Welding Technologies For Repairing Plastic Injection Moulds

Transcription

1 Welding Technologies For Repairing Plastic Injection Moulds B. Silva 1a, I. Pires 1b, L. Quintino 1c 1 IDMEC, Instituto Superior Técnico, Av. Rovisco Pais, Lisboa, Portugal a bmrssilva@yahoo.com, b inespires@ist.utl.pt, c lquintino@ist.utl.pt Keywords: Tig, Laser, Plastic moulds repair, Micro-welding, Tool steels Abstract. The aim of the present paper is to investigate the metallurgical behaviour of AISI P20 tool steel frequently used in injection moulds, after welding. For this purpose Tig and Laser welding tests were conducted to determine the influence of the welding parameters, materials and pre-heating. The welding quality was assessed through the microstructural properties and microhardness evolution. The results achieved with the different processes were compared. This work allows to obtain a deeper knowledge concerning tool steels behaviour after welding. Introduction Polymer injection moulds frequently require some form of repair by welding: (a) during the manufacture, by errors in machining or by some change in the design of the part to be injected and (b) during the service, by the incidence of failures in the mould [1]. Fusion welding is frequently used for rebuilding worn or cracked surfaces and to modify the shape of existing tools. In particular, the repair of plastic injection moulds by welding is routinely performed to remove surface wear, erosion and stress cracking, increasing significantly the tool life [2]. However the application of this repair technology is not simple: complex shapes, hard-to-reachareas, thin blades and small sizes within specified tolerances. Welding in moulds is extremely demanding in what regards the mechanical/metallurgical know how and the welder s skills. Mould materials are generally high alloy steels, the weld deposits must be done in complex geometries, with reduced thicknesses and small tolerances, and access is often difficult [3]. Despite the fact that equipments (TIG/LASER) and filler materials for micro-welding for high performance materials have been introduced in the market, the problems related to their use in moulds are numerous mainly due to the scarce existent know-how (the scientific work published for sheet welding does not contemplate moulds specific requirements) [4]. The required quality of the weld is not only related to the suitable mechanical properties, but also to the weld and the heat affected zone () behaviour when finishing processes are applied to give a uniform and adequate texture to the mould cavity [5]. Nowadays, the welding procedures applied for the repair of moulds are elaborated having as a basis the recommendations given by the manufacturers of steel, but it remains not clear what are the involved metallurgical phenomena, whose knowledge is necessary to define rational procedures, suitable for different materials, sizes and geometries of the moulds to repair. The purpose of the present study is to analyze the influence of different processes and welding parameters on the metallurgical behavior of AISI P20 tool steel.

2 Materials and experimental procedures In order to study the influence of the welding parameters in the welds made by Tig and Laser processes weld beads were made in AISI P20 tool steel. This tool steel is commonly used in the manufacturing of polymer injection moulds and it is furnished in the quenched and tempered state, with hardness between 30 and 34 HRC. The welds were made using a filler wire recommended by the steel manufacture. The compositions of the base metal and filer wire are indicated in table 1. Chemical composition of base metal and filer wires. C Mn Cr Mo V Si Al Cu Ceq P20 base metal TIG filer wire (ø 1mm) Laser filer wire (ø 0.6mm) Table 1. Chemical composition of base metal and filer wires. In what regards Tig welding eight beads on plate were made, on 24mm thick steel, using four levels of welding current and two welding speeds. Two preheating temperatures were used to account for its influence of on the hardness of the weld after welding, (Table 2). The wire feed speed was in each case adjusted to produce satisfactory welds. Multipass welds were also made with the purpose of investigating the effects of the repeated welding thermal cycles in the hardness and microstructure of the welds. After welding, some samples were subjected to a tempering heat treatment to evaluate if it was possible to obtain a similar hardness in base metal (BM), heat affected zone () and weld metal (). TIG welding parameters. Current [A] Travel speed [cm/min] Heat input rate [KJ/cm] Cooling speed [ºC/s] T O =250ºC T O =350ºC , , , , , , , , Table 2- TIG welding parameters (arc voltage = 11V), and estimated cooling speed [6]. In what concerns Laser welds, these were made using a pulsed Nd-YAG laser. The parameters used in these set of experiences are indicated in table 3. All Laser welds were made without preheating, following normal industrial practice, and no post-weld heat treatment were used. Laser parameters.

3 STD *1 Higher Power Lower Power Higher Duration Lower Duration Pulse power [Kw] Pulse duration [ms] Pulse frequency [Hz] Table 3 - Laser parameters. *1 The parameters marked as STD (standard) were chosen by the welder based on his experience, the others (Higher Power, Lower Power, Higher Duration, Lower Duration) are variations of the STD parameters. Travel speed of 0.2 cm/min. After welding the plates were sectioned and metallographic samples were prepared by grinding, polishing and etching procedures (Nital 2%). Hardness profiles in the different regions of the welds were generated using a Vickers microhardness tester with a load of 1 Kgf. Results and discussion Macroscopic aspect of the deposits. In what regards the macroscopic aspect of the weld deposits it was observed that both processes produced welds without detectable superficial defects, although Laser weld deposits were smother, more regular, and show an improved superficial homogeneity when compared to TIG deposits. After cutting and polishing the samples it was observed that some of the laser welds presented defects caused by incomplete fusion of the base material. However, for higher pulse duration (HD) these defects are not present anymore, as shown in figure 1. a) b) c) Figure 1- a) Tig deposit (100A, 15cm/min), b)laser weld deposit (STD), c) Laser weld deposit (HD). After etching the samples it was observed that with laser welding the depth was limited to less than 200 micrometers, wile in TIG this depth was no smaller than 1.8 millimetres. Microstructures and hardness profiles. Microstructural observation of the samples show that in the case of single bead Tig deposit the and the are composed by martensite with high hardness levels, and the BM has a tempered martensite microstructure (Fig 2). Figure 3 represents the evolution of the mean hardness in the and with the heat input, where it can be seen that for this case, hardness values are relatively independent of the heat input. It is also possible to seen, that there is a little hardness decrease with the increase of the preheating temperature. The higher level of hardness in the and can be explained by the high temperability of the BM (the critical cooling speed is 1 ºC/s), in combination of higher cooling speeds. The has lower hardness, compared with the, due to the dilution effect (between 40 and 60%). Figure 4 compares the hardness profiles, measured in a horizontal line parallel to the surface, in single and multipass welds, where it can be seen that the deposit of more than one bead has a tempering effect in some portion of the previous and, lowering the hardness level. However, in the tests made within this work, this effect was not sufficient to lower the harness to values correspondent to the hardness of the base metal ( HV), or to produce a uniform hardness profile. For the welds which were tempered, it was possible to obtain a mean hardness of HV in both and.

4 a) b) c) Figure 2- SEM photomicrography of a) Weld Metal, b) Heat Affected Zone, c) Base Metal. Mean hardness Mean hardness Heat input [KJ/cm] Heat input [KJ/cm] a) b) Figure 3- Mean hardness of TIG single pass welds, a) with 250ºC preheating, b)with 350ºC preheating BM Single pass TIG weld BM a) b) Multi pass TIG weld Figure 4- Hardness profile of a) single pass Tig weld, b) multi pass Tig weld. In the case of Laser welds the is also martensitic with a max. hardness of 570 HV, as shown in figure 5. The weld metal has a mean hardness of HV and a martensitic microstructure with some retained austenite. Laser weld Figure 5- Hardness profile in Laser welds (HD parameters). Conclusions

5 Based on the experimental investigation, the following conclusions can be draw: a) In relation to TIG welding: -Despite the use of preheating, the welds present regions with high hardness values corresponding to martensitic microstructures. In this case preheating is essentially used to reduce the risk of the material to crack during welding. In multipass welds some of these regions were tempered by the repeated welding thermal cycles, lowering the hardness values. However, this procedure did not bring the hardness to base metal values. - The tempering treatment made in some samples has shown that it was possible, with a postwelding heat treatment, to obtain a uniform hardness profile across weld zone, base metal and heat affected zone, even using a filer wire with a composition different from that of the base metal. b) In relation to Nd-YAG laser welding: -Due to is low thermal load, preheating is not used in this type of laser welding. The weld deposits had a homogeneous and defect free surface aspect, with small and regular beads, making this process ideal for repairing small features in polymer injection moulds. -Through the analysis of the cross sections of the weld beads lack of fusions defects were detected in some samples. These defects, that are not detectable by the welder, show the need for adequate parameter setting. The choice of welding parameter should be based in experimental tests and should assure that complete fusion is always achieved. -The heat affected zone microstructure presents some high hardness martensite, but the tinny dimensions of this zone allow not to use post-welding heat treatment. Acknowledgements This work was supported by the POCI 2004/FCT/Portugal. References [1] P.Peças, E. Henriques, B. Pereira, M. Lino, M. Silva: Fostering the use of welding technology in the mould repair, RPD 2006 Building the Future by Innovation. [2] Options for restoring molds, S. Roy, M. Francoeur, September, [3] W. T. Preciado, C. E. N. Bohorquez: Repair welding of polymer injection molds manufactured in AISI P20 and VP50IM steels, Journal of Materials Processing Technology 179 (2006) [4] M. Vedani: Microstructural evolution of tool steels after Nd-YAG laser repair welding, Journal of Material Science 39 (2004) [5] E. Capello, B. Previtali: The influence of operator skills, process parameters and materials on clad shape in repair using laser cladding by wire, Journal of Materials Processing Technology 174 (2006) [6] E. M. Dias Lopes, R. M. Miranda: Metalurgia da Soldadura, Edições Técnicas do ISQ. Lisboa, 1993.