Cross-Wedge Rolling Theory, Technology and Equipment
In large-scale and mass production, the technology of cross-wedge rolling (CWR) of axisymmetric parts made of ductile metals and alloys is virtually unequaled in its economic efficiency. The only exception is parts produced by cold stamping.
The application domains of cross-wedge rolling (CWR) include the manufacture of shafts, axles, fingers and pins for aircraft, automobiles, railroad, tractors, devices, household appliances, and instruments (e.g., instruments for mining and road construction, agricultural equipment, etc.).
The scheme of the CWR process is shown in Fig. 1. A blank that has been previously formed on a press is placed across a lead-in part of a fixed wedge tool. A movable wedge tool moves parallel to the fixed tool and penetrates into the billet, causing its rotation. Both wedge tools have inclined lateral faces (Μ) that make metal surplus move toward butt ends, hence extending the billet. The rest of the metal is profiled by gauge surfaces (Κ) of the tool and acquires their negative profile. Thus, the billet is being continuously rolled along the fixed wedge tool, while the required profile of the part is being shaped from its center to the butt ends. At the final stage of rolling, the metal surplus is cut off from the finished part by specific knives (Η) that are mounted on both sides of the tool. The rolled part is removed, and the movable wedge retracts to the start position.
In order to reduce rolling forces and improve the plastic properties of metal, the billet is heated up to 597-1197°C (1107-2187°F). The heating method is arbitrary, but RF-current is more often used because it provides full automation of heating and little metal loss.
The main CWR parameters are the reduction ratio and tool configuration. It is common practice to define the reduction ratio as:
δ= D/d (1)
where D is the initial diameter of the billet and d is the diameter of the rod after rolling (Fig. 1).
The range of sizes of rolled parts covers almost all industrial requirements: diameters from 1-190 mm (0.039-7.48 inches) and length from 20-1,000 mm (0.787-39.37 inches). The materials applied in rolling are constructional steels, steel alloys, titanium alloys, nickel alloys, copper alloys, aluminum alloys and zirconium alloys. As an example of CWR, Fig. 2 shows parts made from a heat-resistant nickel alloy.
CWR is often used in combination with conventional forging, which can be carried out both prior to and after rolling. In these cases, both processes are usually performed without reheating. In this manner, the forging of aircraft engine blades, automobile universal-joint yokes (Fig. 3), railroad screws, spanners and other extended-axis blanks is carried out. Accurate batching of billets at rolling provides improved durability of the tool at forging, and this makes flash-free forging possible in some instances. Determination of the billet size prior to forging is carried out on the basis of the conventional method. One measures cross sections of the blank together with the flash and determines billet diameters at these points in such a way that the sectional area before forging is equal to the sectional area after forging. The accuracy of these calculations is monitored by computer simulation of forging.
CWR is divided into hot rolling that implies heating up to 897-1197°C (1647-2187°F), warm rolling that implies heating up to 597-797°C (1107-1467°F) and cold rolling that suggests temperatures up to 17°C (63°F). Heating up to 897-1197°C (1647-2187°F) ensures high plasticity of rolled material and, as a consequence, results in a reduction of rolling forces and a reduced risk of an axial cavity opening inside the rolled billet. Warm rolling reduces the power consumption allocated for heating, prevents scale origination and decarburization of the surface of the rolled part, and improves the surface roughness up to 0.6 Ra. While the rolling forces increase, the wedge angle (β) should be decreased to a great extent, which decreases the process output, in order to avoid axial cavity.
Cold CWR has limited application and is used for high-ductile metals such as zirconium (Fig. 2a) or copper and zinc alloys. In the latter case, the wedge angle (β) is made within the range of 0.5-2 degrees.
Not only cylindrical billets but square-section or hexagon-section billets can also be used as starting stock, and these shapes can be partially preserved in rolled parts. Items with hexagonal or square cross sections are manufactured in this manner. Pipes can also be used as initial billets. Depending on its initial size, the thickness of the pipe can increase or decrease during rolling.
CWR technology (the method of reversible rolling) can be designed in such a way that the initial diameter is increased by 1.2-1.4 at specific length-bounded sections. Manufacturing of some eccentric surfaces on rolled steps of shafts is also feasible.
The dimensional accuracy of rolled billets is mostly affected by their size and the heating temperature. Thus, for hot rolling, the accuracy is ±0.2 mm for a diameter of 30 mm and ±0.5 mm for a length of 100 mm. For warm and cold rolling, the dimensional accuracy is ±0.01mm for a diameter of 7 mm and ±0.1 mm for a length of 40 mm.
A distinctive feature of CWR, as compared to other metal-forming processes, is high homogeneity of the stress-strain state in the deformation zone. In the billet-tool contact area, compression stresses and large accumulated strains are observed. In the axial area of the billet, one observes tensile hydrostatic pressure and large alternating strains, which can lead to metal failure in the form of axial cavity opening, known as the Mannesmann effect. In other areas (between the contact area and the billet axis) the hydrostatic pressure approaches zero, and the accumulated strains are smaller than in the two previously mentioned areas.
Compression stresses and large accumulated stresses in the near-contact (surface) area of the billet cause sealing of metal micro-defects, which leads to an increase in fatigue strength and wear resistance of the whole item. Thus, fatigue strength of the rolled automobile ball pins has increased by 2.5 times in comparison to that of the stamped parts, and wear resistance of rolled conveyor axles has increased by 20% as compared to the turned parts.
CWR of the same item can be carried out by various tools that have different angles, α and β. This leads to differences in energy consumption. In order to determine an optimal value of energy consumption for rolling, a notion of the efficiency coefficient has been introduced. This defines the ratio of the theoretical energy consumption by the actual energy consumption in the course of straining. With CWR, the efficiency coefficient varies from 3.1-10.4%, which is much smaller than other metal-forming processes. This factor, strange though it may seem, has a positive effect. The more energy spent at plastic straining, the better the refinement of metal structure.
The CWR process is restricted by three factors:
- Off-contact straining and breakage of a sample caused by axial stresses that exceed the metal yield point
- Loss of bonding between the billet and the tool when the friction forces on the contact are not enough to turn a billet
- Axial cavity opening in a sample due to low metal plasticity
The restrictions can be overcome through the correct selection of values for the tool angles (α and β), the reduction ratio and the rolling temperature. An exception is a low-plasticity material, which can be overcome, for example, by CWR in a high-pressure chamber, as has been theoretically explained by the authors.
The metal utilization coefficient is quite high for a metal-forming process and is within the range of 0.8-0.98. The value of 0.98 is rarely achieved when shrinkage holes are tolerated at the butt ends of the rolled billet and the process is carried out without cutting off the waste ends. The authors have developed a technology that increases the metal utilization coefficient due to decreasing the weight of waste ends. For this purpose, the cutting of piece blanks is carried out when they are heated and cut by two rolls with simultaneous formation of conic surfaces at the butt ends. Immediately after cut-off, the billet is transferred to the CWR mill, and rolling without reheating is performed. The conical butt ends reduce the shrinkage hole and the weight of waste ends by 3-4 times.
Research on the CWR stress-strain state enabled the authors to offer a new way of manufacturing laminar shafts. High compression stresses and accumulated strains in the surface layers of the rolled billet allow welding in this area. If one puts a thin plug made of one material on the base billet made of another material, heats the obtained assembly and rolls it, the two materials become finely welded together (Fig. 4). Analysis of the pull and shear strength of the joint has shown its high strength, which is as good as the strength of the weaker material of the welded pair.
The new design of laminar shafts can be effectively used in order to increase strength and corrosion resistance and reduce weight. The research has shown that the welded joint does not fracture at subsequent plastic straining. Therefore, laminar shafts can be applied in production of laminar blanks as well as laminar sheets. The new technology of CWR of laminar parts is advanced and can be applied in aircraft and spacecraft industries.
The main advantages of cross-wedge rolling are:
- High equipment production capacity (up to 1,000 parts/hour)
- Favorable structure of material fibers
- Ease of maintenance
- High accuracy and maximum proximity to required dimensions of finished products
- Minimum waste (billet-use coefficient is as high as 0.98)
- Opportunities to produce a wide product range using the same equipment
- Die change-over time between profiles is reduced to five minutes
- High die life (up to 1,000,000 parts)
Contact Val Leyzeruk, Nesso Automation Solutions, LLC, Detroit, Michigan; tel: 248-254-3773; e-mail: firstname.lastname@example.org. The authors are V.Y. Shchukin and G.V. Kozhevnikova, Physical-Technical Institute of the National Academy of Sciences of Belarus, Minsk, Belarus.
- Kozhevnikova, G. Cross-wedge rolling / G. Kozhevnikova. – Minsk: Belorusskaya nauka, 2012. – 321c.
- Shchukin, V.Y. Cross-wedge rolling at PTI NAS Belarus / V.Y. Shchukin, G.V. Kozhevnikova, V.V. Petrenko // Applied Mechanics and Materials. – 2012. – Vol. 201–202. – P. 1198–1202.
- Method of Manufacturing of the Axisymmetric Laminar Step Part: patent 13417 Republic of Belarus, IPC B21 H 1/00. / V.Y. Shchukin, G.V. Kozhevnikova; patentee PTI NAS Belarus. – No. a20081431; application of 13 Nov. 2008; published on 30 Aug. 2010 // Official Register / National Center of Intellectual Property. – 2010. – No. 4. – P. 74, 75