LUBRICANT POWDER FOR POWDER METALLURGY

Abstract: The invention concerns a lubricant for powder metallurgical compositions containing 10-60 % by weight of a lithium salt of a fatty acid, 0-40 % by weight of a zinc salt of a fatty acid and 40-90 % by weight of a fatty acid bis-amide. 10-60 % by weight of the lubricant is made up by the lithium and the zinc salt.

Description: LUBRICANT POWDER FOR POWDER METALLURGY The present invention relates to a lubricant for metallurgical powder compositions as well as a metal- powder composition containing the lubricant. The inven ¬ tion further concerns a method for making sintered products by using the lubricant. The powder metallurgy industry has developed iron- based powder compositions that can be processed into integral metal parts having various shapes and sizes for uses in the automotive and electronics industries. One processing technique for producing the parts from the base powders is to charge the powder into the die cavity and compact the powder under high pressures. The resultant green part is then removed from the die cavity and sintered. To avoid excessive wear on the die cavity, lubricants are commonly used during the compaction process. Lubrication is generally accomplished by either blending a solid lubricant powder with the- iron-based powder (internal lubrication) or by spraying a liquid dispersion or solution of the lubricant onto the die cavity surface (external lubrication) . In some cases both techniques are used. Almost all currently used lubricants are de ¬ rived from naturally occurring long-chain fatty acids. The most common fatty acid is stearic acid (C 1 - 7 H- 35 COOH) consisting of an aliphatic chain CH 3 ' (CH 2 )i6 combined with the carboxylic acid group -COOH. When mixed with metal powders, it provides fast flow, high apparent density and good lubricity. Its low melting point (64°C) can lead to softening during blending with the powder causing problems. Therefore, salts of stearic acid, i.e. metallic soaps are more popular. The major drawback of the soaps is their metal content. On burn- CONFIRMATION COPY off, the fatty acid chain volatilizes readily but the metal remains behind as oxide or carbonate, although this may undergo reduction to the metal in a reducing atmosphere . The most widely used metallic soap is zinc stearate because of its good flow properties. In reducing atmospheres, the zinc oxide remaining after initial decomposition is reduced to zinc, which readily volatilizes because of its low boiling point (907°C). Unfortunately, on contacting the cooler parts of the furnace or the outside atmosphere, the zinc tends to condense, forming some zinc oxide as well. A consequence of this condensation is that the production has to be interrupted as the furnace has to be cleaned regularly. The problems associated with metallic soaps can be avoided by the use of completely organic materials such as waxes. The one most widely used in powder metallurgy is ethylene-bissteara ide (e.g. Acrawax C) . This material has a high melting point (140°C) but it burns off at relatively low temperatures and leaves no metallic residue. The most serious disadvantage is its poor flow behaviour in metal powders. Furthermore, mixtures of zinc salts of fatty acids and fatty acid bis-amides have not been accepted the P/M industry because of the poor performance of such mixtures . It has now unexpectedly been found that a lubricant enabling the manufacture of compacted products having high green strength and high green density in combination with low ejecting force can be obtained with a lubricant comprising a lithium and optionally a zinc salt of one or more fatty acids and a fatty acid bisamide product. More specifically the amount of the metal salts of the fatty acids should constitute about 10 - 60 % by weight of the lubricant according to the invention. The amount of the lithium salt is 10 - 60 % by weight and the amount of the zinc salt is 0 - 40 % by weight. Preferably the amount of the zinc salt is at least 10 and most preferably at least 15 % by weight of the lubricant. The amount of the bisamide product is 40 - 60% by weight. Typical examples of lithium salts of fatty acids are lithium laurate, lithium myristate, lithium palmi- tate, lithium stearate, lithium behenate, lithium mon- tanate and lithium oleate which are lithium salts of fatty acids having 12~28 carbon atoms. Typical examples of zinc salts of fatty acid are zinc laurate, zinc myristrate, zinc palmitate, zinc stearate, zinc behenate, zinc montanate and zinc oleate which are lithium salt of fatty acids 12~28 carbon atoms . Typical examples of fatty acid bis-Amides are methylene bis-lauramide, methylene bis-myristamide, methylene bis-palmitamide, methylene bis-stearamide, ethylene bis-behenamide, methylene bis-oleamide, ethylene bis-lauramide, ethylene bis-myristamide, ethylene bis-palmitamide, ethylene bis-stearamide, ethylene bis-behenamide, ethylene bis-montanamide and ethylene bis-oleamide. The lubricant is preferably prepared by mixing and melting the components and the obtained mixture is subsequently cooled and micronized to a suitable particle size . The invention is further illustrated by the following non limiting examples. Examples 1-5 5 different lubrication samples having the composition shown in the following Table 1 were prepared. Table 1 Example No. 1 2 3 4 5 Lithium stearate (% by weight) 10 35 60 20 20 Zinc stearat (% by weight) 0 0 0 15 40 Ethylenebis-stearic acid amide 90 65 40 65 40 (% by weight) Atomized steel powders (10 kg) were mixed with the sample lubricants 1-5(80 g) and each powder mix was investigated as regards apparent density, green density (at 5 and 7 ton/cm 2 ) , ejection force, green strencth and sintered density. The sintering was carried out at 1120°C x 30 min. with base (?) atmosphere. The results are disclosed in table 2. Table 2 Example No. 1 2 3 4 5 Apparent density of raw material 3.16 3.20 3.25 3.25 3.25 before compacting (g/cm 3 ) Ejection pressure Compacting 102 105 106 104 106 of compact pressure (kgf/cm 2 ) 5 ton/cm 2 Compacting 117 114 120 115 121 pressure 7 ton/cm 2 Density of compact Compacting 6.95 6.96 6.95 6.95 6.94 (g/cm 3 ) pressure 5 ton/cm 2 Compacting 7.14 7.10 7.11 7.14 7.10 pressure 7 ton/cm 2 Strength of Compacting 131 135 130 137 130 compact (kgf/cm 2 ) pressure 5 ton/cm 2 Compacting 181 188 182 192 183 pressure 7 ton/cm 2 Density of Compacting 6.94 6.95 6.93 6.96 6.95 sintered compact pressure (g/cm 3 ) 5 ton/cm 2 Compacting 7.14 7.11 7.11 7.13 7.10 pressure 7 ton/cm 2 Subsequently 5 different lubrication samples (comparative examples 1-5) having the compositions shown in the following Table 3 were prepared for comparison. Table 3 Comparative example No. 1 2 3 4 5 Lithium stearate (% by weight) 100 0 0 65 0 Zinc stearat (% by weight) 0 100 0 35 35 Ethylenebis-stearic acid amide 0 0 100 0 65 (% by weight) These samples were tested in the same way as above and the results are shown in table 4. Table 4 Comparative example No. 1 2 3 4 5 Apparent density of raw material 3.44 3.22 3.02 3.09 3.35 before compacting (g/cm 3 ) Ejection pressure Compacting 128 125 118 127 118 of compact pressure (kgf/cm 2 ) 5 ton/cm 2 Compacting 141 140 134 145 135 pressure 7 ton/cm 2 Density of compact Compacting 6.88 6.85 6.77 6.81 6.87 (g/cm 3 ) pressure 5 ton/cm 2 Compacting 7.01 6.99 6.88 6.95 6.98 pressure 7 ton/cm 2 Strength of Compacting 109 105 119 106 120 compact (kgf/cm 2 ) pressure 5 ton/cm 2 Compacting 146 149 162 150 161 pressure 7 ton/cm 2 Density of Compacting 6.87 6.86 6.79 6.83 6.86 sintered compact pressure (g/cm 3 ) 5 ton/cm 2 Compacting 6.99 6.98 6.88 6.96 6.98 pressure 7 ton/cm 2 Example 6 The lubricant used in the production of green compacts by sintering in a large-size sintering furnace (production amount about 200 ton/month) and a medium- size sintering furnace (production amount about 100 ton/month) was changed from zinc stearate which had been used for many years (Comparative example 6) into a powder lubricant prepared with the weight ratios shown in Table 5 (Example 6) . As the result, when the inside of the furnace had been periodically cleaned at the frequency of three times a year when using zinc stearate, the furnaces had not been stopped for cleaning of accumulated matter even after 1.5 years had passed after the change of the lubricant, and no remarkable accumulated matter was noted even after that. Table 5 Comparative Chemical Component Example No. Example No. 6 6 Lithium stearate (% by weight) 20 0 Zinc stearate (% by weight) 15 100 Ethylenebis-stearic acid amide 65 0 (% by weight) Effect of the invention As is apparent fro the Examples 1-6, this invention can provide a powder lubricant for powder metallurgy that can achieve a high bulk density when a metal powder is packed into a metal mould, a low ejection pressure from the metal mould, an improved density and strength of the formed compact, an improved density of the sintered compact, with no contamination of the sintering furnace.

Claims: CLAIMS 1. Lubricant for powder metallurgical compositions containing 10-60 % by weight of a lithium salt of a fatty acid; 0-40 % by weight of a zinc salt of a fatty acid and 40-90 % by weight of a fatty acid bis-amide characterized in that 10-60 % by weight of the lubricant is made up by the lithium and the zinc salt. 2. Lubricant according to claim 1 characterized in that the fatty acid is selected from the group con- sistmg of saturated or non-saturated fatty acids having 12-28 carbon atoms. 3. Lubricant according to claim 2 characterized in that the fatty acid bis-amide is ethylene bis- stearamide. . Lubricant according to anyone of the preceding claims characterized in that the amount of the zinc salt is at least 10, most preferably at least 15 % by weight of the lubricant. 5. Lubricant according to anyone of the preceding claims characterized in that it is in the form of a molten, micronized powder. 6. A metal-powder composition containing an iron- based powder and a lubricant according to any one of the preceding claims.

Inventors: SAITOH HIFOYASO, SERITA TOSHIO, SUZUKI MASAAKI, UKAI NORIO, SAITOH, HIFOYASO, SERITA, TOSHIO, SUZUKI, MASAAKI, UKAI, NORIO
Applicants: HOEGANAES AB, SAITOH HIFOYASO, SERITA TOSHIO, SUZUKI MASAAKI, UKAI NORIO, SAITOH, HIFOYASO, SERITA, TOSHIO, SUZUKI, MASAAKI, UKAI, NORIO
Application year: 1997
Main classification: B22F1
IPC codes: B22F1, B22F3, C10M105, C10N10, C10N20, C10N40, C10N50