals, oils, and other pollutants. Most of this waste cannot be re- used and thus becomes hazardous waste. Mechanical Properties of the Billets
● Zinc phosphate can increase corrosion and diffuse into the workpiece material during heat treatment. This is a common cause of surface embrittlement.
Even though zinc phosphate coating based lu- brication systems are the most widely used, cold forging of carbon steels involves a wide variety of processes and thus a wide variety of lubrica- tion systems. These processes and lubrication systems are summarized in Table 7.2.
Table 7.2 Lubrication systems for cold forging of steel
Process Deformation Lubricant
Upsetting Light None
MiⳭ EP Ⳮ FA
Severe PhⳭ SP
Ironing and open-die Light PhⳭ Mi Ⳮ EP Ⳮ FA
extrusion Severe PhⳭ SP
Extrusion Light PhⳭ Mi Ⳮ EP Ⳮ FA
Severe PhⳭ SP
PhⳭ MoS2
PhⳭ MoS2Ⳮ SP
Mi, mineral oil; SP, soap; EP, extreme pressure additive; Ph, phosphate coating; FA, fatty additives. Source: [Bay, 1994]
Table 7.4 Lubrication systems for the cold forging of copper and titanium
Process Deformation Lubricant
Copper
Upsetting Light Emulsion
Severe Mineral oil
Extrusion Light Emulsion
Mineral oil
Severe Soaps
Titanium
Upsetting Light Emulsion or mineral oil
Severe Copper coatingⳭ soap
Fluoride-phosphate coatingⳭ soap
Extrusion Light Copper coatingⳭ soap
Fluoride-phosphate coatingⳭ soap Severe Copper coatingⳭ graphite
Fluoride-phosphate coatingⳭ graphite
Source: [Schey, 1983]
Table 7.3 Alternative conversion coatings for aluminum billets for cold forging
Bath temperature
Process time,
Phosphating operation ⬚F ⬚C min
Dipping in zinc phosphate solution 130–150 55–65 5–10 Dipping in calcium aluminate solution 140–175 60–80 5–15 Dipping in aluminum fluoride solution 185–195 85–90 5–10
Source: [Bay, 1994] [ICFG, 1996]
Extreme-pressure additives include chlorine, sulfur, and phosphorus. These additives react with carbon steel surfaces to produce excellent barriers (boundary lubrication) against metal-to- metal contact. In severe extrusion operations, the bulk surface temperature may exceed the melting point of the soap. In these cases, mo- lybdenum disulfide (MoS2) is used in place of
the soap. In the most severe operations, the en- tire phosphate coating is replaced by a thin cop- per coating [Schey, 1983].
Stainless Steels. For cold forging of stainless steels and other steels containing more than 5% Cr, an oxalate coating is used in place of the phosphate coating. This is done because it is dif- ficult to phosphate these materials [Schey, 1983].
7.6.2 Nonferrous Materials
Aluminum. Lubrication in the cold forging of aluminum is especially important because of the high adhesion between the aluminum and the die material. The lubrication systems used for the cold forging of aluminum are given in Fig. 7.4. In general, one of three conversion coatings are used with aluminum; namely, zinc phosphate, calcium aluminate, or aluminum fluoride. The lubricants used with these conversion coatings include soaps and molybdenum disulfide. The general treatment sequence for these conversion coatings is the same as described in Table 7.1; however, the type of conversion coating deter- mines the bath temperature and process time as shown in Table 7.3 [Bay, 1994] [ICFG, 1996].
Copper. Lubrication systems for cold forging of copper are summarized in Table 7.4. It should be noted that many EP additives are useless when used in a mineral oil for lubrication of a copper alloy because they do not react with the copper surface to create a barrier (boundary lu- brication) to withstand metal-to-metal contact as they do with carbon steels. In addition, sulfur
additives stain the copper [Schey, 1983] [Gariety et al., 2002].
Titanium. Cold forging of titanium has very limited application. However, in those limited applications, the lubrication systems can be summarized as shown in Table 7.4 [Schey, 1983].