Abstract
	The aim of the submitted work is to evaluate hardness measurement process capability of various qualitative standards of hardenable wrought aluminium alloy - superduraluminium. The standards were simulated by casting (two cooling rates, 4.5°C.s-1 and 20°C.s-1), heat treatment (cooling rate of casting 4.5°C.s-1: solution treatment 6 hours at 500°C ± 5°C with following quenching into water 40°C and natural age hardening 0, 48 and 96 hours at 20°C) and thermomechanical treatment (cooling rate of casting 20°C.s-1: solution treatment 6 hours at 490°C ± 5°C with following quenching into water 40°C and pressing with amount of deformation ??????? and natural age hardening 96 hours at 20°C).
         	The Al  alloy „Superduraluminium“ (STN 42 4203) was used as an experimental material. This type of alloy although being practically the first heat treatable alloy to be discovered still finds wide application for many general engineering and aircraft structural purposes in the form of forgings, extruded bars and sections, sheet, plate, tube and rivets. The excellent properties of this material are exploited in automotive industry. The alloy has good fracture toughness , but shows signs of overheating at about 510°C. 
         	The surface of samples was milled and ground by abrasive paper. Polished metallographic samples were etched with 25 % H2SO4 at 75°C and consequently with 0.5 % HF. The microstructure was evaluated according to STN 42 0491 and STN 42 0462. The average grain diameter was 41.65 ?m for cooling rate 4.5°C.s-1 and 21.55 ?m for cooling rate 20°C.s-1.
           	The quality of alloy was evaluated by Brinell (HBS5/250/30) hardness test according standard ISO 6506  (STN 42 0371). Hardness tester HPO 250, being up to standard STN EN ISO 6506 - 2 (42 0371) for maximum permissible error, was used as measuring instrument.  
           	Because the Brinell ball makes the deepest and widest indentation and the test averages the hardness over a wider amount of material, which will more accurately account for multiple grain structures, and any irregularities in the uniformity of the alloy (typical for cast structure) and soft materials, the Brinell hardness test was applied. 
          	Attempts to standardize the evaluation of the measurement processes capability can be observed for several years.  The capability of measurement processes is searched by analogy with that of production processes. Process capability means the ability of the process to meet technological or other requirements.  Measurement process capability is determined by total variation caused by random reasons influencing the process. 
          	Measurement system analysis (MSA) is an experimental and mathematical method of determining how much the variation within the measurement process contributes to overall process capability. MSA involves gauge repeatability and reproducibility (GRR) studies to evaluate measurement systems. It is an interactive, multimedia, system designed to help engineers and quality professionals assess, monitor, and reduce measurement system variation. MSA helps us conform to ISO 9000 and ISO/TS 16 949:2002 requirements. 
          	To make a GRR study, the same type of parts or samples (10 pieces), two appraisers (people who measure the parts, the appraiser A was “cadet”, the appraiser B “veteran”), one measurement instrument and 3 measurement trials, on each part, by each appraiser were used. The multiple trials were necessary to estimate repeatability %EV and multiple operators to estimate reproducibility %AV. Multiple parts allow to obtain better estimates of repeatability and reproducibility, as well as to estimate part variation %PV.  Index   %GRR represents the overall process capability and ndc (number of distinct product categories) is relative to resolution of measurement instrument. 
           	As a general rule of thumb is the measurement system resolution ought to be at least one-tenth of the process variability. This demand is fulfilled, because the resolution is 0.22 HBS 5/250/30 and  equivalent standard deviation is  between 3.67 and 16.37 HBS 5/250/30. 
          	As   %GRR exceeds 3 0% (95.69 % cast state - cooling rate 4.5°C.s-1, 66.14 % cast state - cooling rate 20°C.s-1, 85.33 % quenched,  90.44 % natural age hardened 48 hours, 93.56 % natural age hardened 96 hours and 93.03 % thermomechanical treated) the used hardness measurement system was not capable. The number of categories (ndc) is less then 2, the measurement system is of no value in controlling the process. It is all noise and one part can not be said to be different from another. The inhomogeneity of tested material at cast state with low cooling rate and partial melting down of grain boundaries and formation of cracks and micro-voids at age hardened and thermomechanical treated states are the causal relation of low capability.