Evaluation of High Pressure Water Atomized Powders for Large Scale PIM Production using Different Binder Formulations

Transcription

1 Evaluation of High Pressure Water Atomized Powders for Large Scale PIM Production using Different Binder Formulations Volker Arnhold, Nicola De Cristofaro, Jack Hamill* GKN Sinter Metals, *Hoeganaes Corporation ABSTRACT Recent developments in high-pressure water atomization provide an economical alternative to both gas atomized and chemically produced powders for PIM applications. The new process achieves fine particles representing near-spherical shape with an appropriate size distribution for both high and low alloy metal powders. The PIM manufacturing process that include molding, debinding and sintering is strongly influenced by the metal powder characteristics and binder formulation. This work presents a comparative analysis for processing of water atomized powders using several binder formulations and debinding methods. The experiments are conducted under laboratory conditions as well as an industrial environment currently producing an assortment of high volume PIM parts. INTRODUCTION Comparing PIM (powder, feedstock, mold, debinding stages, sinter, ) with conventional PM (powder, blend, press, sinter, size, ) we can state that P/M represents much larger volumes and more common, robust and well defined industrial process than PIM. Moreover, PIM has the image of incorporating black magic or secrecy, which limits acceptance and growth potential with end-users or potential producers of parts. It must be the target of the scientific and industrial community to identify optimum processing and strengthen the availability of such best in class processes to grow the overall PIM market. Today, in the relatively early stages of the PIM life cycle, the main competition does not happen between different PIM producers, but with other technologies like investment casting, machining or, very seldom, conventional PM. An economic challenge of PIM is the cost of the fine powders produced either via gas atomization or high pressure water atomization for prealloyed materials or via chemical processing for making base iron or nickel powders. In this paper we want to focus on pre-alloyed grades (316L, 17-4 PH) and especially look at water atomization, which should offer price advantages versus gas atomization. The latest developments indicate these powders appear to be feasible and potentially competitive with gas atomized and carbonyl iron materials. 1

2 There are numerous aspects associated with PIM processing that should be conceptualized strategically before committing to the technology as a viable manufacturing technique: Today s and future needs of customers for such products Possible profits / competitive situation Cost of process Robustness of processes (quality, e.g. for aerospace or automotive applications) Health, Safety, Environment (HSE) impacts (e.g. legislation and internal risk analysis) Availability of raw materials (powders, feedstock) Flexibility for further developments This paper deals with some of the questions from an industrial point of view. What could be the future trend in powders and feedstock systems to grow the overall business worldwide? The main topics to be discussed are: Water or gas atomization PIM feedstock s in an industrial environment Experiments and results (lab and industrial high productivity processing) Evaluation of the findings and conclusions to be drawn for an outlook WATER OR GAS ATOMIZATION? Gas atomized stainless steel powders have been preferred for metal injection molding feedstock production. The attributes which make gas atomized powders most attractive include spherical shape and fine particle size distributions. Both of these characteristics enhance powder-packing performance, which translates into a higher volume of metal powder incorporated with the binders to optimize feedstock rheology and minimize shrinkage during the sintering process. However, high process costs and low yields of gas atomized powders limit their cost-effectiveness for larger and/or higher volume PIM part production. The recent developments in water atomized stainless powders, exhibiting improved characteristics which nearly replicate the size distributions of gas atomized powders, now provide opportunities to reduce material costs thereby extending the size and type of PIM applications. Currently, there are no accepted industry practices for reporting particle size for either gas or water atomized powders. Inconsistencies are commonly evidenced in the way suppliers characterize their various powder grades. The lack of reference standards, difficulties with sensitivity levels of laser diffraction equipment and use of improper testing techniques all share partial responsibility for the inconsistencies. To illustrate this point, Table 1 compares the size distribution of 17-4 PH gas atomized powder and the recently developed water atomized powder tested consecutively using the same laser diffraction unit. The gas-atomized sample in the comparison represents a commercially available powder characterized as minus 22µm particle size. As the comparison indicates, the major distinction in size between the two powders occurs at the d 90 volume fraction. These findings are consistent with a previous 316L stainless comparison [1]. 2

3 Table 1: Microtrac analysis comparing gas and water atomized 17-4PH stainless PIM powder Microns d10d50d90MVstd dev Gas Atomized Water Atomized (Hoeganaes) Inert gas atomized powders generally exhibit improved metal injection molding performance relative to higher tap densities, but have a propensity to slump or distort if not adequately supported during the debinding process. The water atomization development efforts attempted to achieve near-spherical particle shape as a suitable compromise. This allows for reasonably good powder packing, yet maintains adequate green and brown strength after molding and debinding. To illustrate this point, the tap density for standard (low pressure) water atomized 316L fine powder distributions generally approximate 3.8 g/cm 3 with a specific surface area (BET) of 0.6 m 2 /g. Whereas, the newly developed powders, incorporating unique process and/or melt compositions, now represent tap densities of 4.2 g/cm 3 and specific surface area of 0.3 m 2 /g. Table 2, shows the typical properties of 316L and 17-4 PH made by high-pressure water atomization. Table 2: Typical Powder Properties [ 2 ] FPI Grades17-4 PH316L D 10 µm1010 D D Tap Density, g /cm Carbon, w/o Oxygen, w/o Nitrogen, w/o True Density, g /cm Area, m 2 /g The economic advantages of high-pressure water atomization inherently allow for greater flexibility in comparison to other powder processes. Prealloying capability support various grade compositions and provide homogenous metal powder feedstock in contrast to the use of various ferroalloy combinations. Costs related to the atomizing medium, energy consumption, productivity rates, and product yields all favor water atomization in comparison to a carbonyl process or gas atomization technique. With this approach, suitable powders are now available to support development of larger PIM part applications and satisfy the needs of a competitive marketplace. 3

4 PIM FEEDSTOCK SYSTEMS FOR WATER ATOMIZED POWDERS For high productivity industrial PIM processes the following issues are essential and the base of the current PIM philosophy at GKN Sinter Metals: Powders should be readily available (gas / water atomized, carbonyl) Efficient, short cycle 2-step debinding process (chemical catalytic or solvent extraction) Continuous sintering (walking-beam for standard and high temperature sintering) preferably with integrated 1 st step debinding Economics of in-house manufacture or purchased feedstock HSE impact of the PIM processes For high volume production of low alloyed (Fe-Ni-C) or standard high alloyed (316L, 17-4 PH) grades the continuous sintering process is most competitive and robust. Furnace parameters (temperature, speed, atmosphere) should never be changed. For special grades (e.g. Ni or Co base alloys) the need for different temperatures and atmospheres with lower lot size makes batch furnace sintering more favorable. Predominantly gas atomized powders are used for these higher alloyed products. This was influenced in the past by the availability of such powders and the support and readiness of feedstock manufacturers to qualify and supply feedstock based on water atomized powders. Concurrent work at the Center for Innovative Sintered Products - Penn State University involved the evaluation of several custom formulated, polymer-wax binder system iterations designed for use in conjunction with water atomized powders. The testing procedures included capillary rheometry, pycnometer density and laboratory molding trials. The results of that work provided the following conclusions [ 3 ]: Newly developed water atomized powders can be processed successfully when used in concert with an appropriate binder system to compensate for the slight differences in particle shape and tap density. Binder systems formulated for gas atomized powders generally result in severe powder-binder separation when used with water atomized powders. Differences in the surface oxide preclude the use of stearic acid as a surfactant with water atomized powder. Incorporating a high viscosity binder without stearic acid reduces powder-binder separation tendencies. This subsequently identified a promising polymer-wax binder formulation used as the benchmark for comparison to commercial systems. Reasonable green weight variation (<0.3%) was maintained for this binder using lower speed and pressure conditions without the assistance of closed loop feedback control. None of the feedstocks evaluated displayed ideal rheological behavior under the test conditions. This gives some information for further routes to optimize binder formulations for water atomized powders. This work is now in progress. 4

5 BINDER EVALUATION: To evaluate the powders and processes from the viewpoint of industrial applicability a study was done, using six established binder formulation. Feedstocks were made from Hoeganaes 316L and 17-4 PH water atomized powders using the following binder concepts to be compared with standard production grades (Table 3): Table 3: List of binder formulations CodeDescriptionBinder Content 316L (~ wt %) Binder Content 17-4 PH (~ wt %) Debinding time 1 st step ( h ) 1POM - catalytic debinding (patent by BASF) GKN solvent extraction (organic solvent)887 3Penn State solvent extraction (organic solvent)887 4Planet Polymer solvent extraction (water)11-1) 24 5Zschimmer & Schwarz solvent extraction (water)97 2) 30 6Standard production grade (BASF)8 2) 12 3) 4 1) : not available 2) : gas atomized powder 3) : proprietary master alloy system EXPERIMENTS AND RESULTS FOR INDUSTRIAL PROCESSING The feedstocks were manufactured by the different companies listed above. Code 1 was made by GKN using the POM technology. For timing reasons and due to limited volumes of powders no special improvements could be done to optimize the feedstock process. Parts were then molded using the standard mold parameters of a high volume production PIM part (Figure 1, old design). Figure 1. Part 9816 old design, used for the evaluation 5

6 Figure 2. Description of Dimensions I, II, III In principle, tools have to be designed for a certain feedstock system, i.e., to accommodate each binder formulation. As this was not possible in our study, we have to consider this in the interpretation of all results. The tools had been originally designed for the BASF feedstock with carbonyl Fe-Ni powders, which is used for standard production (especially for this part). Still all processes worked satisfactorily, parts were molded to acceptable quality standards. The sintering parameters were identical for all specimens - standard production sintering cycle in a Cremer walking beam furnace at 1365 C with 100% hydrogen atmosphere (i.e. conditions were not optimized for a specific formulation). Table 4 and 5 identify three specific dimensional characteristics (average values) for comparison purposes in the green and sintered state. Table 4: Dimensions of 316L part # 9816 CodeDimensions, green (mm)dimensions, sintered (mm) Dim. IDim. IIDim. IIDim. IDim. IIDim III XSXSXSXSXSXS

7 Table 5: Dimensions of 17-4 PH part # 9816 CodeDimensions, green (mm)dimensions, sintered (mm) Dim. IDim. IIDim. IIDim. IDim. IIDim. III XSXSXSXSXSXS Table 6: Properties of parts made from 316 L PropertyUnitCode Carbon cont. % Oxygen cont. % Av. shrinkage % Weight greeng Weight sinteredg Av. weight loss % Densityg / cm Table 7: Properties of parts made from 17-4 PH PropertyUnitCode Carbon cont. % Oxygen cont. % Av. shrinkage % Weight greeng Weight sinteredg Av. weight loss % Densityg / cm

8 Figure 3 a 316L water atomized (1365 C) Figure 3 b 316L gas atomized (1365 C) Figure 4 a 17-4 PH water atomized (1365 C) Figure 4 b 17-4 PH gas atomized (1365 C) Figure 4 c 17-4 PH, master alloy system (1365 C) Table 6 gives properties of the parts after sintering (316 L), Table 7 shows these properties for 17-4 PH. Typical microstructures for the different grades are shown in Figures 3 (316 L) and 4 (17-4 PH). 8

9 DISCUSSION OF THE RESULTS The sintered densities for 17-4 PH are equivalent for water and gas atomized powders ( g/cm 3, at 1365 C ). However, the 316L gas atomized powders show at 1365 C higher sintered density, 7.6 g/cm 3 vs. 7.3 g/cm 3 as average, compared to water atomized powders. The weight of the green part (combined with the dimensions, see Table 6) indicates a very high powder load of code 6 (BASF, 316 L, gas atomized). This leads to the high density of 7.60 g / cm 3. The feedstocks for water atomized 316 L have to be optimized for standard production or the sintering temperature has to be slightly increased. Obviously the sinter activity of 17-4 PH is much higher than 316L. Carbon and oxygen contents are equivalent in all cases. The 316L microstructures are comparable; however, the gas atomized grade exhibits somewhat larger grain size (Figures 3a, 3b). The microstructures of 17-4PH reflect the different manufacturing methods; homogeneous structures for the atomized grades (Figure 4a, 4b), and 2-phase structure for the master-alloy grade (Figure 4c). There is no clear winner in the feedstock systems related to the above described data. The dimensional stability is equivalent for all systems. The large differences in shrinkage, leading to different dimensions and densities after sintering clearly indicate the need to optimize the process for each part (including possibly some tool changes to optimize the molding process for a given binder). CONCLUSIONS AND OUTLOOK Given the context of both laboratory and production evaluation, the study netted the following results and/or observations: The results show, that water atomized powders are a valid technical alternative to gas atomized powders. No problems in making good parts exist. The primary driver will be cost. The economics have to be evaluated in detail and depend also on the feedstock process. The ideal binder system does not exist (yet). Catalytic feedstock has the highest green strength (easy to handle). The other binders exhibit somewhat improved molding characteristics (fine shapes and contours). The catalytic process has the shortest processing time and is the only process realized in a production chain. It also has shown its robustness in mass production. On the other side, catalytic debinding shows principally some potential risk in the handling of concentrated acids and the need of running a controlled combustion of reaction gases. The solvent extraction methods are all applicable. Organic solvents may lead to some difficulties under HSE aspects. The optimum feedstock should be made via a water based or a solvent extraction process using water as solvent with reasonably short debinding cycle. Further R&D work is necessary and in progress. 9

10 The authors want to thank BASF, Planet Polymer and Zschimmer & Schwarz for supporting the work with samples or their input on discussions and analysis of results. Also we want to thank Randall M. German (Pennsylvania State University) and Neal Myers (Center for Innovative Sintered Products) for their contribution, analysis of materials and discussion of results. REFERENCES [ 1 ] Jack Hamill, Christopher Schade, Fine Particle Atomization Technology EUROMAT 99 proceedings, September 1999, Munich, Germany [ 2 ] Jack Hamill, Christopher Schade, Neal Myers, Water Atomized Fine Powder Technology 2000 Powder Metallurgy World Congress proceedings, November 2000, Kyoto, Japan [ 3 ] Neal S. Myers, Randall M. German, Binder Selection for PIM of Water Atomized 316L, to be presented at PM 2 TEC 2001, New Orleans, LA. 10