Papers by Author: Kari Mäntyjärvi

Abstract: Corrugated core panels contain a formed, corrugated core bonded between two skin sheets. These panels are typically used in applications, where a low weight is required with integrity in stiffness. This paper demonstrates the mechanical properties of a simple panel structure (SPS), constructed using strips of work-hardened, austenitic stainless steel (ASS) grade 1.4310 (type 301) with the yield strength (YS) of ~1200 MPa. The 0.5 mm thick strips were formed into a C-shape and subsequently laser welded together by lap joints to form a SPS. The thickness of the SPS was 50 mm. The bending tests for the SPS were carried out transverse and 45-degrees related to the orientation of the web sheet. The results showed that the SPS, as loaded in the transverse direction, has about the same bending stiffness prior yielding as that of the previously tested 6 mm thick, low carbon S355 plain steel sheets, but the SPS is three times lighter than 6mm thick plain steel sheet. Compared with a corrugated core panel made of an annealed ferritic stainless steel (SS-panel) with the YS ~ 250 MPa, the weight of the both panels are roughly the same, but the bending resistance of the SPS is 45% higher. Experimental tests also verified that the benefit in the stiffness is quickly reduced if the load direction differs from transverse. In the 45-degrees loading direction, the SPS and the SS-panel had almost the same bending strength. On the other hand, the SPS and the SS-panel stiffnesses are much better than that of the carbon steel (the YS ~ 300 MPa) panel (CS-panel) in the both loading directions – the SPS being twice as stiff as the CS-panel.

Abstract: The laser welding is usable method for joining thin plates with low energy input and precise penetration control. When joining of very thin sheets such as 0.5 mm the shape of the weld must be complete in order to achieve a good strength of the joint. The part of the test welds were welded without shielding gas and other part of the test welds by using 65 mm shielding gas nozzle behind the key hole. The strength of the laser weld of 0.5 mm Austenitic stainless steel (ASS) plate was measured in welds without shielding gas and Ar shielded weld. The strength of the shielded weld joints was significantly better than the joint weld without shielding gas due to convex shielded welds. In addition the shielded welds were bright which improves the corrosion properties of the joint.

Abstract: The focus of this study was to test a low-cost level plastic printer in the multi-material printing application using principles of design for additive manufacturing (DFAM). Two sample structures were designed in the project. One of the main planning principles of the examples was to integrate multiple functions into one part and intelligently utilize a variety of materials and reduce parts count. The most common material used in the experiments was the basic PLA, which is widely used, easy-to-print and economical alternative. As special materials, electrically conductive PLA-based graphene filament and highly flexible polyurethane-based filament was used. The results show that multi-material printing is also possible with lower cost devices and it makes it easier for smart products to be manufactured cost-effectively. It has also been found that multi-material printing can be technically challenging and that further research and experiments in this subject are needed. In the future, the research topic will be even more interesting as equipment and materials will develop. This paper presents detailed printing parameters for all the materials used in the printing tests.

Abstract: The study was performed to investigate the bending resistance of laser-welded sandwich panels (Vf-core). The main aim of the study was to determine the effect of the tensile strength on bending strength of the panel structures. Panels were manufactured using an ultra-high strength (UHS) and low strength (LS) steels with yield strengths of 1200 and 200 MPa, respectively. Secondly, the bending strength of the panel structures was compared with the conventional sheet steels to estimate the possibilities for weight reduction. Results showed that the UHS steel panels had significantly higher bending strength than panels of the LS steel. The bending strength in the weakest loading direction of the UHS panel was approximately four times higher than the one of LS steel panel. The panels made with UHS steel faceplates and LS steel cores had better bending strength than LS steel panels. In comparison to UHS sheet steel, 30% weight saving is estimated by using the geometry optimized UHS steel panel.

Abstract: Additive Manufacturing (AM) does not yet have a standardized way to measure performance. Here a AM machines dimensional accuracy is measured trough acceptance test (AT) and AM machines capability is tested trough test parts. Test parts are created with specific geometrical features using a 3D AM machine. Performance of the machine is then evaluated trough accuracy of test parts geometry. AM machine here uses selective laser melting (SLM) process. This machine has done Factory acceptance test (FAT) to ascertain this machine ́s geometrical accuracy with material AISI 316L. Manufacturer promises accuracy of ±0.05 mm. These parts are used as comparison to AT parts made in this study. After installation two AT parts are manufactured with AM machine. One with AISI 316L and one AlSi10Mg. Dimensional accuracy of geometrical features on these parts are then compared to FAT part and to one another. Machines capability is measured trough two test parts done with material AlSi10Mg. Two of the test parts are done at the same time using same model as the FAT. Parts are printed without supports and with features facing same directions. Features of these parts were then evaluated. Another test to find out AM machines capability was to create part consisting of pipes doing 90˚ angle resulting in horizontal and vertical holes. Dimensional accuracy and circularity of holes was measured. Through these tests machines capability is benchmarked.

Abstract: Metal 3D AM (Additive Manufacturing) has been becoming a more common production method for larger variety of parts. In this review the current situation and future development trends of the 3D metal AM are presented, concentrating on the SLM (Selective Laser Melting) technology. A holistic approach to the AM as a digital manufacturing method is presented and different manufacturing aspects of the AM production are identified. The most promising aspects for the future development are the automatization of the AM design tasks and automatization of the production. With the development of these aspects the production and cost efficiency of the metal AM can be increased to a more competitive level compared with other manufacturing methods.

Abstract: To get all outstanding mechanical properties from ultra-high strength steel (UHSS) transformed into a lightweight and durable product, a comprehensive design process must be performed. The general DFMA method has been extended to get good design solutions with limited designer resources and within a relative short time period. Key steps of the extended DFMA design process are innovative brainstorming, the agile utilization of the optimization software, the heavy use of finite element analysis and fast design iteration. In this paper, the modified DFMA design method is described step by step. This method has been successfully used in the design process of the small lightweight rally car frame. The design process of the rally car is introduced as a case example of implementation of the extended DFMA.

Abstract: This study was employed to investigate the buckling effect for a single Vf corrugated core. Brief and simple designing method is developed for UHS sandwich structure. This method is based on simplified calculation of slenderness ratio integrated with FEM simulation. Method is developed for studying the local buckling resistance of sandwich structures to optimize the panel dimensions for maximal stiffness. Five different core dimensions were tested (angles of 110-135 ° and height of 35 - 55 mm). Buckling tests were made using two different steel grades; ultra-high-strength (UHS) ARS400 and DC01 mild steel. For comparison, FEM simulations were carried out for the ARS400. The results showed that even 600% higher bending resistance can be achieved for the panel structure using the ultra-high strength steel instead of the low strength counterpart. The comparison showed that the FEM-simulations can be used reliably estimating the buckling effects in UHS panel structures. The difference between the empirical and simulation results was 5.3% in average (S.D. 4.1). In practical tests, best angle and height for the ARS400 was 110 ° and 35 mm respectively. For the DC01, the best dimensions were 125 ° and 35 mm.

Abstract: As a new manufacturing method, Additive Manufacturing has begun to get a foothold in the manufacturing industry. The effective exploitation of the technology requires many times a re-design of the product or re-considering the manufacturing technology. Design for additive manufacturing differs considerably from design to other manufacturing methods, therefore design guidelines for additive manufacturing has been developed. The purpose of this paper is to present a new variant of the Design for Manufacturing and Assembly (DFMA) method which supports additive manufacturing.

Abstract: This study describes design and construction of a novel flexible heat treatment line for processing customer-oriented small batch steels. The induction heater (600 kW) developed is suitable for the sheet thickness in the range 3.2 30 mm and the width of 85 1250 mm. Sheets are fed using an electrical motor (1.5 kW) and a chain drive, the speed being in the range 0.3 7 m/min, depending on the power and the sheet dimensions. At this study, 4.5 (WR-1) and 10 mm (WR-2) thick wear resistant steels were tempered at different peak temperatures to compare the effect of rapid tempering on mechanical properties. Results showed that the heat treatment line is capable of producing tempered steel grades with adequate properties at industrial product rate. For example, 4.5 thick WR-1 tempered at 550 oC provided a yield strength (YS) over 1000 MPa with minimum bending radius of 6 mm (in the delivered condition YS = 1605 MPa and R_min = 12). Tempering of WR-1 at 700 oC provided YS of 762 MPa and R_min of 1 mm. Results were similar between two test materials, but the enhancement in bendability was slightly more effective with the thinner sheet.