DE102005027259B4 - Process for the production of metallic components by semi-hot forming - Google Patents

Abstract

Process for the production of metallic components by semi-hot forming of blanks of alloys with superplastic structure, characterized in that the elongation of the blank in the half-warm forming below 50% of the achievable by a superplastic forming strain values and at strain rates (ε ') performed which is at least 100 times that for superplastic forming.

Description

The invention relates to a method for the production of metallic components by semi-hot forming of blanks from alloys with superplastic structure, in particular of UHC steels, steels, Al or Ti alloys at high strain rates, according to the features of claim 1.

In mechanical engineering but also in the automotive industry a wide variety of shaping processes for the production of complex shaped components made of metals are known. These include, but are not limited to, extrusion, cross rolling, hole pressing, rotary swaging, gear cutting, swaging or hydroforming, and forging.

If these shaping processes are carried out at low temperatures, then only very slow forming speeds can be achieved in some cases. If forming is faster, the forming pressures must be increased to very high levels at the expense of tool wear and overall tooling costs. On the other hand, if the shaping processes are carried out in the high-temperature range, in particular at the forging temperature, the tool costs increase considerably, since only expensive high-temperature tools, possibly ceramic ones, can be used. Where appropriate, multi-step procedures are required. In addition, the raw materials to be reshaped are usually oxidatively damaged without special precautions. This leads, for example, to scaling of steels or excessive oxidation of titanium alloys. Before the further processing of the components produced thereby must be reworked at least on the surface.

Of further importance for the economy of the molding process is to realize the highest possible degree of deformation corresponding to a high elongation (ε). Only then are complex components made available in a single forming step.

Superplastic metals offer a high potential in mechanical engineering and the automotive industry to produce components with high degrees of deformation. Superplastic alloys are for example from the FR 274 1360 A1 . US 567 2315 . EP 1 252 352 A1 , or the US 2001 020 502 known. Further state of the art for superplastic alloys is from the published patent application DE 21 62 533 A known.

Superplasticity is the ability of a material to withstand only very low flow tensions without constriction and virtually no work hardening to withstand degrees of deformation, which compared to those usual in the normal plastic materials about 10 to 40% by some 100 to more than 1000%. There is an extremely high Gleichmaßdehung. An essential feature of the superplastic behavior of materials is the strong dependence of the flow stress on the strain rate or strain rate (ε ').

The superplastic deformation proceeds via temporally controlled diffusion processes in which the very fine and uniform, often also roundish crystallites slide past each other and rotate. Therefore, only a narrow process window of temperature and strain rate (strain rate (ε ')) is given to achieve the superplastic strain strain values at some 100 to 1000%. Typical examples are an elevated forming temperature above about 50% of the melting temperature and a very low forming speed of about 10 ^-2 to 10 ^-5 s ^-1 .

For aerospace applications, superplastic titanium alloys are of particular interest. For example, titanium alloys TiAl6V4 are transformed at about 925 ° C and TiAl5Fe2.5 at 870 ° C. In this case, pressures of about 10 bar, or 1 N / mm ² are used. The temperatures required for superplastic forming necessitate forming tools with good creep resistance at high forming temperatures. Due to the very high reactivity of titanium alloys at high temperatures, the forming must be carried out in vacuo or under protective gas. Since the risk of welding the workpiece with the tool is very large, the tools must be protected.

In contrast to the aerospace industry, mass-produced components, in particular of steels, are of great interest in mechanical engineering and the motor vehicle industry. In mass production, a high process speed plays an essential role in keeping the costs for mass production low. The forming speeds of the superplastic forming are generally not acceptable for the series production of components.

It is therefore an object of the invention to show cost-effective method for the mass production of highly formed metallic components.

The object is achieved by a method for the production of metallic components by semi-hot forming of blanks from alloys with superplastic structure, wherein the forming pressure in the forming tool at least 20% below that required for forging Forming pressure of the corresponding alloy is maintained and the strain rate (ε ') of the semi-hot forming is set to values above 0.1 / s, and according to the invention by a method for producing metallic components by semi-hot forming of blanks of alloys with superplastic structure, wherein the elongation (ε) of the blank in the semi-hot forming is carried out below 50% of the elongation values obtainable by superplastic forming and at strain rates (ε ') which are at least 100 times that for the superplastic forming. The forming pressure is often referred to as the corresponding yield stress.

The method according to the invention thus makes use of the superplastic structure of metallic alloys in order to utilize high forming speeds in comparison to the maximum values of the superplastic forming of comparatively low strains. On the high strains that would be possible in forming in the superplastic region is omitted according to the invention in favor of a high process speed.

As a rule, the metallic alloys are not in a structural state after their metallurgical production, which would have superplastic properties. Only by a special thermo-mechanical treatment, a structure is formed, which has superplastic properties, hereinafter referred to as superplastic structure. For example, to achieve the ultrafine crystallites required for the superplasticity of UHC steels, two phases must be formed to prevent grain growth. The corresponding phases are ferrite and cementite or other carbon-rich phases. In order to adjust this structure, initially relatively homogeneous material is made from pearlite, which is a lamellar mixture of ferrite and cementite. In a second step, this perlite structure is converted into the superplastic microstructure, in which the carbides are predominantly spheroidal (spherical) and the ferrite ultrafine-grained.

The method according to the invention thus offers the advantage that a considerable reduction of the forming steps can be achieved for a large number of alloys, in particular high-strength steels.

The transformation can be carried out already at the middle temperatures of the half-warm forming. These are temperatures well below the forging temperature of the respective alloy, which makes the difference between about 600-900 ° C and about 1200-1400 ° C for steels, for example. These lower temperatures have a significant advantage for the forming tools, since now usually can be worked with simpler steel tools and not only with high temperature resistant steels or even ceramic tools.

The lower temperatures also cause even less scaling or oxidation of the metallic surfaces in the components to be produced. This can be a very significant advantage depending on the alloy.

In general, with the method according to the invention, even for complex components, no or at least significantly reduced machining further process steps are required, which also results in better material utilization. Likewise, where appropriate, can be combined as a plurality of separate rear-mounted stored forming processes to a single inventive forming process. The process chain for producing the finished components is shortened in an advantageous manner.

Another advantage of the method according to the invention is the comparatively low forming pressure in the forming tool. This leads to significantly improved tool life. Likewise, the space requirement of Umformapparate is reduced.

It is envisaged to keep the forming pressure well below the forming pressure of the corresponding alloy needed for forging, working explicitly outside the superplastic region. Although the temperature of the semi-hot forming usually corresponds to the temperature range in which superplasticity is observed, the selected strain rate is clearly outside the superplastic region. The strain rate (ε ') of the semi-hot forming is set to values above 0.1 / s and the forming pressure in the forming tool is set at least 20% below the forming pressure required for forging. The forging pressure refers to the corresponding alloy composition which does not have a superplastic structure. The temperature and pressure conditions of forging usually correspond to those of hot working.

Surprisingly, the superplastic structure of the alloys can be formed even at high speeds of deformation at relatively low process pressure. The process pressure required for forming is dependent on the respective alloy, the exact microstructure setting and in particular also on the forming speed. Process pressure and forming speed are in opposite directions.

For UHC steels with superplastic structure it is preferred to work at a process pressure below 150 to 180 MPa and a deformation rate (strain rate (ε ')) above 0.1 / s. Particularly preferred is a process pressure of the forming chosen below 100 to 150 MPa. The design of the process can be optimized for low process pressure or high forming speeds. Particularly preferred transformation rates are above 0.5 / s. For Al alloys with superplastic structure, the strain rate at lower pressures is significantly higher. Preference is given to working at pressures below 80 MPa and strain rates above 1 / s.

In the embodiment according to the invention, it is provided to carry out the expansion of the blank in the half-warm forming below 50% of the elongation values achievable by superplastic forming and at strain rates (ε) which are at least 100 times that for the superplastic forming. This explicitly dispenses with the high achievable superplastic strains, in favor of a high strain rate. This corresponds to the very advantageous short process times. Surprisingly, with the very high strain rates, a degree of deformation can be achieved with which even complex component geometries can be achieved. For steels with a superplastic structure, it is preferable to work at expansions (degrees of deformation) above 100% and particularly preferably in the range from 100 to 500%. The stated values are to be understood as values averaged over the component geometry. The preferred strain rates are in a range of 100 to 1000 times the low strain rates required for superplastic forming.

In a particularly preferred embodiment of the method according to the invention, the semi-warm forming takes place in a temperature range which corresponds to the process window for the superplastic forming. The process window is essentially given by the parameters temperature, strain and strain rate. For steels, in particular UHC steels, the preferred temperature range is 600-900 ° C., more preferably 750 to 850 ° C. UHC steels with an Al content above 2% are particularly preferably converted at 800-950 ° C. The Al alloys are preferably formed in a temperature range of 300-400 ° C.

Preference is given to using alloys which have a strain rate sensitivity (m) above 0.4 for the corresponding superplastic forming. Particularly preferred are alloys with m> 0.7.

Preferred alloys include those whose maximum superplastic elongation is above 500%. Particularly preferred alloys are used with strains above 800%. This ensures that strains in the range of 100 to 400% can be achieved without problems during the forming of the blank. These degrees of deformation already cover a very large range of technically relevant components.

In a preferred embodiment of the invention, the process time of the forming is set to values below 10 s. Depending on the chosen forming process, the preferred range is between 1 and 5 s. The process parameters such as forming temperature and process pressure must be adapted accordingly. Lower forming times are also possible, but are not essential, since the set-up times for the forming tool become the speed-determining factors of the total process time.

The inventively preferred alloys are steels, Al and Ti alloys.

The steels are particularly preferably selected from the UHC steels (Ultra High Carbon steels). The C content of the steels is above 0.9 wt .-% and particularly preferably in the range of 1.2 bi 2%.

• – C: 0,9–2,2; Al: 1,5–10; Cr: 0,5–7; Mn 1–10; Rest Fe und Verunreinigungen
• – C: 0,9–2,2; Al: 1,5–10; Cr: 1–16; Mn 0–2,2; Rest Fe und Verunreinigungen
• – C: 0,9–2,2; Al: 1,5–10; Cr: 0,25–5; 0,5–5 Mo; Mn: 0–2; Rest Fe und Verunreinigungen
• – C: 0,9–2,2; Al: 1,5–3; Cr: 1–7; Si: 0,5–5%; Mn 0–2,2; Rest Fe und Verunreinigungen
• – C: 0,9–2,2; Al: 1,5–10; Cr: 1–7; Ni: 0,25–5; Mn 0–2,2; Rest Fe und Verunreinigungen

Particularly suitable UHC steels have the following compositions, the data being in% by weight:

• - C: 0.9-2.2; Al: 1.5-10; Cr: 0.5-7; Mn 1-10; Residual Fe and impurities
• - C: 0.9-2.2; Al: 1.5-10; Cr: 1-16; Mn 0-2.2; Residual Fe and impurities
• - C: 0.9-2.2; Al: 1.5-10; Cr: 0.25-5; 0.5-5 Mo; Mn: 0-2; Residual Fe and impurities
• - C: 0.9-2.2; Al: 1.5-3; Cr: 1-7; Si: 0.5-5%; Mn 0-2.2; Residual Fe and impurities
• - C: 0.9-2.2; Al: 1.5-10; Cr: 1-7; Ni: 0.25-5; Mn 0-2.2; Residual Fe and impurities

Typical representatives of the corresponding group are:
1.3 C, 1.5 Al, 7 Cr, 0.5 Mn, balance Fe,
1.3 C, 1.5 Al, 1 Cr, 2 Mo, 0.5 Mn, balance Fe;
1.3 C, 1 Al, 3 Cr, 2 Si, 0.5 Mn, balance Fe;
1.3 C, 1.5 Al, 4 Cr, 1 Ni, 0.5 Mn, balance Fe;
1.3 C, 3 Al, 1.5 Cr, 2 Mn, balance Fe;
1.3 C, 7 Al, 3 Cr, 3 Mn, balance Fe.

Another well-suited UHC steel, in addition to iron and common steel-accompanying impurities, the following alloy components in wt .-%:
0.8 to 2.5% C,
0.1 to 0.85 Sn,
3.5 to 15% Al,
0.5 to 4% Cr,
0.01 to 4% Si,
up to 4% Ni, Mn, Mo, Nb, Ta, V, and / or W, and
up to 3% of Ti, Be and / or Ga.

Preferably, an Al / C ratio above 2/1 is selected. This ratio is particularly preferably above 3/1.

Another advantage is the high Al content UHC steels. Al contents above about 3% are to be understood. The Al further reduces the scaling of the steel. When semi-hot forming usually takes place in these steels so little surface oxidation or scaling that can be dispensed with a surface treatment to remove the scale layer. The surface available after forming substantially corresponds to the ready-to-use state.

Due to their Al content, the listed UHC steels are not dependent on a special protective gas atmosphere. The semi-warm forming therefore takes place preferably under air access.

Conversely, the transformation of the Ti alloys is typically carried out under protective gas, in particular under Ar.

As a further low-carbon steel and without Al, the following composition can be selected for the process according to the invention (data in% by weight): C <0.05%, Si <1.2%, Mn <2.8%, Cr : 15-23%, Ni: 3-9%, Mo: 1-1.9%, N: 0.09-0.25%, remainder Fe and impurities.

• – 70–79 Zn, Rest Al;
• – 6–8 Al, 4–6 V, Rest Ti;
• – 4–6 Al, 2–3 Fe, Rest Ti.

Particularly suitable Al or Ti alloys include the following compositions in weight percent:

• - 70-79 Zn, balance Al;
• 6-8 Al, 4-6 V, balance Ti;
• - 4-6 Al, 2-3 Fe, balance Ti.

The method according to the invention is preferably carried out as a near-net-shape method, so that the component is obtained in the most ready-to-use state after the forming and only has to be subsequently reworked on special functional surfaces. Furthermore, it is expedient to use the process heat of the semi-warm forming in order to set the appropriate structure of the component. In this case, the component is cooled regulated after the forming. Steels and Al alloys are hardened in a known manner during cooling. Since no further forming process step is connected to the semi-warm forming, this procedure is only possible. In comparison, this procedure is unsuitable in the conventional approach, since shaping steps, for example by turning or drilling, are expediently carried out only on the uncured component; the curing is carried out after the final shaping by a further thermal process step.

The preferred process variants of semi-hot forming include extrusion, cross rolling, bore pressing, rotary swaging, gear rolling, upset kneading, forging or hydroforming, which are substantially adapted to the process of the invention in terms of process temperature and process pressure.

The combination of the inventive semi-warm forming with the known shaping processes can be used to produce gear shafts, extrusions, forgings or drop forgings, which are otherwise only accessible in several process steps, with higher investment or operating costs.

By means of the semi-hot extrusion molding according to the invention, a great variety of mass components of mechanical engineering are preferably produced. These include, for example, gears, pinions, short profile parts and so on.

In the following, the different methods of semi-hot forming on the example of transmission hollow shafts are exemplified.

Among the most suitable methods is transverse rolling. The known methods are merely adapted to the process parameters of the semi-warm process according to the invention. For example, in the DE 199 05038 A1 a transverse rolling process is described in which in the direction of a workpiece axis of rotation, a movable profiled mandrel is arranged, which has a defined geometry. This is acted upon by a coupled to the mandrel mandrel feed device for guided movement of the mandrel along the workpiece axis of rotation with an axial force. The relative movement of the mandrel is matched to the movement of the cross rolling tool. Other transverse rolling methods are for example from DE 100 66 177 A1 in which a rotationally symmetrical blank, for example a round rod, is unidirectionally rolled or reciprocated between two or more tools until the tooth structure formed on the tools has been transferred to the outer surface of the blank.

The half-hot extrusion molding according to the invention makes it possible to produce solid shafts with ready-to-use surfaces of demanding surface geometry. So it is also possible already to provide gears on the surface. In particular, several gear rings can be introduced independently of each other in this process step. In a subsequent process step, the drilling of the solid shaft to the hollow shaft is required.

The transverse rolling of gear shafts made of steel can be carried out by the novel process with very productive cycle times of 1 to 5 s. The temperature of the semi-warm forming makes it possible to transform the steel shafts by means of steel tools. Preferably, hollow transmission shafts are produced; either from solid shafts with subsequent drilling or from pipes. It is necessary that the inner contour of the tube is supported during cross rolling. This can be done through mandrels, in particular movable mandrels. Due to the low process temperature, the mandrels can also be made from low-cost steels. Compared to conventional cross rolling, considerably thicker walled tubes can be used as starting material. Particularly preferably, the entire outer contour of the gear shaft is introduced ready for use in cross-rolling invention. The outer contour may, for example, carry gears that are introduced during the semi-warm transverse rolling. Optionally, the finished surface structure of the pipe outside is not yet introduced during transverse rolling, but only in a subsequent process step of the semi-warm Radialschmiedens. As a result, larger geometric degrees of freedom can be realized in particular in the shaft inner contour.

For the production of gear shafts with wall thickness enlargement in the area of the flanges, a combination method of rotary kneading and upset kneading at room temperature is conventionally used. In the semi-hot upset swaging (or axial-radial-forming) according to the invention, the gear shaft can be produced in a single method step with high precision and with a complex geometrical configuration. While in the conventional method must be heated several times and compressed to the flange, or repeated clamping is required to reduce the long and the short shaft shank, the entire transformation can be carried out in a semi-rotary swaging / upset kneading according to the invention in one process step. Optionally, gears may also be introduced in the same or a subsequent semi-hot forming step by means of suitable tools. The degrees of deformation of the method according to the invention are generally sufficient to already introduce teeth of the usual geometry and depth on the gear shafts.

In the case of the highest geometric requirements, it may also be expedient to apply gearwheels or other surface structures by means of cutting processes, such as turning or milling, or by electrochemical processes, such as, for example, the PECM process. A coarse contour can already be provided in any case by means of the semi-warm forming and considerably simplify the work of fine machining.

Another variant of the method is the half-warm hydroforming. To produce a hollow shaft, an internal pressure is applied to the output tube and the material is tracked in the direction of the tube length. In the conventional method for typical transmission shafts pressure ratios of up to 10,000 bar are required, which makes a cost-effective mass production impossible. By contrast, with the method according to the invention, process pressures of 1000 to 3000 bar are already sufficient for gear shafts. As a rule, gears must be introduced in a further process step.

For the half-warm forming according to the invention, the known basic processes can also be suitably combined or carried out in succession. In particular, the already formed components still have considerable expansion reserves, since only a fraction of the maximum possible elongation is carried out in each of the forming steps according to the invention. Only after several forming steps, the maximum superplastic strain is achieved. Depending on the alloy used, it is necessary to allow process steps to follow one another directly, so that no recrystallization of the superplastic microstructure can take place by cooling and reheating.

For example, it may be expedient first to produce a blank of a gear shaft by means of semi-hot extrusion of solid material and then to convert this by means of semi-hot bore pressures into a hollow shaft with fine inner and outer contours. In this case, in particular, the inner contour may already be the ready-to-use contour, which otherwise eliminates the need for elaborate or even impossible post-processing inside.

Claims (18)

Process for the production of metallic components by hot forging of blanks of alloys with superplastic structure, characterized in that the elongation of the blank in the half-warm forming below 50% of the achievable by a superplastic forming strain values and at strain rates (ε ') at least 100 times that for superplastic forming. A method according to claim 1, characterized in that the semi-warm forming is carried out in a temperature range corresponding to the process window of the superplastic forming. Method according to one of the preceding claims, characterized in that the semi-warm forming takes place below the forging temperature of the alloy. Method according to one of the preceding claims, characterized in that alloys are used which, in the presence of a superplastic structure, have a strain rate sensitivity (m) above 0.4. A method according to claim 1 or 2, characterized in that an alloy is selected whose maximum superplastic strain is above 500%. A method according to claim 1 or 2, characterized in that the deformation leads to an elongation of the blank in the range of 100 to 400%. Method according to one of the preceding claims, characterized in that the semi-warm forming is set to a process time below 10 s. Method according to one of the preceding claims, characterized in that UHC steels used as alloys which have essentially the following compositions (in% by weight): - C: 0.9-2.2; Al: 1.5-10; Cr: 0.5-7; Mn 1-10; Residual Fe and impurities - C: 0.9-2.2; Al: 1.5-10; Cr: 1-16; Mn 0-2.2; Residual Fe and impurities - C: 0.9-2.2; Al: 1.5-10; Cr: 0.25-5; 0.5-5 Mo; Mn: 0-2; Residual Fe and impurities - C: 0.9-2.2; Al: 1.5-3; Cr: 1-7; Si: 0.5-5; Mn 0-2.2; Residual Fe and impurities - C: 0.9-2.2; Al: 1.5-10; Cr: 1-7; Ni: 0.25-5; Mn 0-2.2; Residual Fe and impurities - 0.8 to 2.5 C; 0.1 to 0.85 Sn; 3.5 to 15 Al; 0.5 to 4 Cr; 0.01 to 4 Si; up to 4Ni, Mn, Mo, Nb, Ta, V, and / or W, and up to 3 on Ti, Be and / or Ga; Residual Fe and impurities. A method according to claim 8, characterized in that the semi-warm transformation takes place with access of air. A method according to claim 6 or 7, characterized in that the process heat is used to cool the component regulated. A method according to claim 6, 7 or 8, characterized in that is tempered during cooling. Method according to one of claims 1 to 7, characterized in that Al alloys are used which have substantially the following compositions (in wt .-%): - 70-79% Zn, balance Al; 6-8% Al, 4-6% V, balance Ti; - 4-6% Al, 2-3% Fe, balance Ti. Method according to one of claims 1 to 7, characterized in that as alloy, a steel of the following composition is chosen: C <0.05%, Si <1.2%, Mn <2.8%, Cr: 15-23%, Ni: 3-9%, Mo: 1-1.9%, N: 0.09 -0.25%, balance Fe and impurities. Method according to one of the preceding claims, characterized in that the semi-warm forming as extrusion, cross rolls, bore pressures, swaging, Verzahnungswalzen, upset kneading, hydroforming or combinations thereof is performed. Method according to one of the preceding claims, characterized in that a plurality of semi-warm transformations are carried out on the same component in succession. Transmission shafts, gears, pinions or profile parts obtainable according to one of the preceding methods. Transmission shafts, gears, pinions or profile parts according to claim 16, characterized in that they are formed from UHC steels with a superplastic structure. Gear shafts, gears, pinions or profile parts according to claim 17 for use in a method of claims 1 to 14.