The demands of market in an area of engineering materials are getting greater. Hence, it is important that the designed and fabricated materials could fulfill desirable requirements in specific applications. It is obvious that none of the materials fully satisfy all requirements but a combination of different properties is possible to obtain in composites. They are very attractive for lots of applications due to unique properties.
It can also be seen the rapid growth of interest in the area of joined materials (Ref 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26). These materials are increasingly used in engineering applications because of their special properties, for example high strength-to-weight ratio, corrosion or erosion resistance, high-temperature resistance or high-temperature strength (Ref 1). The joined materials are usually composed of layers made of two similar or dissimilar (in terms of chemical composition) materials (predominantly metals/alloys or ceramic), which are permanently connected to each other. Using appropriate joining techniques, it is possible to produce finished elements in the form of the whole uniform product. An ability to join dissimilar materials enables to creation of lightweighting structures for automotive industry, modern medical implants, useful tools, equipment for consumers, and many other products (Ref 2,3,4,5,6,7).
Joining techniques may be also applied in powder metallurgy technology (Ref 1, 3, 5, 7,8,9,10). It is important to note that the most significant property of sintered materials is porosity, because of the influence on the choice of joining process and quality of obtained joint (Ref 3, 5, 7, 11). Sintered parts with density below 6.5 g cm−3 have the large pore volume. That is why, they should not be joined using methods related to local remelting. The recommended joining processes of these parts are diffusion welding, sinter-bonding, adhesive joining and brazing. The sintered parts with density higher than 7.2 g cm−3 can be welded. A wide variety of processes can be used to join in the case of intermediate density level (Ref 5). However, resistance projection welding and friction welding are preferred (Ref 3,4,5, 7, 11). It should be emphasized that using an additional joining operation (such as welding) has some disadvantage. It is a further step of production and needs appropriate devices so it entails an increase in product manufacturing cost (Ref 2, 5, 13).
In the case of the sintered parts, besides universal joining techniques such as welding or brazing, sinter-bonding and sinter-brazing are increasingly used (Ref 3,4,5,6,7,8, 12,13,14,15,16,17,18,19,20,21,22,23,24,25,26).
Sinter-bonding (also called sinter diffusion bonding) is a specific joining method applied only in powder metallurgy technology. It is increasingly being used on an industrial scale, especially by the largest manufacturers of sintered parts (Ref 3, 5, 6, 8). Usually, sinter-bonding relies on simultaneous sintering process of two green compacts, which are placed in furnace one on top of the other (Ref 3, 5, 6, 9). They can be prepared using the same or different powders (or powder mixtures). The connection between these green compacts is created during sintering as the result of solid-state diffusion, liquid-state diffusion or mechanical interlocking (Ref 3, 5, 8). Furthermore, the connection can be created using a bonding agent (which is placed between green compacts) or without it. It is also possible to joint green compact with part made of wrought material (Ref 5, 14). Sinter-bonding may be achieved by using different PM techniques such as conventional co-pressing and co-sintering (Ref 6, 8,9,10, 16), cold or hot isostatic pressing (HIP) (Ref 6), Spark Plasma Sintering (SPS) (Ref 1) or co-injection molding (Ref 17,18,19,20,21,22,23,24,25,26). It is considered that co-pressing and co-sintering of two materials are a beneficial solution because of simple production and lower cost in comparison with other fabrication methods (with addition joining operation). Furthermore, this method is also used to fabricate the sintered functionally gradient materials (Ref 3, 5, 6, 8,9,10).
The sinter-bonding behavior was studied by several authors (Ref 8,9,10,11, 13,14,15,16, 18,19,20,21, 23,24,25,26). In order to fabricate a desired microstructure and properties, different materials including iron (Ref 8,9,10, 23, 24, 26), low-alloy steels (Ref 8), high-alloy steels (Ref 8, 11, 13, 15, 16, 18,19,20,21, 23,24,25,26) as well as hardmetals (Ref 9, 10) were joined together during co-sintering. The studies focused on the influence of the techniques and conditions of sinter-bonding on an interface formation between two materials as a result of co-sintering, the densification, an evolution of microstructure as well as the microstructural characterization in order to achieve the crack-free complex products. It can be stated that sintering conditions have a significant effect on the densification and the co-sintering behaviors. Thus, they must be properly selected and controlled during co-sintering process (Ref 15, 24).
Using powder layering technique and co-pressing (for example, compaction with floating die, single-action as well as double-action mode of uniaxial pressing and cold isostatic pressing) or co-injection molding and then co-sintering, bi-materials components can be produced from AISI 316L and 17-4 PH stainless steels powders (Ref 6, 15, 16, 20, 22,23,24). In this way, the joining of the two different materials can be combined with the production of complex-shaped components. It is an attractive and economical production method of the two-body products. AISI 316L and 17-4 PH steels have great industrial applications. They exhibit a good corrosion resistance and high strength. AISI 316L is non-magnetic, while 17-4 PH is magnetic. Products made as AISI 316L and 17-4 PH components could be magnetic properties at one area and non-magnetic properties at another area. In order to achieve such products, it is necessary to select the appropriate parameters of co-pressing and co-sintering processes which enable to obtain a good solid connection in combination with the stability of the product shape.
Not much published information (Ref 6, 15) is available on the sinter-bonding of AISI 316L and 17-4 PH steels using a conventional PM, namely co-pressing and co-sintering processes. It should be noted that it is one of the most advantageous solutions because of low cost and simple production of elements. Based on a literature review, it can be stated that AISI 316L/17-4 PH components are usually produced by other PM techniques, mostly co-injection molding and then co-sintering (Ref 16, 20, 22,23,24).
The densification and microstructure of 316L/17-4 PH bilayers were studied (Ref 15). They were produced using commercially available gas atomized stainless steel powders by co-pressing and co-sintering at temperatures ranging from 1100 to 1340 °C for 120 min in hydrogen and vacuum. The mismatch strain and strain rate of bilayer during co-sintering process were calculated. It was found that the strain rate of 17-4 PH was higher than that of 316L during sintering. This caused mismatch strain between two layers and affected the densification rate of the bilayer. It should be pointed out that too high densification rate increases the danger of interface cracking and product deformation. It was found that materials should be sintered in a hydrogen atmosphere due to the higher density and the lower mismatch strain and strain rate between layers. 17-4 PH and 316L steels show a similar sintering response with a maximum strain incompatibility of 0.5%.
Based on a literature review, it can be stated that the influence of the pressing pressure was not analyzed and particular analysis of the influence of all sintering process parameters on densification, microstructure evolution or sintering compatibility of AISI 316L and 17-4 PH steels was not performed.
The present work concerns the application of sinter-bonding method to join dissimilar materials. The commercially available water atomized stainless steel powders of AISI 316L and 17-4 PH and the powder layering technique in combination with co-pressing and co-sintering were used to produce AISI 316L/17-4 PH components. The effect of co-pressing and co-sintering parameters on the densification, sintering compatibility and microstructural evolution of AISI 316L/17-4 PH components was investigated.