Lead (Pb), as in steels in general, makes brass easy to work (with low fusion point), reduces friction between chip and insert, as well as wear and tear, and finally improves chip control.
Laboratory tests show the characteristics of the materials being examined as the composition of the alloy changes, in particular of lead.
Once the cutting parameters, tool type and cooling lubricant are fixed, the first aspect examined is the one linked to the formation of the chip which, in a high-volume production, may condition its efficiency.
The formation of short chip is optimal in CW614N brass, irrespective of the cutting parameters used.
For those as CW510L and CW511L classified brasses, on the other hand, as the percentage of lead decreases, the chip gets longer, reaching a continuous, tubular shape.
In recent years, new brass alloys have been created containing other alloy elements with the aim of finding an alternative to lead, such as, for example Silicon (Si), Arsenic (As), Carbon, etc.
The most widely-used and most successful alternative is represented by CW724R (Ecobrass), where the presence of Silicon (Si) partially compensates for the absence of lead. This influence is both negative and positive. In fact, Silicon (Si) forms oxide inclusions which result in an increase in wear and tear. The inclusions, however, have a considerable influence on workability, even if they are present in very small percentages in the total composition.
In the same way, through the tests, the Specific cutting force (kc) was obtained, the increase of which, due to greater frictional force in the contact area between the tool and the workpiece, due to the absence of lead, entails the use of greater power, causing high temperatures in the cutting zone which reduce the tool life.
Consequently, the efficiency of the machine, caused by unscheduled shutdowns due to the presence of long chip, is compromised, as is the efficiency of the tools.
After analysing the effects of lead reduction in brass alloys, a parameter which can define, in first approximation, all the critical factors examined, is the index of workability metal cutting.
The reference index for workability represented here has a value equal to 100% represented by the CW614N brass, with a quantity of lead from 2.5% to 3.5%.
Alloys with a lead content of 0.2% or less, which also comply with US regulations, are CW509L and CW510L, with a workability index of 50%.
CW511L alloys, with a lead content of 0.2% or less, on the other hand, have a workability index of 30%.
Finally, CW724R, traded under the name of Ecobrass, reaches a workability index of 70%.
The workability index highlights the characteristics of CW724R (Ecobrass), but the negative aspects must also be considered.
In fact, the main reasons why CW724R “Ecobrass” is not used are:
When working these materials, some people restrict themselves to only using more powerful machines, which allow relative working, even if not in an optimal way. For making lead-free brass products, (CW511L, CW510L, CW509L, etc.), the ideal approach must necessarily envisage new working technologies, new tools and new mandrels with the right characteristics.
To counteract the abrasive power of the new materials, harder tools are needed which are more resistant to wear and tear. Since they are more fragile, however, they need to be installed on mandrels which enjoy higher rigidity, and possibly on work units with hydraulic vibration damping systems, such as, for example, the Picchi “High performance HPLM” units.
With the gradual increase in the sizes of the chip, the necessary power increases, but production efficiency decreases. Therefore, it is often necessary to manually remove the chip which cannot be evacuated. It is indispensable to reduce the size of the chip and the tool must be reviewed, and when this is not enough, resorting to PRT511 technology is always possible.
