Method of manufacturing a metal sheet component

Abstract: A sheet metal component, obtained by press-forming a one-piece metal sheet comprised of two metal sheets into a desired shape, has an expanding portion having an external appearance varying in dependence on a weld state of a weld portion of the one-piece metal sheet at which the two metal sheets are welded at their confronting ends. In manufacturing the sheet metal component, the one-piece metal sheet is transferred between upper and lower dies of a pressing machine and is positioned such that its metal sheet section including part of the weld portion is disposed between a punch formed in the lower die and a recess formed in the upper die so as to face the punch. The metal sheet section is pressed by the punch to be subject to plastic deformation, whereby the expanding portion is formed.

Description: Technical Field The present invention relates to a method of manufacturing a sheet metal component according to the preamble of claim 1 (see, for example, EP-A-0 713 746), which permits easy and reliable inspection of its weld quality. Related Art Vehicular components such as a frame and a cross member affecting the safety of a vehicle are in some cases constituted by sheet metal components. Some of these sheet metal components are obtained by welding two metal sheets at their confronting ends and by press-forming a resultant one-piece metal sheet into a desired shape. For example, a side frame of a vehicle is constituted by a front member comprised of a first metal sheet and a rear member comprised of a second metal sheet whose thickness and mechanical strength are larger than those of the first metal sheet, so that it may be deformed to absorb the impact upon head-on collision. The first and second metal sheets are subject to butt welding or lap welding, to be united with each other at their confronting ends, and the resultant one-piece metal sheet is subject to press forming to be formed into a side frame of a desired shape to which important security parts and/or specially guaranteed parts such as suspension unit, engine, transmission unit of a vehicle are attached. Thus, a weld failure caused in the side frame can result in a problem of safety of the vehicle. In order to determine the weld quality in respect of important sheet metal components, such as side frame, greatly affecting the safety, it is advisable to carry out the total inspection of these components at the stage just after the components are fabricated or after entire cars each incorporating therein the sheet metal component are obtained. Thus, methods for carrying out such total inspection have been investigated. The weld quality of a sheet metal component can be inspected accurately, if the component can be cut at its weld portion so that the cut portion may be subject to visual inspection for weld quality determination in term of, e.g., a magnifying glass. Although this method is applicable to a sampling inspection, it is of course impossible to apply such method to a total inspection. On the other hand, a weld quality determination of a sheet metal component such as a side frame cannot be made in term of a visual inspection at the stage of obtaining an entire car whose side frame is already painted. Weld tests involving deformation of a welded zone of a test strip are disclosed in Patent Abstracts of Japan vol. 004, no 019 (C-073), 16 February 1980&JP 54 155143 A (Nippon Steel Corp.) & Dec. 1979. Butt welding operations on metal sheets followed by a welded area deformation, for height alignment with an adjacent area, are disclosed in EP-A-713 746. Summary of the Invention The aim of the present invention is to solve the problems set forth above by providing a method of manufacturing a metal sheet component according to claim 1. With the manufacturing method of this invention, a sheet metal component whose weld state is determinable by appearance inspection can be manufactured properly. Since the sheet metal component has the expanding portion which has an external appearance thereof varying in dependence on the weld state, the weld state can be properly determined based on the external appearance of the expanding portion. Incidentally, the present invention has created by taking note of phenomenon which takes place in a test piece during Erichsen test in which a spherical punch is gradually pressed against a plate material (test piece) and the depth of pressing the punch observed just when a crack appears in the plate material is determined as an Erichsen value for the evaluation of formability of the plate material. As distinct from the Erichsen test in which the test piece is subject to plastic deformation until a crack actually appears in the test piece, the present invention forms the expanding portion by subjecting the metal sheet section, including part of the welded portion, of the one-piece metal sheet to plastic deformation until a predetermined amount of deformation is caused in the metal sheet section. The predetermined amount of plastic deformation is restricted to the extent that no crack appears in the expanding portion if the weld state of the welded portion of the one-piece metal sheet is proper as usual. In the present invention, the steps (a) and (b) are carried out, with use of the second pressing machine which also serves as the first pressing machine, at the same time when the one-piece metal sheet is press-formed into the sheet metal component of a desired shape, whereby the expanding portion is formed simultaneously when the one-piece metal sheet is press-formed. With this method, the efficiency of manufacturing the sheet metal component having the expanding portion can be improved, and in particular, sheet metal components subjected to total inspection in respect of weld quality can be manufactured efficiently. Preferably, the two metal sheets are butt-welded or lap-welded at the welded portion. Preferably, the steps (a) and (b) are carried out by using the punch formed into a semi-spherical shape, and the expanding portion is formed into a semi-spherical shape. Preferably, the step (a) includes positioning the one-piece metal sheet so that the part of the welded portion is in alignment with a distal end of the punch, and the step (b) is carried out in this positioning state, thereby forming the expanding portion whose top is formed in alignment with the part of the welded portion. Preferably, the second press-die half does not apply a force onto the metal sheet section as the metal sheet section is subject to the plastic deformation within the recess of the second press-die half by being pressed by the punch of the first press-die half in the step (b). With this preferred method, the expanding portion having its external appearance representing the weld state of the welded portion more accurately can be formed. Brief Description of the Drawings Fig. 1 is a schematic perspective view showing an automotive side frame manufactured with use of mush-seam welding; Fig. 2 is a fragmentary enlarged sectional view showing an expanding portion of the side frame taken along II-II line shown in Fig. 1; Fig. 3 is a view showing manufacturing processes according to the invention and other processes in respect of the side frame shown in Fig. 1, the uppermost part on the left of Fig. 3 showing a process of subjecting two metal sheets to mush-seam welding to obtain a one-piece metal sheet, the second-upper part thereof showing the side frame obtained by press-forming the one-piece metal sheet, the upper right part thereof showing a process of positioning the one-piece metal sheet in a pressing machine, the lower right part thereof indicating a process of press-forming the expanding portion in the one-piece metal sheet, the second-lower part on the left of Fig. 3 showing a process of appearance inspection, and the lowermost part thereof showing a process for assembling the side frame into a vehicle body; Fig. 4 is a fragmentary enlarged view showing several elements of a mush-seam welding machine for the mush-seam welding shown in the uppermost left part of Fig. 3, together with the one-piece metal sheet; Fig. 5 is a schematic perspective view showing an automotive side frame manufactured with use of laser welding; Fig. 6 is a fragmentary enlarged sectional view showing an expanding portion of the side frame taken along VI-VI line shown in Fig. 5; Fig. 7 is a view, similar to Fig. 3, showing manufacturing processes according to the invention and other processes in respect of the side frame shown in Fig. 5; and Fig. 8 is a block diagram showing a manufacturing method together with assembling and inspecting processes, which does not fall under the scope of claim 1. Detailed Description With reference to the appended drawings, a sheet metal component and a method for manufacturing the same will be explained by taking, as an example, an automotive side frame which is one of sheet metal components constituting a vehicle. A side frame shown by reference numeral 10 in Fig. 1 comprises a main body 10a which is obtained by press-forming a one-piece metal sheet into a desired shape, the one-piece metal sheet being obtained by welding first and second metal sheets 1 and 2 together. The first metal sheet 1 serves as a front member of the side frame, and the second metal sheet 2 serves as a rear member of the frame. As best shown in Fig. 2, a rear end portion of the first metal sheet 1 and a front end portion of the second metal sheet 2 overlap each other at a welded portion 3 of the frame main body 10a. The first and second metal sheets 1 and 2 are lap-welded (mush-seam welded in this embodiment) at the welded portion 3. In order to permit the weld quality (weld state) of the welded portion 3 to be determined by appearance inspection, the frame main body 10a is formed with an expanding portion 4. This expanding portion 4 is formed by subjecting a section (hereinafter referred to as metal sheet section or deformed section) of the frame main body 10a to plastic deformation, the metal sheet section including a vertically central part of the welded portion 3. The expanding portion 4 projects from an outer side surface of the frame main body 10a outwardly in the width direction of a vehicle (not shown). In this embodiment, the expanding portion 4 is formed into a semi-spherical shape, and has a top thereof formed in alignment with the central part of the welded portion 3. The amount of projection (height H) of the expanding portion 4 corresponds to an amount of plastic deformation of the metal sheet section for the formation of the expanding portion 4. The height H (plastic deformation amount) of the expanding portion 4 is set such that an unusual appearance (e. g. , crack) indicative of weld failure does not appear in the expanding portion 4 if the expanding portion is formed by plastic deformation of the metal sheet section including the central part of the weld portion 3 which is properly welded, whereas such an unusual appears if the expanding portion 4 is formed by plastic deformation of the metal sheet section which includes the central part of the weld portion 3 improperly welded. With reference to Fig. 3, a method of manufacturing according to the invention the side frame 10 shown in Figs. 1 and 2 will be explained below. First, a steel plate having, e.g., 1.4 mm thickness is cut into a desired shape to thereby obtain the first metal sheet 1, and a steel plate having, e.g., 2.3 mm thickness is cut into a desired shape to obtain the second metal sheet 2. Next, the first metal sheet 1 is placed on the second metal sheet 2 with the lower face of the rear end portion of the first metal sheet 1 abutted against the upper face of the front end portion of the second metal sheet 2, to thereby overlap the confronting ends (more specifically, the confronting end portions) of these sheets 1 and 2 each other. Then, the overlapping portion is held between two circular plate electrodes 21 and 22 of a much-seam welding machine shown in Fig. 4, these electrodes having both a pressing function and electric power supplying function. The first and second metal sheets 1 and 2 are clamped by clamping mechanisms, not shown, of the mush-seam welding machine. The width of the overlapping portion of the metal sheets 1 and 2, i.e., the width of the welded portion 3, is set to be smaller than the width of the electrodes 21 and 22. In Fig. 4, reference numeral 23 denotes a weld transformer having the primary winding thereof connected with a power source (not shown) and the secondary winding thereof connected to the electrodes 21 and 22. As shown in the uppermost part (Step S31) on the left of Fig. 3, respective lengthwise sections of the overlapping portion of the metal sheets 1 and 2 are pressed and heated in sequence by the circular plate electrodes 21 and 22 while supplying a weld current from the weld transformer 23 (Fig. 4) to the electrodes 21 and 22 and while rotating the electrodes 21 and 22, so as to weld the overlapping portion of the metal sheets 1 and 2 straight by means of mush-seam welding, thereby obtaining a one-piece metal sheet 1, 2. Next, the one-piece metal sheet 1, 2 is press-formed into the frame main body 10a (more generally, the side frame 10) and at the same time the expanding portion 4 is formed at and around the central part of the welded portion 3. A pressing machine for both the press forming of the one-piece metal sheet and the formation of the expanding portion 4 has an upper die (first press-die half) 11 and a lower die (second press-die half) 12, as shown in the upper right part of Fig. 3. Although an illustration is omitted in Fig. 3, the upper and lower dies 11 and 12 have press-forming faces suitable for press-forming the one-piece metal sheet into the side frame 10 of a desired shape. In relation to the formation of the expanding portion 4, the press-forming face of the lower die 12 is provided with a punch 13 formed into a semi-spherical shape, and the press-forming face of the upper die 11 is provided with a recess 14 facing the punch 13 of the lower die 12. The recess 14 provides a space for permitting the intrusion of the punch 13. In this embodiment, the recess 14 is arranged to provide a free space on the side opposite to the punch 13 with respect to the metal sheet section of the one-piece metal sheet even in a state where the facing faces of the upper and lower dies 11 and 12 abut against each other so that the punch 13 finishes pressing the metal sheet section of the one-piece metal sheet 1, 2. For the simultaneous formation of the side frame 10 and the expanding portion 4, the one-piece metal sheet 1, 2 is first transferred between the upper and lower dies 11 and 12 of the pressing machine, as shown by Step S32 in Fig. 3, and is then positioned so that its metal sheet section is located between the punch 13 of the lower die 12 and the recess 14 of the upper die 11. In this embodiment, the one-piece metal sheet 1, 2 is positioned such that the central part of the welded portion 3, included in the metal sheet section, is disposed in alignment with the distal end of the punch 13. Under this positioning state, the pressing machine is operated to make a press-forming action as shown in the lower right part (Step S33) of Fig. 3. During the press-forming action, the metal sheet section of the one-piece metal sheet 1, 2 is pressed at its lower face by the semi-spherical punch 13 to thereby subject the metal sheet section to plastic deformation, in a condition that the one-piece metal sheet is held, at its part around the metal sheet section, between the upper and lower dies 11 and 12, whereby the semi-spherical expanding portion 4 is formed. Since the recess 14 is formed in the upper die 11 so as to provide a free space between the metal sheet section and the upper die 11 on the side away from the punch 13 with respect to the metal sheet section, the upper die 11 does not apply any substantial force onto the metal sheet section while the same section is pressed by the punch 13 to be subject to plastic deformation. That is, no undesired force is applied to the metal sheet section during the formation of the expanding portion 4, so that the expanding portion 4 has its external appearance which accurately represents the weld quality of the welded portion 3. More specifically, an unusual appearance (crack, for instance) indicative of weld failure does not appear in the expanding portion 4, if the same portion 4 is formed by the plastic deformation of the metal sheet section including part of the welded portion 3 at which the first and second metal sheets 1 and 2 are properly welded. On the other hand, such an unusual appearance is caused, if the expanding portion 4 is formed by the plastic deformation of the metal sheet section including part of the welded portion 3 at which the metal sheets are welded improperly. The external appearances of expanding portions of side frames 10 (preferably, all the side frames) each manufactured in the above manner are visually inspected or subject to visual inspection in term of a magnifying glass, as shown by Step S34 in Fig. 3. Those side frames 10 free from an unusual appearance such as a crack and thus having an adequate weld quality are mounted to vehicle bodies, respectively, in a body assemble process (shown by Step S35 in Fig. 3). Alternatively, the weld quality of side frames 10 are inspected based on external appearances of their expanding portions 4 at the stage of obtaining entire cars into which the side frames are respectively assembled, as generally shown by Steps S84 and S85 in Fig. 8. With reference to Figs. 5-7, a side frame and a method for manufacturing the same according to the invention will be explained below. As compared to the example in figure 1 having two metal sheets 1 and 2 are lap-welded, a side frame 100 of this embodiment is different in that two metal sheets 101 and 102 are butt-welded at their confronting ends, in term of, e.g., laser welding. The side frame 100 has a basic construction which is substantially the same as that of the side frame 10 of the first embodiment. In this regard, the side frame 100 and the manufacturing method will be explained briefly with reference to Figs. 5-7 in which each frame element is denoted by reference numeral corresponding to the sum of 100 and reference numeral of an associated element of the first embodiment. As best shown in Fig. 6, the first and second metal sheets 101 and 102 of the side frame 100 are laser-welded at their confronting ends (welded portion 103) to be formed into a one-piece metal sheet. The side frame 100 is formed with a semi-spherical expanding portion 104 by subjecting a section of the one-piece metal sheet, including a central part of the welded portion 103, to plastic deformation. The expanding portion permits the weld quality of the welded portion 103 to be determined by appearance inspection. The top of the expanding portion 104 is in alignment with the central part of the welded portion 103. The height H of the expanding portion 104 is set such that no unusual appearance (a crack, for instance) appears in the expanding portion 104 if the welded portion 103 is properly welded, whereas such an unusual appearance appears if the welded portion 103 is improperly welded. For manufacturing the side frame 100, the first and second metal sheets 101 and 102 are abut against each other at their confronting ends, and these metal sheets are laser-welded into a one-piece metal sheet by use of a laser welding machine which includes a weld torch 120, as shown by Step S71 in Fig. 7. Next, the one-piece metal sheet is press-formed into a side frame main body 100a (more generally, into the side frame 100) and at the same time the expanding portion 104 is formed in the metal sheet section which includes the central part of the welded portion (weld line) 103 of the one-piece metal sheet. A pressing machine for both the press forming of the one-piece metal sheet and the formation of the expanding portion 104 has an upper and lower dies 111 and 112 as shown in the upper right part of Fig. 7, the upper and lower dies having their press-forming faces corresponding to an outer shape of the side frame 100. Further, the lower die 112 is formed at its press-forming face with a semi-spherical punch 113, and the upper die 111 is formed at its press-forming face with a recess 114 which faces the punch 113 of the lower die 112. For the simultaneous formation of both the side frame 100 and the expanding portion 104, the one-piece metal sheet is transferred between the upper and lower dies 111 and 112 of the pressing machine and is positioned such that the metal sheet section is disposed between the punch 113 and the recess 114 and that the central part of the weld line 103 included in the metal sheet section is in alignment with the distal end of the punch 113, as shown by Step S72 in Fig. 7. Under this positioning state, the pressing machine is operated to make a press-forming action, as shown in the lower right part (Step S73) of Fig. 7. External appearances of expanding portion of side frames 100 manufactured in the above manner are inspected (Step S74), preferably in term of total inspection, and those side frames which are adequate in weld quality are assembled into vehicle bodies, respectively, in a body assembling process (Step S75). Alternatively, the appearance inspection is made at the stage of assembling the side frames 100 into the vehicle bodies, respectively, to thereby obtain entire cars (see, Fig. 8). The present invention is not limited to the first and second embodiments of the manufacturing method according to the invention explained above, but may be modified in various manners. For instance, the present invention is applicable to a variety of sheet metal components obtained by butt-welding or lap-welding two metal sheets into a one-piece metal sheet and by press-forming the one-piece metal sheet, although a case where this invention is applied to an automotive side frame has been explained in the preferred embodiments of the inventive method. In the first and second embodiments, the lap-welding in term of mush-seam welding and the butt-welding in term of laser welding have been explained, respectively. However, other methods for butt welding or lap welding may be used in the present invention. The expanding portion may be formed into a U-shape or a polygonal shape in cross section although it is formed into a semi-spherical shape (a semi-circular shape in cross section) in the embodiments. It is not inevitably necessary to form the expanding portion such that it has a top formed in alignment with part of the welded portion (weld line). That is, the expanding portion may be formed to have an off-center part which includes part of the welded portion. In the preferred embodiments, the expanding portion projects outwardly from the surface of the sheet metal component, so as to make it easy to perform the appearance inspection of the expanding portion. Alternatively, the expanding portion may project inwardly. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A method of manufacturing a metal sheet component in which two metal sheets (1, 2; 101, 102) are welded at facing ends thereof to provide a one-piece metal sheet with a welded portion (3, 103) which is press-formable into a main body (10a, 100a), the method comprising the steps of: (a) transferring said press-formed one-piece between first and second press die halves (11, 12; 111, 112) so as to position the welded portion (3, 103) between a punch (13, 113) formed at the first die half (12, 112) and a recess (14, 114) formed at the second die half (11, 111), the punch facing said recess and being actuatable to enter thereinto; (b) actuating said punch (13, 113) to press the metal sheet so as to form by plastic deformation at said welded portion (3, 103) a press-formed expanding portion (4, 104) projecting from the surface of the main body (10a, 100a) by a predetermined amount (H) set so as to avoid producing in a properly weld portion (3, 103) the unusual appearance indicative of a weld failure, and so as to produce in an improperly weld portion the unusual appearance of a weld failure, wherein said press-forming step is performed in a single phase on a single press-forming machine to press form simultaneously said main body with said expanding portion; and (c) inspecting the external appearance of said expanding portion (4, 104). The manufacturing method according to claim 1, wherein the two metal sheets (111, 112) are butt-welded at the welded portion (103). The manufacturing method according to claim 1, wherein the two metal sheets (1, 2) are lap-welded at the welded portion (3). The manufacturing method according to claim 1, wherein said steps (a) and (b) are carried out by using the punch (13, 113) formed with a semi-spherical shape, and the expanding portion (4, 104) is formed with a semi-spherical shape. The manufacturing method according to claim 1, wherein said step (a) includes positioning the one-piece metal sheet so that a part of the welded portion (3, 103) is in alignment with a distal end of the punch (13, 113), and said step (b) is carried out in such positioning state, thereby forming the expanding portion (4, 104) whose top is formed in alignment with the part of the welded portion (3, 103). , The manufacturing method according to claim 1, wherein the second press-die half (11, 111) applies no force onto the metal sheet section as the metal sheet section is subject to the plastic deformation within the recess (14, 114) of the second press-die half by being pressed by the punch (13, 113) of the first press-die half (12, 112) in said step (b). The manufacturing method according to claim 1, further comprising the step of assembling into a vehicle body the inspected component that has an adequate weld quality.

Inventors: ENOMOTO MASAYUKI, GOTO KIYOSHI, KOMIZO TOSHIYUKI, ENOMOTO, MASAYUKI, GOTO, KIYOSHI, KOMIZO, TOSHIYUKI, Enomoto, Masayuki, c/o Saidaiji Sangyou Co., Ltd,
Applicants: MITSUBISHI MOTORS CORP, MITSUBISHI JIDOSHA KOGYO KABUSHIKI KAISHA
Application year: 1998
Main classification: B23K31
IPC codes: B62D25, B23K31