Impact Joining of Metallic Sheets and Evaluation of its Performance

Abstract

Similar or dissimilar metallic sheets were joined at their edges by the original impact joining method developed by one of the authors. Surface layers of both sheet edges activated by high-speed shear are immediately contacted with sliding motion in the joining process. The whole processing time is within a few milliseconds. The materials tested were mild steel and titanium sheets. Drop-weight impact testing machine was used. Joining performance of the fabricated sheets was evaluated by tensile test, etc. The joining was not available all over the thickness between sheets, in which sharp notch was observed near both sheet surfaces. The central portion was successfully joined without cavity. The joined specimen of mild steel and titanium was sliced to remove surfaces with such notch. Fracture occurs at the part of mild steel whose strength is lower, then the joining boundary was not damaged. Introduction It is well known that time and temperature effects have important role in solid state joining by atomic diffusion at elevated temperature. On the other hand, under cold condition, if the surface expansion is relatively large, two metal parts can join at the newly created surface, in which the brittle oxidized surface layer fractures. Joining strength in solid state welding was found to be approximately equal to the normal applied stress during the process in the absence of oxide films for the case of aluminum welded together in 1970 [1]. The film theory of such kind of welding or bonding was established, in which roll bonding was applied in 1983 [2]. Recently the film theory was used to derive a model that quantifies the relevance of these parameters to the weld strength [3]. Cold bonding may have a potential for recycling scrap aluminum [4]. The diffusion bonding is usually achieved by very high compressive stress with large plastic deformation. The shape drastically changes from the initial one and the joining strength also depends on the initial surface condition. Surface treatment is necessary for removal of the dirty surface layer. Experimental results in diffusion bonding were summarized for various metals including superplastic alloys [5]. Joining of different metals were tested [6] and experiments were carried out using super plastic materials [7, 8]. Hot isostatic pressing was also effective for the diffusion bonding of the nickel powder onto alumina tubing [9]. Divergent extrusion was used for bonding of aluminum by means of two opposing punches and finite element simulations was conducted [10]. However, the method requires very special conditions in temperature, atmosphere, surface treatment, etc. and they are very time consuming. One of the authors proposed a novel joining method for sheet metal [11]. The edge of the sheet is joined to another edge, where the sheet thickness is unchanged, because the plates are not plastically compressed. In the present study, the materials are mild steel and pure titanium sheets. Main objectives are to observe the motion of the tools and the materials in the device, and to check the Explosion Shock Waves and High Strain Rate Phenomena Materials Research Forum LLC Materials Research Proceedings 13 (2019) 91-96 https://doi.org/10.21741/9781644900338-16 92 deformation performance of the sheet composed of different materials by tensile and bending tests. The boundary was also inspected by elemental analysis. Experimental device and materials tested The impact joining setup is shown in Fig.1. The device is driven by an impact of drop-weight. The mass was 22 kg and the impact velocity was 10 m/s. The left half of the lower sheet is supported by the counter punch, whose reactive force is given by compressing the circular pipe (A6061, 12 mm diameter and 1 mm thick wall). The top edge of punch A is impacted then the simultaneous shearing commences. The upper sheared face slides to fit the lower sheared face. The motion stops at the prescribed position. The device is mounted on the low-elastic rubber that is pre-compressed to avoid the damaging excessive force. Test materials are mild steel sheet SPC of 1.0 or 3.2 mm thickness, and pure titanium sheet TP340 of 1.0 or 3.0 mm thickness. Their tensile strengths were 303, 317 and 427, 401 MPa, respectively. Overlap length in sliding stage was varied. Fig.1 Experimental setup Motion of tools and sheets in joining device Motion of the tools and the sheets were observed with a high-speed video camera, in which the joining of TP340 (Upper specimen) and SPC (Lower one) was carried out. Progressive pictures are exhibited in Fig.2. The shear deformation and fracture of SPC are captured at time t: 87.7 and 439 μs, respectively. The left half of SPC moves downwards, then the TP340 also moves downwards after shear fracture. The TP340 appears at 2456 μs thereafter the edges of both materials slide each other with the prescribed overlap length at 3018 μs. The sliding stage terminates at 3333 μs. The device sags due to the redundant energy of the dropweight, where the low-elasticity rubber is compressed. It recovers at 6368 μs. Repulsion and contact between the tool and the drop-weight may occur in addition to the deceleration of the drop-weight, this causes the differences in moving distance of the tools calculated the impact velocity of 10 m/s. Experimental result and discussions Examples of joined specimens are exhibited in Fig.3 for the joining of different materials. No warping of the joined sheet with 1.0 mm thickness is observed, though the joining is not achieved all over the thickness. Sharp gap is observed at both surfaces of 1.0 and 3.0 mm thick sheets. The (a) Photo of setup (b) Joining device Lifting hook