Melting and casting
Copper is melted either in fuel-heated or induction furnaces. Neutral or oxidising melting is recommended for copper. In the latter case, hydrogen absorption (high hydrogen solubility of the melt) is prevented by an excess of oxygen. Subsequently, deoxidisation must be carried out with suitable means, usually with a copper-phosphorus master alloy. Casting is the fastest process to get from raw metal to finished product. For shaping by casting, the same moulding and casting processes are used for copper materials as for iron and other non-ferrous metals. In principle, the copper casting materials are suitable for all conventional casting processes. For technical and economic reasons, by far the largest number of all castings made of copper materials are produced by the sand, chill-mould, centrifugal, continuous casting and – to a certain extent – die-casting processes.
Casting in permanent moulds
For the production of shaped castings made of copper and copper alloys, casting processes with permanent metal moulds are used on a large scale. Metal moulds achieve better dimensional accuracy and a better surface of the castings. In addition, a grain refinement and thus an improvement of the mechanical properties is achieved by the more abrupt cooling, due to a steeper temperature gradient in the metal mould. The main focus is on the gravity die casting process, known as precision gravity die casting when using drawable steel cores, and the centrifugal and continuous casting processes.
Gravity die casting
Besides the sand casting process, the gravity die casting process is the most frequently used casting process for copper and copper alloys. The moulds are mainly made of steels with high thermal shock resistance. Moulds made of cast iron and copper beryllium are also used. If, as in the case of precision gravity die-casting, the drawable cores are also made of metal, this is referred to as solid gravity die-casting. It should be noted, however, that certain undercuts in the mould are not possible with drawable cores. In gravity die casting, the die is tilted to achieve a low drop height and a quiet, turbulence-free, i.e. laminar, die filling. At the same time, the escape of air from the mould must be ensured. As the mould fills, the mould is raised. Mould filling must take place as quickly as possible, but must not exceed a maximum casting speed. The gravity die casting process offers the following advantages: Finer structure and higher strength than sand casting, better, smooth casting surfaces, dimensional accuracy and pressure tightness, avoidance of mechanical reworking and the possibility of recasting castings made of other metals. The only disadvantage is the higher moulding costs. As with other casting materials, the low-pressure die casting process is also used, but because of the larger castings it has no technical significance for automotive construction. The gravity die casting process is suitable for all copper casting materials with a narrow solidification range. These are copper, unalloyed and low-alloyed, copper-zinc and copper-aluminium casting alloys.
High-pressure die casting
In the high-pressure die casting process, the mould and cores are also made of a hardened hot-work steel. The mould is filled in a die-casting machine at high speed and solidification takes place under high casting pressure. As the copper casting materials have relatively high melting temperatures compared to other non-ferrous metals, the thermal shock stress in the mould is extremely high and the mould service life is relatively low when using copper alloys. For this reason, this casting process has not been able to establish itself for cast copper materials. It is only used to a limited extent for the comparatively low-melting copper-zinc casting alloys. The economic efficiency of the process is greatly impaired by low mould service lives. Of the copper casting materials, only the materials CuZn39Pb1Al-C-GP (GD-CuZn37Pb) and CuZn16Si4-C-GP (GD-CuZn16Si4) are suitable for the high-pressure die casting process.
Centrifugal casting process
The centrifugal casting process is only suitable for the production of rotationally symmetrical castings. In the centrifugal casting process, a rotary mould is set in rotation and the horizontally or vertically rotating mould is charged with liquid metal. The wall thickness or inner diameter of the solidifying tubular casting is determined by the amount of metal supplied. The cooling of the molten metal is very fast and can be increased by additional cooling. Due to the influence of the centrifugal force and the extremely fast solidification, a dense, fine-grained structure is achieved. This casting process is excellently suited for the production of plain bearing bushes. In addition to the continuous casting process, this casting process is of outstanding importance for the manufacture of initial moulds for plain bearing production. Centrifugal casting is particularly suitable for copper-zinc, copper-tin and copper-tin-zinc, as well as copper-aluminium casting alloys. The casting process is only conditionally suitable for copper-lead-tin casting alloys due to possible lead increases under the influence of centrifugal force.
The continuous casting process also belongs to the casting processes in permanent moulds. However, it is also of great technical importance for the manufacture of semi-finished products for the casting of certain moulds, such as rolled plates, extrusion billets, etc. Furthermore, the continuous casting process is used for the production of castings with finished part character, especially as starting formats for the production of plain bearings, bushings and sliding strips. In terms of casting technology, the continuous casting process is superior to other processes, since both continuous and semi-continuous continuous casting involve stationary casting and solidification conditions.In the continuous casting process, as much liquid metal is poured into a short, intensively cooled mould as is solidified and drawn off in the same time in the mould. The mould consists of either copper, low-alloyed copper or graphite. The “endlessly” solidifying metal strand, which is only solidified at the outer edge zones, continues to be intensively cooled (secondary cooling) during drawing and is cut into fixed lengths with a so-called flying saw. Stripping takes place in non-uniform steps, interrupted by small standstill or recoil times. A distinction is made between horizontal and vertical, as well as continuous and semi-continuous continuous casting. In the case of vertical continuous casting, the investment costs are higher due to the required high headroom; in the case of horizontal continuous casting, with thicker cross-sections there is a risk that the hot, not yet completely cooled cast strand will deform under the influence of gravity. In semi-continuous continuous casting, casting is interrupted each time the specified strand length is reached and then restarted each time. It is possible to cast hollow strands by means of mandrels that are fixed in the mould. All strands with a uniform cross-section can be produced, e.g. round, square or rectangular cross-sections, solid, hollow and flat sections. Since, similar to the centrifugal casting process, a strictly directed and rapid solidification takes place, the cast strand is continuously fed, resulting in a dense, fine-grained microstructure. The good mechanical material properties that can be achieved are identical for continuous casting and centrifugal casting formats. The process is excellently suited for the production of starting formats for plain bearing production. The continuous casting process is suitable for all cast copper materials. Starting shapes for plain bearing production are mainly continuously cast from copper-tin, copper-tin-zinc and copper-lead-tin casting alloys.
Beginning in the 1930s, the strip casting process for the production of thin-walled rolled bushings with cast-on copper-lead-tin alloy for internal combustion engine construction became established. In this process, so-called rolled three-material bearings are produced with an approx. 0.35 mm thick layer of lead bronze made of the alloy CuSn5Pb20-C-GS (G-CuPb20Sn). A so-called running-in layer of white metal (lead/tin 91/9 %) is then applied to this layer as an “overlay”. In the continuously operating strip casting plant, the strip of soft carbon steel, which later serves as a steel support shell, runs from the ring into an annealing and casting line at a constant speed in thicknesses of 1.1 to 3.3 mm and widths of 100 to 150 mm. The strip is bevelled on both sides to prevent the lead bronze from flowing off during pouring, then bright annealed and then poured with lead bronze. The composite strip is rewound as it leaves the annealing and casting section. On the processing machines, the strip is milled, re-rolled, ground and punched out to the strip size for the bearing or bushing to be produced. Recently, worm wheel rims made of copper-tin alloys have also been cast onto hubs made of grey cast iron or steel using the centrifugal casting, shell moulding or chill casting processes.
Powder metallurgical forming (sintering)
For sintered parts made of copper materials, suitable alloys are put together by mixing the corresponding powders. Pre-alloyed powders such as copper-zinc, copper-tin or copper-lead-tin alloys can be used. Pre-alloyed powder mixtures with a high lead content have the advantage that good mixing and fine lead distribution help to avoid lead sweating out during sintering. The powders are pressed into mouldings on mechanical or hydraulic presses. Pressing at room temperature is preferred. If very high compaction is desired, double pressing must be used. The moulded parts already have a certain strength due to mechanical clamping. The moulded parts obtain their final strength by sintering. This means that they are heated to temperatures below the melting point of the alloys to be sintered. In this process, alloy components with melting temperatures below the sintering temperature, such as lead, favour the sintering process, as the liquid phase increases the strength of the sintered parts. After sintering, the parts are calibrated, i.e. remoulded in a cold state, as the dimensions of the moulded parts change during pressing and sintering. The main area of application for sintered parts made of copper materials are oil-impregnated sintered bearings, so-called self-lubricating bearings, with a pore space of up to 30 %. The pores can also be filled with other lubricants, e.g. graphite.
Materials produced by powder metallurgy can be divided into conventional sintered materials and dispersion-hardened copper grades produced by powder metallurgy. Strictly speaking, the material “GLIDCOP” belongs to this group. There are no material standards for conventional sintered materials either. PM materials have the greatest significance for the topic to be dealt with here for the production of “self-lubricating plain bearings” from sintered bronze. This is based on mixed powder consisting of 90 % copper powder (electrolytically deposited or atomised) and 10 % tin powder. The powder is only compressed to the extent that a pore space of approx. 25 % remains. After sintering and calibration, the bushings are then impregnated with lubricating oil. In addition, copper or copper alloy powders are used for friction linings. As the high lead copper-lead-tin casting alloys tend to lead segregate and are difficult to cast, processes have also been developed for the production of steel-lead-bronze strips, in which the lead bronze is sintered on. This strip is also used for the production of rolled three-material plain bearings. However, since the strip casting process has been developed considerably further in the meantime and, in addition, plain bearings with cast-on lead-tin alloy have a higher load capacity than bearings with sintered-on lead bronze, this process is increasingly taking a back seat to the strip casting process.
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